Today’s Standard Model describes our Universe’s particles and forces.

Six quarks, six leptons, and antiparticles comprise the fermions.

Eight gluons, one photon, the weak W-and-Z, plus one Higgs represent our bosons.

Gravity, electromagnetism, and strong and weak nuclear constitute the forces.

Most particles possess non-zero rest masses.

Today’s particle and field structure only emerges after electroweak symmetry breaking.

Above energies of ~100 GeV, electroweak symmetry was restored.

Those energies were exceeded during the cosmos’s first 100 picoseconds (10^-10 seconds).

Three key properties differed back then.

1.) All fermions and bosons were massless: with zero rest mass.

Only neutrinos could’ve possessed mass, potentially from a separate mechanism.

2.) The W-and-Z, photon, and Higgs bosons didn’t yet exist.

Instead, four massless bosons, W¹, W², W³, and the B, plus a Higgs field, existed.

3.) The electromagnetic and weak forces were replaced.

Rather, we had electroweak nuclear (W¹, W², W³) and hypercharge (B) forces.

The Higgs field also possesses electroweak and hypercharge.

When electroweak symmetry breaks, W¹ and W² gain mass and charge, becoming W⁺ and W^–.

The B and W³ combine to become the (massive) Z⁰ and (massless) photon, with one (massive) Higgs boson emerging.

These unbroken symmetries were short-lived, but existentially important.

Mostly Mute articles tell a scientific story in images, visuals, and no more than 200 words.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

If you take everything we know of and can directly observe in the Universe — stars, stellar remnants, galaxies, gas, dust, plasma, and black holes — we find that it’s insufficient to explain what we see on the grandest of all cosmic scales. Unless you hypothesize some novel form of matter, something that’s not included in the Standard Model of elementary particles, you cannot explain a whole suite of evidence. This includes:

• the abundances of the light elements and isotopes found in the most pristine environments,

• the temperature (and polarization) fluctuation patterns found in the cosmic microwave background,

• the correlations between distant galaxies imprinted in the Universe’s large-scale structure,

• and astrophysical systems, like colliding galaxies and galaxy clusters at high speeds, where gravitational effects are observed to separate from the locations of normal matter.

Luckily, there’s a single addition (or modification) to the Universe’s contents that we can make to explain all of these and more: dark matter. However, dark matter has yet to be directly detected through the experimental attempts we know how to conduct. Even though its properties may ultimately render it fundamentally undetectable via those means, many use this as evidence against dark matter’s existence, instead promoting a gravity-modifying alternative idea that typically fails (unless a dark matter-mimicking effect is also introduced) on scales larger than individual galaxies: MOND (modified Newtonian dynamics).

For the first time, a completely new test of dark matter vs. MOND has been conducted: one using the kinetic Sunyaev-Zel’dovich effect. The results are in, and what we observe agrees with dark matter while significantly departing from MOND’s predictions. Here’s what it means, and what the evidence shows.

Let’s start by understanding the dichotomy between the two main approaches to cosmology: adding the ingredient of dark matter or adding a modification of gravity consistent with MOND. In the dark matter scenario, Einstein’s general relativity, the established law of gravity across the Universe, remains unchanged, but a new ingredient is added: dark matter. This ingredient outmasses the full sum total of all normal matter by about a 5-to-1 ratio, clumps and clusters like (slow-moving) matter does, but doesn’t interact with normal matter or light in any measurable way. All it does is gravitate, and those gravitational effects should show up across a wide variety of cosmological tests.

On the other hand, modifying gravity — according to the MOND prescription — introduces a new fundamental acceleration scale that shows up importantly at small cosmic accelerations. This should be unmeasurable within the Solar System, but should show up profoundly on larger scales, like the scales of individual galaxies. MOND describes the internal behavior of most (but not all) individual galaxies very well, but cannot explain many phenomena seen on larger cosmic scales (colliding galaxy clusters, fluctuations in the CMB, the large-scale structure of the Universe) at all, unless an additional ingredient or modification, one identical to including dark matter, is added.

People have looked for “natural laboratories” (in space) to directly test dark matter’s predictions against MOND’s, but the ones they’ve found so far, like the tests on wide binary star systems, suffer from selection effects and underlying assumptions; results cannot be uniformly reproduced by independent teams.

But the kinetic Sunyaev-Zel’dovich effect is a little bit different, and a lot more compelling. When you have a background source of light that passes through an intervening cloud of ionized matter, the properties of that light can be altered by that matter as it travels along its line-of-sight on its way to your eyes and instruments.

Normally, the focus is on what we call the thermal Sunyaev-Zel’dovich effect: where the intervening normal matter is hot and ionized, and photons passing through that region get boosted — via interactions with those charged particles — to higher energies. This means that if you look at a background of radiation (like the CMB) that’s largely uniform in a certain temperature range, the thermal Sunyaev-Zel’dovich effect will create an observed deficit (or cold spot) in your radiation at the expected wavelengths, because those photons have instead been excited to higher temperatures and energies: where you would find a photon excess (or hot spot) if you looked at those shorter wavelengths instead.

But there’s also a kinetic (sometimes called kinematic) component to the Sunyaev-Zel’dovich effect: where the electrons, whether part of a hot or cold population of matter, are also in motion. As the photons interact with the moving electrons, they also get boosted (or, potentially de-boosted) due to those interactions as well.

(To avoid writing “Sunyaev-Zel’dovich effect” so many times, we’re going to abbreviate it from hereon out the same way professional cosmologists do: as the SZ effect.)

While the thermal SZ effect is usually much larger than the kinetic SZ effect, it typically is only useful when studying galaxy clusters. Many galaxy clusters have a large population of internal, ionized matter within them, as was first revealed through studying their X-ray emissions. If you know enough about X-rays and gases/plasmas, you can determine what fraction of the cluster’s total mass is present in the form of this ionized state. Then, if you add in thermal SZ data, you can construct a temperature profile and the thermal structure of the cluster, and use it to learn a variety of cosmic lessons. Similarly, the thermal SZ effect is usually the dominant effect in creating some of the observed “cold spots” in the CMB: the ones that line up with these plasma-rich galaxy clusters.

However, galaxy clusters aren’t exclusively found “at rest” with respect to the Hubble flow. Even though these objects represent large collections of mass, they are also influenced by the distribution of matter around them: both overdense (cluster-rich) regions and underdense (void-dominated) regions. This can lead to galaxy cluster speeds that move at hundreds or even thousands of km/s relative to the Hubble flow. Typically, in order to extract the kinetic SZ effect and its imprint on observed CMB temperatures, you need to understand and separate out the thermal effects first. In the post-Planck era of cosmology, this has been measured directly for many individual galaxy clusters.

In fact, measuring the SZ effect is so powerful that you can actually take a CMB map and look for signs of the presence of galaxy clusters that would leave an SZ imprint on the data. Ground-based CMB telescopes like the Atacama Cosmology Telescope and the South Pole Telescope led to the first detection of the kinetic SZ effect in an individual object back in 2012, and the subsequent discovery of hundreds of new galaxy clusters via the SZ effect. Looking at the CMB in gory detail, particularly at small angular scales, provides an outstanding natural laboratory for finding, measuring, and quantifying both the thermal and kinetic SZ effects.

In theory, you could also use the kinetic SZ effect for a cosmic purpose: to test the effects of gravitation on large scales, and to see whether the observed effect is consistent with dark matter’s predictions or MOND’s predictions, which should differ significantly on distance scales separating galaxy clusters. The reason is simple: when you have two galaxy clusters in close proximity to each other, they gravitationally attract one another. If you have a good enough map of the large-scale structure in this region of the Universe, you can know how far away these clusters are from you, and also from each other. And then, if you can tease out enough information about the kinetic SZ effect, you can constrain the gravitational acceleration between them. That’s where the critical test lies.

Think about this for a moment. In the Solar System, it’s very, very difficult to measure extremely small accelerations, owing to the fact that we have a large number of massive objects that it’s very difficult to get far away from. The Sun, in particular, is extremely massive; even at the orbital distance of Pluto, acceleration due to the Sun’s gravity is somewhere around a few microns-per-second². You’d have to go all the way out into the Oort cloud, far beyond any spacecraft we’ve ever launched (even the ones that have left the Solar System), to have the acceleration drop to values below the MOND scale: of just 0.12 nanometers-per-second².

If you go beyond the Solar System and look at the spaces between stars, you can sometimes get those vast separations, which is what makes wide binary star systems such an interesting laboratory for study: at very large separation distances, the predicted accelerations drop below that critical value, enabling us to test MOND’s predictions. But, despite what its proponents assert, these systems cannot be well-measured individually. We do not know their orbital trajectories, whether these are bound systems or just passing stars, or whether there are more than two stars present inside of them. It would take thousands of years to determine this for any individual system, and therefore, it’s very difficult to draw conclusions. (Indeed, every new study on this topic has yielded a new answer.)

But on extremely large cosmic scales — on scales of the separation of galaxy clusters — the tests we can perform are a lot cleaner, and a lot less ambiguous. If you take two galaxy clusters in proximity to one another (but that haven’t collided or merged), they’re still going to be separated by large distances: megaparsecs at least, where one megaparsec is a little over 3 million light-years. In reality, these clusters may be tens or even hundreds of megaparsecs away from one another: typical of the separation distances we find in our local Universe between the Virgo, Coma, Fornax, Leo, and Antlia clusters, for example.

Even though these galaxy clusters are incredibly massive, often weighing in at hundreds of trillions (or more) of solar masses apiece, containing thousands of times the mass of a Milky Way-like galaxy, they’re separated by vast cosmic distances: tens or hundreds of million of light-years. At these expansive distances, the expected accelerations between two nearby galaxy clusters is tiny, even with these incredibly large masses. This is important, because:

• In a Universe governed by Einstein’s general relativity where dark matter is present, the gravitational force law at large and small distances, and at large and small accelerations, is Newtonian: behaving as a 1/r² force law.

• But in a Universe governed by a MOND-compatible law of gravity, the gravitational force law is only Newtonian-like at small distances where the acceleration is larger than that critical value set by the MOND scale; at larger distances (and smaller accelerations), the force law behaves as 1/r, not as 1/r².

In other words, we should be able to tell these two scenarios apart with sufficiently good data.

The method for conducting this test is as follows.

• You conduct a large survey of galaxies and galaxy clusters spectroscopically: so that you can measure the positions and line-of-sight velocities of a large number of galaxies across a wide volume of space very accurately.

• You then compute a pairwise velocity inference from the galaxy-galaxy correlation function: where you infer, for any two galaxies in your sample, how quickly they’re moving either toward or away from each other, and “bin” large numbers of these galaxies as a function of separation distance to make your data points.

• Then, independently, you conduct a CMB experiment to measure temperature variations as a function of distance and redshift, and then use those variations (and a physical understanding of the relationship between momentum and pairwise velocity) to infer the magnitude and sign of the effects of the kinetic SZ effect.

While there are many other details that go into such an analysis — selecting the objects you’re using in order to minimize biases and uncertainties, for example, or restricting your CMB maps to a single frequency — this is precisely how the April 2026 study, led by astrophysicist Patricio Gallardo, proceeded.

The graph above, which represents the most important plot from Gallardo’s paper, showcases the measured pairwise kinetic SZ effect, which only shows up, overall, at the level of hundredths of microkelvin in the CMB data. (For comparison, the CMB’s overall temperature is about 2.7 K, or 100 million times greater.) The black points represent the measured kinetic SZ effect as a function of separation distance between galaxies (or, equivalently, on the separation scales of the objects in the CMB at a particular distance away from us), while the black dotted line is the best fit in a dark matter-and-dark energy dominated Universe from a previous, related study.

The blue band represents the predictions from a dark matter-and-dark energy dominated Universe that follows the rules of Einstein’s general relativity, while the yellow band represents the predictions from a dark matter-free Universe that follows the rules of MOND. As the study clearly shows, the data is in strong agreement with the dark matter-and-dark energy prediction (and the Newtonian notion of a 1/r² force law at all scales), and strongly disagrees with the MOND-ian notion that there’s ever a transition, even at large distances or low accelerations, away from that: to a 1/r force law instead.

In fact, there’s another figure in the paper (shown below) where they directly test these results against different values of the exponent in the gravitational force law.

To date, this represents the largest-scale direct test of MOND, with scales ranging between 30 Mpc (about 100 million light-years) to 230 Mpc (about 750 million light-years). Leveraging this method has long been awaited by cosmologists, but only in recent years has data for both the CMB and for large-scale structure surveys become robust enough to yield a meaningful result like this.

In the future, optical spectroscopic surveys that provide large galaxy catalogues — including DESI, Euclid, Rubin, SPHEREx, and Roman — will far surpass what Sloan Digital Sky Survey data has delivered here, while next-generation CMB observatories, such as Simons Observatory or the Japan-led LiteBIRD mission, will improve our estimates of the kinetic SZ effect while reducing errors and uncertainties.

Today, this first analysis rules out MOND on these large cosmic scales by a modest 3.3σ: strongly suggestive, but still far from the 5σ “gold standard” of significance demanded by modern astrophysics and cosmology. However, the future surveys that are already planned forecast the ability to rule out MOND (and the n=1 force law scaling) on cosmic scales by a whopping 10σ significance. While no one can be sure that, ultimately, our current understanding of gravity as a manifestation of Einstein’s general relativity won’t need modification, this study clearly shows that attempting to do away with dark matter and replacing Einstein’s laws with a modification to Newtonian gravity simply doesn’t match our observable reality.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Back in 1997, a joint venture between NASA, ESA, and the Italian Space Agency (ASI) was launched with the explicit purpose of studying the most distant naked eye planet of the Solar System: Saturn. The Cassini-Huygens mission, unlike the predecessor missions that visited Saturn — Pioneer 11, Voyager 1, and Voyager 2 — wasn’t simply a fly-by mission, but rather flew to Saturn with the intention of remaining there. After a seven year journey to get there, Cassini arrived, where it had many close encounters with Saturn’s rings, a large number of Saturn’s moons, and of course, the planet itself.

It didn’t just fly around Saturn, but rather above and below it as well, capturing views from 2004-2017 that maximized what we could learn about this prominent planet. One of the most surprising finds came early on in the mission, when the Cassini orbiter flew over the south pole of Saturn, and found something that had only ever been seen on Earth before: a hurricane with a well-defined eye wall to it. That was back in 2006, when Saturn’s south pole was tipped toward the Sun. With an axial tilt of 26.7 degrees, that means that Saturn has equinoxes and solstices, but because Saturn takes nearly 30 years to complete one orbit around the Sun, seasons last more than 7 years on Saturn apiece.

If you’re thinking, “hey wait a minute, I’ve seen pictures of Saturn’s poles before, and this doesn’t look anything like what I’ve seen,” you’re no doubt correct. Just as Earth has two very different poles — the Antarctic south pole, marked by a massive continent, and the icy north pole, whose ice floats atop the waters of the Arctic ocean — so too does Saturn. But on Saturn, we can only take images of whichever pole is tipped toward the Sun at that very moment. From the beginning of Cassini’s arrival at Saturn in 2004, it was the south pole that was tipped toward the Sun. Only in August of 2009 did Saturnian equinox occur, and over the next eight years, until May of 2017, the north pole of Saturn tipped more and more severely toward the Sun, until Saturnian solstice arrived.

For the first five years of the Cassini mission around Saturn, the north pole of the planet was not visible, as the northern hemisphere of Saturn was still experiencing winter on that planet. But with the passing of the equinox and the arrival of the northern hemisphere’s spring, the north pole gradually began to not only come into view, but to heat up and “thaw,” in a sense, as the Sun’s direct rays began falling on it. Although half of the pole was visible in 2009 imagery, it wasn’t really until 2012 that the region surrounding the pole would come into full view. And when it did, what Cassini scientists saw was both surprising and spectacular.

Sure, Saturn is famously golden-colored and exhibits a banded structure across it, like many gas giant worlds do. Rings of cloudy features appear at specific latitudes: also a common feature among gas giants. But very close to the north pole itself, a stack of features arose that were entirely unexpected.

• There’s a big color change as you move toward the north pole, as the planet transitions from golden colored to a dimmer brown color to a still darker brown, and finally turns blue.

• Additionally, while there are many circular features at more equatorial latitudes, the north pole itself exhibits a six-sided hexagonal feature, separating the browns from the blues of the pole.

• And at the very center of the pole itself, a similar hurricane to the feature spotted at Saturn’s south pole appears as well: with a deep blue color and a dark “eye” to the center.

What was going on with that hexagon? And was it always there?

After all, Cassini wasn’t the first mission to arrive at Saturn. While you might not be able to get a very good look at its north pole from a telescope here on Earth, scientists working on the Voyager 1 mission did in fact acquire a series of photographs of Saturn’s north pole, stitching them together to make a map. What they found, all the way back in 1980 (just as Saturnian spring was beginning), was that there was indeed a strange hexagonal shape near the pole. For 30 years, scientists — including some scientists who worked on both missions — awaited this data from Cassini to see just what was going on around Saturn’s north pole.

That hexagonal shaped storm was not only real, it was incredibly persistent as well. The storm was determined to begin at 78º north latitude on Saturn, and extends downward for approximately 100 kilometers (62 miles) in depth. From end-to-end, the storm itself is huge: over 32,000 kilometers (20,000 miles) wide, or approximately 2.5 times the diameter of Earth. While other atmospheric features can vary or fluctuate in latitude over time, this hexagonal feature is remarkably constant, changing by only negligible amounts in terms of its distance from the pole.

The hexagon posed a mystery to scientists, and it was only by performing fluid dynamics experiments here on Earth with a specific set of conditions that we were able to reproduce that unusual hexagonal feature. This led to our current model of how this hexagon was created and sustained itself. The polar storm around the north pole continuously rages: a hurricane in every sense of the word. However, if there’s an eastern-moving current around the hexagon’s outline, roughly moving at 360 km/hr (220 mph), then when that current interacts with airflow at more equatorial latitudes, a six-sided, hexagonal pattern does indeed emerge.

The above image, acquired in 2013, shows a few slight changes as more of Saturn’s north polar area becomes illuminated. Saturn’s two hemispheres are clearly asymmetrical: the hexagon in the north does not have a matching counterpart in the south, suggesting underlying differences between the two hemispheres that have not fully been revealed. After more time in the Sun, the blue color is a little less deep, and has begun to yellow. The leading thought behind why is that methane forms a blue color, so as the north pole emerges from winter, much of that methane is still present, giving the polar area its blue color initially.

As direct sunlight shines on methane molecules — even at Saturn’s impressively large distance from the Sun — those molecules can then get broken apart. When they do, there’s always a chance that they’ll simply find their “missing pieces” and revert back to being methane. But they also have a chance of encountering other broken-apart methane molecules, in which case they can link up to form longer-chain hydrocarbons: the same types of hydrocarbons found ubiquitously across the rest of Saturn. That’s the leading thought: that the initially blue hexagon turns yellow as it spends time in the Sun, as methane gets slowly converted into heavier, longer-chain hydrocarbons.

An incredible decision was made for December of 2012: that Cassini would swing over Saturn’s poles, allowing mission scientists to construct a movie condensing 10 hours of time down to seconds.

These false-color filters reveal a wide variety of features. Sure, there’s the polar vortex at the center, as well as the large hexagonal ring outlining and bordering it. But there are also a wide variety of (reflective) cloud features present, which show up as pink spots near the pole itself. Smaller vortices, appearing as reddish-white ovals, including a large one near the bottom of the image: an estimated 3500 kilometers (2200 miles) wide. According to Andrew Ingersoll of the Cassini imaging team,

“The hexagon is just a current of air, and weather features out there that share similarities to this are notoriously turbulent and unstable. A hurricane on Earth typically lasts a week, but this has been here for decades – and, who knows – maybe centuries.”

Earth can’t form features like this hexagon, as our jet stream slithers and meanders around in complex shapes, owing to how air flows given the distribution of the continents and oceans, along with heat transfer and weather patterns. On Earth, sunlight and the planet’s rotation are the dominant factor that determines how air flows, but on Saturn, the thick atmosphere, with no known solid surface, combined with the extreme distance from the Sun and its uniform composition, enables patterns like this hexagonal shape to emerge.

As the summer solstice neared, and as Cassini neared the ultimate end of its mission, the north polar region continued to change in appearance. The region bounded by the hexagon began to turn a brownish-yellow, losing all trace of its initially blue appearance. In fact, the last vestiges of blue only remained inside the interior of the storm directly above the north pole itself: suggesting that only within this region, where sunlight could not strike the methane inside very well, does photodissociation (and re-binding into longer chain molecules) not occur. As solstice approaches, the entirety of the polar region changes color.

Periodically, the scientists making decisions about Cassini’s navigation plan decided to put it into polar orbits: where it would fly directly over the poles of Saturn, enabling a “bird’s-eye view” of either the north or south pole. (The illuminated pole is always the more interesting one from an observational perspective.) As a result, shortly before its demise in 2017, one of the last things Cassini did was acquire a movie of the north pole one last time, enabling an “apples-to-apples” comparison of the polar change from June 2013 to April 2017, as shown in the animation below.

Even though features vary tremendously over time, including color, cloud cover, and particular instabilities both inside and outside Saturn’s polar hexagon, many key features remain constant.

• There’s always a rapid hurricane, shifting only slightly in position, directly above Saturn’s north pole.

• There are always thick, cloudy eye walls around that hurricane, and winds that rotate more quickly, in terms of angular velocity, than the winds at lower, surrounding latitudes.

• And there’s always a hexagonal feature at about 78 degrees north latitude, not seen to vary in any appreciable way, surrounding the system.

That’s what Saturn looks like as seen from the only orbiting spacecraft we’ve ever sent up to see it. Since 2017, when Cassini met its demise, we’ve had only these archival photos to look at, plus whatever images we’ve been able to take from afar with telescopes such as Hubble, JWST, and the vast array of cutting-edge observatories on the ground. While they have indeed shown us remarkable views of Saturn in many ways, including at wavelengths that have only been rarely probed before, we haven’t been able to acquire any bird’s-eye views of Saturn’s poles since.

Which is part of what makes AI slop like the image shown below so upsetting: it’s not only factually incorrect, but it detracts from all the good, solid work that the entire Cassini team actually conducted for so long.

Looking at this image after seeing the real thing, it’s incredible how many details are incorrect. Let’s go through a few of them.

• The center of the vortex is dark; it shouldn’t be.

• There is a deep purple swirl at the center, whereas the actual images show only blues, whites, yellows, and browns, with a bit of green where the blue and yellow mix together.

• The cloud patterns are far too regular; realistic imagery shows a much more mottled vortex.

• The sharpness of the cloud patterns is too exaggerated given the imaging capabilities of Cassini and the tremendous distance over the poles that were always maintained.

• The blue color extends outside of the hexagon to a series of swirls: something not observed by Cassini.

• The actual hexagon on Saturn has rounded edges, whereas the view shown here has many sharp angles all throughout the polygon, showing several “points” at each vertex.

• And perhaps most maddeningly, eight does not equal six, and this AI slop displays an octagon, not a hexagon!

There are plenty of other things you can complain about, like the wrongness of the scale for showing the upper perhaps ~20 degrees of the planet and then having it surrounded by rings, but the whole point is that we are currently living through a period where AI slop — of things that aren’t real and should never be confused with the real thing — are being shared at a record rate, and most people can’t tell what’s real from what’s simply being hallucinated by an often-incorrect feature-guessing machine.

Because of a combination of measurements made with several instruments, including with gravitation-sensitive instruments, scientists were able to determine that the high winds at the uppermost reaches of Saturn’s cloud-tops, which reach up to 1440 km/hr (895 mph), actually extend downward for up to 8000 km (5000 miles) toward the planet’s interior. Even though Cassini met its demise back in 2017, an incredible wealth of data has led to continued research on Saturn, with scientists recently suggesting that a supersonic jet stream, extending far below the limits of where Cassini’s instrument suite could reach, could be being pinched by smaller cyclones in Saturn’s interior, giving rise to the hexagonal shape around one pole only.

As with any successful mission, Cassini’s wealth of riches should be spurring us on to future investigations. We should be planning orbiter missions to Uranus and Neptune; neither of which has ever received one. We should be planning a follow-up mission to Saturn, as we still aren’t sure about the size, properties, or even the presence of a rocky core at the center of Saturn. We have theories about how and when the main rings formed, and a new in situ mission could test many of them. But if we replace our dreams of conducting bona fide science and getting actual answers to today’s big questions with unrealistic pictures hallucinated by a machine, that’s a generational loss for humanity.

We must not allow ourselves to succumb to the false allure of a pretty picture. In the scientific endeavor, reality is the only picture that matters.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Of all the planets, star systems, and galaxies we’ve ever discovered, the only one that displays any yet-detected signals of life is right here: planet Earth which orbits the Sun right here in our own Milky Way. While there are:

• hundreds of known planetary bodies in our own Solar System,

• more than 6000 known exoplanets detected so far,

• approximately 400 billion stars located within the Milky Way,

• and trillions of galaxies within the observable Universe,

each one of them only represents a chance for life and living beings here in 2026. At present, only Earth, of all the known worlds, and only our Solar System, out of the 2 × 10²¹ stars suspected to exist in the visible Universe, has been demonstrated to have living organisms thriving upon it.

But any world that’s home to life is also, inevitably, going to be home to death as well. Earth may be the only known planet with life on it, but it’s also the only known planet with war, conflict, and murder on it as well. From up close, these tensions are palpable, and their effects on the creatures inhabiting our world can be profound, from humans to animals to every organism in every ecological system in the world. But from afar, the Earth looks deceptively peaceful. Moreover, the farther away you go, the more peaceful our planet appears. At greater and greater distances, the reality of the destruction that we’ve often inflicted on one another — and on ourselves — fades away.

Here’s a look at how a more distant perspective changes what one is capable of seeing occurring on our world.

The longest sustained presence of humans in space, ever, is showcased in the above image: taken from the International Space Station. From November 2, 2000, up through the present day here in April of 2026. For over 25 continuous years, humans have been orbiting the Earth from hundreds of kilometers up: above the majority of our atmosphere, with the ability to look and see whatever portion of our planet we’re currently flying over from a top-down perspective.

From this height, no individual humans can be seen. Only the collective glow of cities, the eruptions of volcanoes, the strikes of lightning bolts, and large-scale wildfires appear to human eyes.

With advanced, high-resolution cameras, of course, we can see details that are far greater: farmland, the effects of tornadoes, the color of an individual house’s roof, and the location of objects to a precision of less than a single meter. The effects of combat or even a localized trail of destruction can easily be pulled out with modern technology: enabling our successes in global satellite imagery. However, if we go to greater and greater distances, our world — often rife with violence — appears much more stable and peaceful.

As we travel to greater and greater distances — and remember, with Artemis II’s recent fly-by of the Moon, they traveled the farthest that any human has ever traveled from Earth — the Earth’s angular size drops, its apparent brightness falls off, and in order to resolve the same size features, a larger telescope is required. From the great distances that humans have achieved, first in the Apollo era and once again now here in the 21st century, national borders and boundaries are all but invisible. Only clouds, continents, glaciers, icecaps, oceans, and other very large-scale, bright features can be resolved.

One can always envision the construction of an arbitrarily large, powerful telescope: one that would enable you to see detailed features on Earth from any distance imaginable. However, practicality issues always emerge whenever you’re imagining any device made of matter: there are physical limits to the size, stability, and precisions that are obtainable with physical systems.

From these great distances of thousands, tens of thousands, or hundreds of thousands of kilometers, a telescope the size of JWST, the largest space telescope ever launched, would only be able to detect brief flashes of luminous energy outputs, like the detonations of nuclear devices, for a brief moment, and only if they occurred on Earth’s night side at that.

At even greater distances, such as the maximum distance spacecraft have ever traveled from Earth and still looked back to successfully image it, the Earth appears as no more than a single pixel: impossible to resolve. But hope is not gone for detecting our impacts on the planet, or from searching for our impacts on the world and its atmosphere and surface. The same science that goes into investigating the properties of exoplanets can indeed be applied to Earth from afar, and even a single pixel encodes a wealth of information.

For instance, simply by observing the light from Earth and breaking it up into its component wavelengths spectroscopically, we could:

• infer the chemical composition of our atmosphere,

• detect the percentage of the world that is covered in ice, water, clouds, and land,

• watch the continents green and brown with the seasons, revealing life’s presence,

• observe the seasonal variations in carbon dioxide and water vapor, further revealing life’s properties,

• and detect events that vastly increase the cloud cover/opacity/reflectivity of our planet, including massive fires, the fallout from nuclear device detonation, or volcanic eruptions.

With spectral signatures, we can even potentially discern what’s being added to our atmosphere and when. A tremendous amount of science can be done, concerning Earth, from afar, just as easily as we can conduct that sort of science for exoplanets.

Of course, that’s looking for a very small signal against the background of “noise” that is the rest of the Solar System. If you want to image Earth at a great distance, you have to take care that your observations don’t get swamped by Earth’s much larger and brighter neighbor: the Sun. The Sun is tens of billions of times intrinsically brighter than Earth across all wavelengths, and coronagraphy or a starshade must be used to great effectiveness if one wishes to pick out the signal of the Earth that’s just separated by one astronomical unit (around 150,000,000 kilometers) from the relatively much more brilliant star that we orbit.

Even if we can do that, and extract the signal of Earth against the background of the Sun’s light, it’s still a tremendous challenge to pick out the signatures that showcase war, violence, or other sources of conflict or disaster on our world. The leading signals of Earth, at least in the form of electromagnetic radiation, showcase our planet’s oceans and other bodies of water, clouds, icecaps, continents, and atmospheric composition. Extracting the tiny variations in those properties that occur over time is a much greater achievement: one we have yet to reach for any Earth-sized exoplanet at Earth-like distances from a Sun-like star.

It can be done, with sophisticated enough technology and enough continuous observing time, but that all assumes that one would even bother to look at planet Earth on a continuous basis.

If you were an intelligent alien who had already discovered the Earth and had detected signs of life on it, you might be motivated to come back to it over and over again. After all, it remains possible that life is exceedingly rare; we only have constraints on how common it is, without a second positive detection (beyond our own planet) to guide us. However, if you were an intelligent alien from another world who discovered the Earth, it would indeed provide at least a second example, teaching you that life was not unique to your world. If you’re the only one you know, you might be alone, but if there’s a second example, you can be certain that there are many.

The same type of science that allows us to detect changes in the night sky — what we call transient events, like supernovae, tidal disruption events, or fast radio bursts — can be applied to planets like Earth, even from afar. If there’s a large event that:

• changes the reflectivity of the planet,

• adds a new species of gas to the atmosphere in copious amounts,

• causes the release of a large amount of energy,

• or changes the color of a particularly large region of the world,

these can all be detected by a dedicated-enough observer, even from interstellar distances away.

However, these signals aren’t the only ones that human activity, including war, can create. There are, on a fundamental level, nuclear reactions that can be observed even from a distance: owing to the generation of neutrinos.

Whether from nuclear power or from nuclear weapons, nuclear reactions that either fuse light nuclei into heavier ones or that split atomic nuclei apart in fission reactions emit neutrinos and/or antineutrinos: reactions that impart a specific energy spectrum onto those neutrinos. With a large-enough directional neutrino detector, such as a large-enough tank of water sufficiently lined with photomultiplier tubes, the nuclear activity that humans have conducted on Earth could be detected from afar. If one could coordinate optical/electromagnetic signals with those nuclear signals from neutrinos, one could even obtain multi-messenger signals of nuclear activity on Earth.

Of course, that isn’t necessarily easy to do: there are natural backgrounds of neutrinos that can easily swamp the terrestrial signals we generate here on Earth. The Sun, for example, emits many more neutrinos than even the full sum of all the nuclear events on Earth: reactors, bombs, physics experiments, etc. But in the nuclear reactions that generate neutrinos on Earth, there are specific signatures, such as:

• the type/species of neutrino created,

• the nature (neutrino or antineutrino) and spectrum of those neutrinos,

• and the energy properties of those neutrinos,

that can be teased out of the data. From even far-enough away, the individual sources of those neutrinos can be determined, enabling a faraway alien species to detect the type of activity we’re conducting.

But, as important and informative as those signals all are, there’s one more problem that comes along with them: the finite speed of light.

• It’s been seven years since the dawn of the era of satellite megaconstellations, which means that only the Sun, the Alpha Centauri system, the Barnard’s star system, and the Luhman 16 system can detect them. Anyone farther away would have no evidence Earth’s orbit is becoming so heavily polluted.

• It’s been 81 years since the dawn of the atomic bomb era: with the first detonation on Earth (Trinity). Even though neutrinos (and antineutrinos) travel at nearly the speed of light, there are only a few thousand — likely somewhere between 4000 and 10,000 — stars within 81 light-years of the Sun. That means, from farther away, no alien, no matter what technology they have to look, could detect our discovery of nuclear technology.

• It’s only been 131 years since Guglielmo Marconi sent the first human-made radio signals (in Morse code), meaning that someone more distant than 131 light-years away wouldn’t be able to even detect human-generated radio or television waves.

Similarly, it’s been only about 250 years since the start of the industrial revolution, about 1200 years since the invention of gunpowder, and about 12,000 years since the dawn of agriculture. Even in most locations within our home galaxy, there exists only scant evidence for the presence of humans.

In fact, for most of planet Earth’s history, it didn’t necessarily even bear much of a physical resemblance to the “blue marble” we recognize today. Our planet has changed dramatically in appearance over time in ways we don’t often think about.

• 65 million years ago, a catastrophic asteroid impact caused a great mass extinction, blanketing the planet in a pall of ash lasting weeks or more: dramatically changing the planet’s appearance from afar.

• From 180 million to 300 million years ago, the superocean Panthalassa covered roughly 70% of Earth’s surface, with the supercontinent Pangaea concentrating Earth’s land masses into a single object.

• Prior to about 400 million years ago, the continents did not become green, but rather remained the color of the rock beneath them, as plants had not yet colonized the land.

• And several times throughout Earth’s history, including for about a 300 million years period from between 2.5 billion to 2.2 billion years ago, the entire planet was frozen over: creating a Snowball Earth scenario.

From far away, these would be the notable features about Earth: not any sign of whether it’s inhabited and certainly not any sign of human-caused conflict on the surface. In fact, from a distance of more than 4.5 billion light-years away, Earth would not even be detectable at all, as our Solar System, Sun, and planet would not have yet formed.

The farther away you go, in fact, the less you can see and detect about what’s happening here on Earth. Only the very nearest perspectives can give us a glimpse into the activity we think of as human: industrial and nuclear activity, as well as our most destructive weapons of war. Only within the Milky Way, and perhaps the Magellanic Clouds, can we detect any evidence for the presence of humans at all. Only within 400 million light-years — just one-millionth of the volume of the visible Universe — can we detect signs of plant life on the planet’s surface; for over 90% of our planet’s history, the continents were barren.

From far away, Earth not only appears as just a simple planetary point, but nothing at all appears remarkable about it. We weren’t even a “pale blue dot” for much of our history, but took on a variety of appearances across time. The farther away we go and imagine what Earth would look like, the less and less Earth-like it appears, at least, by modern standards. Humanity’s rise to prominence on Earth, and our ensuing transformation of the planet, has been a swift but transient phenomenon. If we aren’t careful in our stewardship of this world, it will erase the legacy of our presence just as quickly as we constructed it.

It might take a bit of imagination to envision our planet as seen by alien eyes, but to nearly all aliens in the Universe that happened to view our planet as they see it today, they would indeed be viewing an alien Earth: with no hint of the violent changes we’ve wreaked upon it in recent years.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

For most of us, the Universe doesn’t appear to change all that much on the scale of even a human lifetime. Sure, the stars move relative to one another, our Sun burns through a little more of its fuel, and the Moon slowly spirals away from the Earth as our rotation rate gradually slows down. Meanwhile, on grander scales, older stars across the Universe run out of fuel and die, new episodes of star-formation are triggered, and the Universe continues to expand, driving individual galaxies, groups of galaxies, and clusters of galaxies mutually apart, faster and faster, as time goes on.

But all of these events take time: enormous amounts of time. Stars occasionally pass through our Oort Cloud, but that only happens a couple of times every million years. The Sun will continue burning for 5-7 billion more years before it evolves to the next stage of its life. The Moon spirals away from the Earth so slowly that we’ll continue to have total solar eclipses for hundreds of millions of years. And star-formation will continue for trillions of years, providing new lights in the sky and new chances for cataclysms like supernovae, kilonovae, and tidal disruption events.

Still, when you read, “the Universe has changed by the time you finish this sentence,” just over three seconds have likely elapsed. And in those three seconds, the Universe changes in some profound ways that truly add up over time. Here are some of the most important and relevant ones.

Let’s start with the brightest thing in our skies: the Sun. The Sun shines with a relatively constant power output — also known as its luminosity — of a humongous 3.8 × 10²⁶ Watts. The ultimate source of that power, and the reason the Sun doesn’t slowly contract, cool, and/or dim over time, is because there is a nuclear furnace burning in the Sun’s core: one that converts bare protons into helium-4 nuclei. This process, relying on several nuclear fusion chain reactions but dominated by the proton-proton chain, converts a small amount of the net initial mass of those protons, about 0.7%, into pure energy via Einstein’s most famous equation: E = mc².

This leads to a few profound changes, even over the incredibly small timescale of just three seconds or so.

• First, the Sun loses mass: the equivalent of about 12 million tonnes (or 12 billion kg) over the span of reading a single sentence.

• Second, the Sun very, very slowly heats up: becoming slightly more luminous (increasing its overall energy output) by a tiny factor of ~5 × 10^–¹⁶.

• And third, the tiny loss of mass causes the Earth to very slowly spiral outward in its orbit around the Sun: increasing our orbital radius by approximately 5 nanometers during the time it took you to read the title sentence of this article.

Just by having a Sun with the mass and energy output that it exhibits, these changes are inevitable.

Then, we have to reckon with the fact that it isn’t just the Earth that orbits the Sun, but the Earth-Moon system. Compared to the Earth, the Moon is relatively large and massive: 27% of the Earth’s diameter and about 1.2% as massive as Earth. Those are the largest ratios of a moon-to-planet in the entire Solar System, with one needing to move down to the Pluto-Charon dwarf planet system to find a more significant ratio. The Moon is exceptionally large compared with Earth for a natural satellite, and at only 30 Earth-diameters away from the Earth, its gravity is significant enough to be the dominant factor in determining the oceanic tides that our planet experiences.

But the gravitational interactions between the spinning Earth, the Moon, and the Sun all lead to an additional set of important effects: they cause the transfer of angular momentum from the spinning Earth to the orbit of the Moon around the Earth. This means that:

• the Earth’s rotation slows ever so slightly, lengthening the day by around 4 picoseconds over the three seconds it takes you to read the title sentence,

• and the Moon spirals out from Earth a little bit as well, migrating outward by a net of just over 10 nanometers during those same three seconds.

While these changes might sound tiny, remember that they are cumulative: happening every three seconds atop the changes of the prior three seconds, and so on. Over very long timescales, they lead to a substantial evolution.

Overall, across the Universe, a process continues that has been happening for all but the earliest few million years in cosmic history: the formation of stars. Thus far, 13.8 billion years into our cosmic story, a whopping 2 × 10²¹ stars have already formed: a truly cosmic, mind-bogglingly large number. And yet, the star-formation rate is just a trickle of what it was at its peak some 11 billion years ago: just 3% of that maximum rate. It’s steadily fallen ever since that maximum, known as “cosmic noon,” was reached all those billions of years ago, and is steadily decreasing even now, ensuring that most of the stars that will ever form in the Universe have already been born.

But even with those diminished numbers, it means that in the span it took you to read the title of this article — just around three seconds — more than 4000 solar masses worth of new stars were formed throughout the Universe. Given that the average estimated mass of a newborn star is right around 40% of the Sun’s mass, that equates to approximately 10,000 new stars formed every three seconds within our observable Universe. Very rarely is one of those stars within our Milky Way, which only forms an estimated 5 new stars per year, but overall, the Universe keeps on forming them, leading to the total number of stars growing ever larger over time.

Of course, the stars that we form don’t live forever; eventually they will all burn through the entirety of the fuel in their cores and then die: either gently for lower-mass stars, or violently and cataclysmically for other (mostly higher-mass) stars. The most massive stars will die in a core-collapse supernova, often within merely a few million years after their birth. Less massive stars can also experience violent cataclysms, either from tidal disruption events (where the star is torn apart by the gravitational influence of a black hole, for example) or when a stellar remnant, like a white dwarf, collides with another white dwarf, producing a “second chance” (or type Ia) supernova.

All told, supernovae are common events throughout the Universe, and even during the sentence-reading span of just over three seconds, an estimated 16 new supernovae occur somewhere within the observable Universe. About 80% of those supernovae, or around 12-13 of them, are likely of the core-collapse variety, with the remainder being type Ia, or emerging from the collision of two white dwarfs. Occasionally, one of those 12-13 supernovae may indeed be a peculiar type, coming from a star with a stripped gas envelope from its interactions in a binary system. For those brief moments where the supernova goes off and its energy output peaks, it can rival or even exceed the brightness of the entire host galaxy where it occurred.

But the most subtle changes are sometimes the most profound of all, and these happen not just within the observable Universe, but on the cosmic scales that push the limits of what we can see, measure, reach, or ever hope to interact with. The earliest signal that we can directly see from the Universe at all is the cosmic microwave background (CMB), also known as the leftover glow from the Big Bang. Back when the Universe was young, there were no neutral atoms: just an ionized sea of protons, atomic nuclei, and electrons, amidst a bath of neutrinos and photons. Early on, for about 380,000 years, those photons were numerous enough and hot enough that any time a neutral atom formed, a photon smashed into that electron and kicked it off of that short-lived atom, ionizing it once again.

But as the Universe expands, it cools: stretching the wavelength of each photon traveling through it. After a few hundred thousand years, neutral atoms can stably form, and by the present day, the temperature of the CMB has fallen to a chilly 2.7255 K, or less than three degrees above absolute zero. This cooling process continues even today, where in the span of just three meager seconds, the CMB is now around 60 attokelvin (or about 6 × 10^–¹⁷ K) cooler than it was three seconds prior. Again, it’s not much on such a short timescale, but all of these changes are cumulative, and in time, we’ll be able to measure the cooling of this omnipresent background glow.

Meanwhile, the scale of the observable Universe continues to grow. Because of cosmic expansion — and the fact that space itself expands, in addition to signals traveling at up to the speed of light through it — our 13.8 billion year-old Universe allows us to detect signals that are up to 46.1 billion light-years away. This cosmic expansion doesn’t actually consist of anything exceeding the speed of light, as the limits of special relativity (which limit speeds to a limit of the speed of light) are confined to two objects passing each other at the same location in space. Special relativity says nothing about the rate of expansion itself, enabling our dark energy-dominated Universe to grow much faster than one would expect using special relativity alone.

Even in the span of just over three seconds, our cosmic horizon grows tremendously: by approximately 6 million kilometers. This is much, much greater than the distance that light travels in just over three seconds, which is just over 900,000 km! The expansion of the Universe, particularly at the limits of what can be observed, are impressive, important, and profoundly different from how light behaves as it moves in our local vicinity: where the Universe, at least locally, doesn’t expand.

As time marches onward, there are two other profound changes that occur, in addition to the already-profound change of the most distant signal we can perceive being progressively farther away.

The first is that the number of objects that we can see continues to increase over time. Right now, we can see for about 46.1 billion light-years in all directions, but there are more distant objects (and locations) that have already emitted light, long ago, that’s been on its way toward us for most of cosmic history. Because of the two factors of:

• the expansion of space in our expanding Universe,

• and the finite speed of light, where light always travels through the vacuum of space at 299,792,458 m/s,

the light from objects within 61 billion light-years of us will eventually reach our eyes, with more of that already-on-the-way light arriving as time marches on.

In the span of reading just one sentence, over a timescale of just over three seconds, that implies that about a thousandth of a galaxy becomes newly visible. That may not sound like much in terms of galaxies, but when you remember that an “average” galaxy has about a billion stars within it, that’s a million new stars that become visible over the span of those three-or-so seconds.

But just as more and more objects become visible over time, fewer and fewer objects become reachable over time. That’s because the Universe is dominated by dark energy, as opposed to being dominated by matter or radiation: a transition that occurred about six billion years ago. For the first 7.8 billion years of cosmic history, if you were to look at a galaxy that was receding from us within the expanding Universe, you’d find that its recession speed appeared to slow over time. The expansion rate was dropping, and so was the apparent speed that a galaxy moved away from you.

Since dark energy rose to domination, however, the opposite is true. A distant object that recedes from you in the expanding Universe recedes at faster and faster speeds, progressively: a consequence of the expansion accelerating due to dark energy. At a distance that’s somewhere between 15 and 18 billion light-years away, a galaxy transitions from being “reachable,” which means that if you left today at the speed of light, you could eventually reach it, to “unreachable.”

As a consequence of cosmic expansion, in the span of just three seconds, two additional stars transition from reachable to unreachable: a dramatic illustration of how consequential dark energy is even on extremely human timescales.

With every sentence you read, or with every just-over-three-seconds that elapse, these changes happen again and again. They compound atop one another. If you spend ten minutes reading this article, or approximately 600 seconds, then you should multiply everything you learned by reading this by a factor of 200: that’s how much the Universe has changed over this amount of time. With each year that passes, since a year contains roughly 31.56 million seconds, multiply the numbers you’ve read so far by 10,000,000; those are the Universe’s annual changes. And if you multiply those numbers by a billion, you’ll get the changes in the Universe with each century, many of which add up quite significantly.

In reality, everything in the Universe changes with time. Wait a few hundred-thousand years, and another star will drift into our Oort cloud, perturbing it and potentially triggering new comets arriving in our inner Solar System. Wait a few million years, and the number of days in a year will change, removing the need for leap years altogether. Wait a few hundred million years, and total solar eclipses will be no more. Wait a few billion years, and the number of reachable galaxies will have halved from what it is today, while our Sun reaches the end of its life and dies. These are slow changes from just one limited perspective, but they’re happening all across the Universe. If you’re willing to wait for long enough, just about anything you can fathom will someday disappear. Even though it hasn’t happened yet, the sciences of physics and astrophysics allow us to predict when and exactly by how much: a tremendous achievement of human investigation into the very nature of nature itself.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

For as long as people have been putting effort into anything, there’s been a push to get the most “bang” for your “buck.” More simply, it’s to get the greatest amount out by putting the least amount in. And if that’s what you’re after — that type of optimization — you might have heard of the term BrainMaxxing: the evolution of the mental part of what was once called lifehacking. Unfortunately for those of you who are truly seeing to grow your minds, that arena is full of what can only be rightfully called grift:

• detoxing your body from activities that give you the “hit” of a mental reward,

• giving your brain a regular schedule of structured activities that stimulate your cognitive efforts,

• supplements to “wake your brain up” optimally and “give your brain rest” at the appropriate time,

• and using digital tools to, as one dedicated website calls it, maximize your brain potential.

While the BrainMaxxing movement does have a large amount of extraneous, profiteering aspects surrounding it, the idea at the core of it is sound: you can, in fact, through sustained hard work, grow your mind and become better at any task and more proficient at any set of skills that you’re willing to practice at. In the era of AI, the notion of learning something for yourself is a quiet, internal act of rebellion against a world that seems to be flowing in the wrong direction. Here’s how the road less traveled can lead you to long-term success, while your peers’ shortcuts mortgage their long-term futures.

The trap is as follows: you need a short-term result, and because that’s the thing you value, you take the quickest and easiest path toward getting that result. If there’s an assignment due, you might:

• look up the answers to the assignment online,

• find someone else who’s done the assignment and take advantage of their hard work,

• outsource the assignment to someone else,

• or prompt an AI agent to complete the assignment for you.

Certainly, without having the expert-level knowledge needed to complete the assignment, whatever it may be, these methods are not only faster than attempting to figure out the solutions for yourself, they’re actually more likely to give you an accurate, correct set of solutions than you’d get if you simply did your best with the knowledge and resources you already possessed.

If this is for a school assignment, the consequence of eschewing those short-cuts likely means a lower grade on the assignment than your peers, which translates into a lower GPA and (what’s perceived as) making you a less competitive candidate for the next steps in life. If this is for a work assignment, the consequence could be getting reprimanded for either taking too long or producing work of sub-par quality. In the short-term, the advantage goes to the person who maximizes their path toward “getting the right answer,” as completing the assignment quickly and correctly is usually what gets externally rewarded.

But then the next step arrives: the subsequent assignment. In most situations — including both work and school situations — the demands of what comes next builds upon the skills that the prior assignment was designed to teach you, and the problem-solving abilities that were intended to grow inside of you. If you took the short-cut earlier, you’re going to feel the pressure to take another, similar short-cut once again. And just like before, you’ll probably meet with some success.

• If the assignment is to translate something from a foreign language, there are plenty of tools and resources that can do it for you, including by recognizing and figuratively translating idioms.

• If the assignment is to solve a math problem, again plenty of tools and resources can find the solution, with many such resources even showing you the step-by-step pathway to derive the answer.

• If the assignment is to write an original composition on a particular topic, plenty of such compositions already exist, and many tools can generate essays that even a plagiarism-checker will deem to be original.

If you attempted to figure out the solution for yourself earlier, and attempt to figure it out on your own again this time, you might still struggle, and you might produce work that isn’t up to the standards of your peers who do take those short-cuts. It might seem like using your own mind is disincentivized.

But this is the trap. The person who outsources their critical thinking and problem solving becomes reliant on whatever tools they can lean on to overcome each obstacle in their path. The more complex the problems become, the greater the demands on those external resources, while those resources themselves — due to the increasing specialization of the nature of the assignments — become less available, less comprehensive, and less sufficient overall.

Meanwhile, the person who’s been putting in the hard work — who’s been willing to struggle, persevere, study, all while slowly but steadily increasing their skill set — might still not be up to the more difficult assignments that they’re currently facing, but has now built up a solid foundation based on the earlier material. They know how to approach the problems they encounter, and have a number of tools at their disposal to pick it apart.

Going back to the problems encountered initially, the person who’s been putting in that hard work all along can now solve them skillfully and often easily, while the person who leaned on external resources will very likely still need to do so, not having developed the foundational skills that the hard worker, through their willingness to struggle and persevere, inevitably acquires for themselves.

The problem is the same as it’s always been: making the choices that undermine the construction of a sturdy foundation of knowledge, skills, and problem-solving abilities. You learn physics and math by setting up and solving problems. You learn a foreign language by learning the alphabet, the vocabulary, the grammar, and by practicing reading, writing, speaking, and listening. You become skilled at athletic and artistic endeavors by practicing them, with a coach or trainer or tutor to help you develop your fundamentals and to help you grow and progress to the next level. The goal isn’t to get the answer; the goal is to develop and expand that foundation of skill and knowledge and ability.

When people talk about “the education crisis” or “the workplace crisis” what they’re really talking about is that those who take short-cuts — through, for example, outsourcing the foundation-building struggle to AI chatbots — wind up cheating themselves out of that foundational knowledge. It’s the same sort of problem that people worried about when the calculator was invented: what about the student who never learns to add, subtract, multiply, or divide? How diminished will their opportunities for success be in the real world? What of the author who never learns to write, the computer programmer who never learns to code, or the artist who never learns to draw?

The short answer is this: the person who achieves any modicum of success in any profession or career, but who lacks the foundational skills and abilities that underpin that profession or career, will find themselves ill-prepared to meet the challenges they’re likely to face in the future. By contrast, the person who’s put in the hard work, and who has built themselves a solid foundation in the process, is the person who’s going to be better prepared to not only face whatever foreseeable challenges lie ahead of them, but to face whatever novel problems await them once they’ve graduated to the next steps that lie beyond their schooling, their apprenticeship, or their days as a junior person in their chosen field.

To put it more simply, you can choose to build that foundation for yourself, confident that the ultimate payoff will come later, or you can choose to get the maximum result with the least effort today, and cheat yourself out of the long-term reward of having grown your mind.

That’s the secret behind actually maximizing the potential of your brain: putting it to use in a way that grows it. It isn’t just the ability to have an idea that’s key — ideas are a dime-a-dozen — it’s the ability to discern a good idea from a bad one, and to know whether a good idea is original or whether it’s already been put forth previously. Beyond that, it’s the ability to develop an idea into a finished product, in whatever relevant arena the idea takes place in: what Edison referred to as the “99% perspiration” component of success.

It turns out that the recipe for growing your mind is actually simple, and only involves a very small selection of what the modern BrainMaxxing movement touts as necessary to success. All you need to do is:

• decide that you are serious about wanting to get good at something,

• start by learning the elementary fundamentals of that thing,

• practice them for a sufficient amount of time, every day, until you become very good at them,

• then move on to the next, more challenging steps that build upon those previous fundamentals,

• practice them every day, again for a sufficient amount of time, until you become skilled at those next steps,

• then continue to challenge yourself both conventionally and creatively at an even higher level in that endeavor,

• and continue iterating in exactly this fashion, over a long, sustained period of time.

Whether it’s a sport, an art, a language (including a programming language), mathematics, a science, music, or any other endeavor, the key to growing your brain is to devote the time and energy into struggling with the material and engaging with it directly. That is the secret: there is no shortcut to proficiency, you must put in the time and effort yourself to build those mental connections. Anything else will not lead to proficiency, the best it can do is lead to the illusion of proficiency: an illusion that will be exposed sooner or later, most often as soon as you do encounter someone with bona fide expertise.

You cannot succeed merely by “keeping a streak alive” or by practicing only at your current level over and over again. That might be sufficient to build a foundation at that current level, but it won’t help you progress beyond that at all. It’s important to rest, eat, and sleep well, but that’s important for any body and mind to maintain your overall health. And it’s important to mentally engage with what you’re doing, but that should go without saying: if you want to get good at something, you need to devote your attention to it. The more distracted you are — and yes, what’s often called “multitasking” is tantamount to not giving the endeavor your full, undivided effort — the less you’ll get out of it.

But that doesn’t mean you can’t enjoy the things that give you “dopamine hits,” like video games, junk food, or social media, even though the BrainMaxxing “movement” derides them. You simply shouldn’t count the time that you’re doing those things as time that you’re actually devoting to growing your mind. (We all need downtime, and these can all serve as a form of mental rest.) You don’t have to optimize your lifestyle by structuring your entire routine; you simply have to be consistent about the time and effort you put into learning what you value getting good at.

You can use tools if you’d like help, particularly if you’re someone who isn’t skilled at organizing your schedule or knowing how to study and review information, but no single tool is right for everyone. And it’s vital to understand that you aren’t rewiring your brain so that you prefer engaging deeply with difficult work and the acquisition of skills, even though that’s what people often state that the ultimate goal of BrainMaxxing is.

What you are attempting to do — and, for many of you, this will be your first time doing it — is to develop healthy and regular habits that lead to you putting in a large, sustained effort over long periods of time. Once something becomes habitual, it becomes easier to keep doing it, because that’s what you’ve trained yourself to do. That is not a lifehack or any special strategy: it is simply its own form of practice.

• If you want to get good at swimming, get in the water and swim.

• If you want to get good at painting, pick up a brush and start creating art.

• If you want to get good at playing the piano, sit down in front of your keyboard and start making music.

• And if you want to get good at physics, you’ll need to encounter problems that you work to set up and correctly solve.

The more you do it, the better you’ll get at it, particularly if you have external feedback from someone who’s more skilled at it than you and who can help you improve.

That’s the part of BrainMaxxing that’s real, and that actually works. In today’s world, it’s likely an approach that’s more uncommon than ever, as:

• in schools, good grades are rewarded over honest effort,

• in workplaces, junior-level positions are being eliminated in favor of AI agents,

• and even at the executive level, sycophancy and short-term gains are more often heavily rewarded than adherence to demonstrable truths and long-term sustainability over time.

But that’s the problem with chasing the wrong goals. If your goal is short-term success, you can often achieve it by mortgaging your long-term future. If your goal is long-term success, there is really no substitute for doing the necessary hard work.

That means that, when it comes to schooling, you should prioritize growing your mind over the “reward” of a good grade. When it comes to your career, you should prioritize skill development and the long-term growth of your capabilities over getting the fastest acceptable result. You should prioritize producing something of the maximum quality you can in the time you have to do it, over the commonly-sought alternative of a “minimum viable product.” Your life is not a sprint, and your mind is not a fuel tank to be emptied. Grow it. Nourish it. And do it scrupulously and sustainably. The reward of a skilled, capable mind is one that will stay with you as long as you live.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

The closest supermassive black hole pair, inNGC 7727, was discovered in 2021.

Just 89 million light-years away, these 154,000,000- and 6,300,000-solar-mass black holes are just 1,600 light-years apart.

We’ve also discovered pairs of “double quasars,” with two supermassive black holes each.

Approximately 0.1% of young quasars are expected to be doubles, with typical separations of ~10,000 light-years.

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Until 2015, when PKS 1302-102‘s was identified, only one double supermassive black hole was known.

That’s OJ 287: still the most extreme supermassive binary, 3.5 billion light-years away.

First spotted in 1887, it flares with a double burst every 12 years.

Its main black hole is enormous: 18.35 billion solar masses.

Its event horizon is 12 times the size of Neptune’s orbit.

It also has a companion black hole of “merely” 150,000,000 solar masses.

The periodic double burst arises when the smaller black hole punches through the larger’s accretion disk.

With a 12-year orbit, it varies from 0.05 to 0.28 light-years away from the primary.

The secondary black hole precesses 39° with every orbit: a fantastic confirmation of general relativity’s predictions.

In only ~10,000 years, these behemoths should merge.

Hopefully, humanity will be watching when it happens.

Mostly Mute articles tells an astronomical story in images, visuals, and no more than 200 words. This article was first published in December of 2021. It was updated in March of 2026.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Ever since the final years of the original space race, NASA has been unrivaled as the world leader in space sciences and space exploration. In particular, NASA astrophysics has brought us a wide range of space telescopes that have pushed the frontiers of humanity’s knowledge across the electromagnetic spectrum, from the highest-energy gamma-rays through X-rays, ultraviolet, optical, infrared, and even microwave wavelengths. Whenever we consider building a new observatory, the big thing that scientists focus on is what we call discovery potential, or “how much” ability there is to see beyond the limits of our current instruments and observatories.

Not all wavelengths have received equal attention, however, and some wavelengths have been woefully neglected in recent years. In particular, the most powerful X-ray observatory in human history remains NASA’s Chandra, despite the fact that Chandra was launched all the way back in 1999: back in the 20th century. Plans for a future X-ray flagship were developed, laid out, and approved, with the next big step toward that goal being NASA’s AXIS mission: a true 21st century X-ray facility that would help answer all sorts of questions that Chandra cannot. It was intended to also pave the way for a Lynx-like mission after that, with unprecedented power, resolution, and energy sensitivity.

Unfortunately, on Monday, March 9, 2026, NASA HQ informed the entire AXIS team that:

• their mission is not eligible for selection,

• their submitted Concept Study Report will not be subjected to the full review process,

• and that this decision was entirely programmatic, without concern for the technology or science of the mission.

This effectively sets X-ray astronomy back by a decade, with the earliest realistic timeline for an X-ray flagship now looking like the 2050s or even 2060s. Here’s what AXIS was all about, what happened, and what it means.

There are always big, burning science questions that often serve as the motivation for any new facility or observatory. Whenever current facilities are insufficient to answer those questions, of course you’ll want to build a new one that’s capable of collecting the data needed to provide those answers. In X-ray astronomy, that includes big questions like:

• Where do the seeds of supermassive black holes come from, how large are those seeds and how do they form, and after they form, how do they grow?

• How do key components of matter — gas, dust, and heavy elements — flow into, get transported throughout, and ejected out of galaxies over time?

• What powers the explosive and transient phenomena that appear across the electromagnetic spectrum, including in high-energy X-rays?

These questions are beyond the capabilities of current X-ray observatories such as Chandra for a variety of reasons. Chandra is a relatively small area, with a correspondingly small light-gathering power. It has good resolution but only moderately high contrast, meaning it’s not great at separating faint and bright sources that are right atop one another. And it’s slow with a narrow field-of-view, meaning that you have to sit on a certain location for a while to get a good image, and that you have to be pointed precisely at your target of interest. As a result, Chandra is good for a lot of things, but its limitations matter when considering those big questions above.

That’s why an X-ray telescope that would be superior in those regards has so much discovery potential just awaiting it. Instead of a small-area telescope, a larger-area telescope would have more light-gathering power, enabling it to see fainter sources, and to reveal intricate details in a fraction of the time it takes Chandra to do so. X-rays provide a unique probe of the high-energy Universe, able to measure the emissions from gas heated to hundreds of thousands or even millions of degrees: emissions that routinely occur in and around active galaxies, or other processes powered by black holes. Superior spectral energy resolution, a superior point-spread-function, and high-contrast imaging can help enable all of these, above and beyond Chandra’s current limits.

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And beyond that, speed and a large viewing area are also key concerns. When a transient event occurs — a gamma-ray burst, a tidal disruption event, a supernova, etc. — there’s a lot of information encoded in the X-ray portion of the spectrum, but getting your telescope to point to the right place quickly is key. Chandra doesn’t have those capabilities, but AXIS was designed to respond quickly to alerts, improving our ability to conduct both time-domain astronomy as well as multi-messenger astronomy. These improvements in technology translate into new capabilities in science, which is exactly what we’re after. That’s what AXIS is (or, I should say, was) all about.

But there’s a bigger prize that we’re after, too, even though scientists don’t often talk about it. Sure, we know a new observatory is going to both find:

• more examples, fainter examples, and deeper examples of the classes of objects and phenomena that we already know,

• as well as greater amounts of sophisticated details about those objects and phenomena.

We saw this when JWST came online and surpassed the limits of Hubble and Spitzer; we saw this when ALMA came online and surpassed the limits of Arecibo and the VLA; we saw this when Hubble came online (and especially after its first servicing mission) and surpassed the limits of all prior ground-based and space-based observatories. For the things you already know, you’re going to see them better, see more examples of things just like them, and see a greater set of details within them than was previously possible.

But the reason we call it “discovery potential” is that these new capabilities often bring with them the ability to literally discover things you didn’t expect: things that you had no way of knowing were out there before you looked with these novel capabilities. We don’t know what those things are going to be, that’s part of the motivation for building them and looking. We didn’t know, when we built ALMA, that it would be the key facility in imaging a black hole’s event horizon directly. We didn’t know that JWST would teach us that an asteroid-like belt and Kuiper-like belt aren’t universal, and reveal that some systems even have a third belt. And we didn’t know that deep-field imagery was even possible until we tried it with Hubble. We had to look in order to discover, and to be able to look, we had to build a facility that was capable of looking in a novel, more powerful way.

In fact, one could argue (and I have) that the two most underrepresented wavelengths here in the middle of the 2020s, as far as our arsenal of space telescopes go, are the X-ray and the far-infrared. Both of those wavelengths were considered for flagship missions in the Astro2020 decadal survey — Lynx (for X-rays) and Origins (for far-infrared) — but instead of simply choosing a mission from among the four candidates, the decadal committee recommended a path where:

• all three main types of proposals, X-ray, optical/UV/infrared, and far-infrared, should be developed,

• that the optical/UV/infrared option should take aspects from both proposals (HabEx and LUVOIR) and cobble together our next flagship mission from that, in what has now become the Habitable Worlds Observatory,

• and that the X-ray and far-infrared teams should develop the technologies needed for those future flagships, and that the way to do that next is to plan an explorer-class mission for each one.

The X-ray explorer class mission was AXIS, while the far-infrared explorer class mission is PRIMA. Both missions received a small amount of funding (about $5M each, or ~0.5-1% of the total mission cost) to develop their concept, with the understanding that both of them would be competing for one spot. They would each submit a comprehensive mission concept study for review, and then after the panel at NASA headquarters reviewed them both, one would be selected to go forward.

Then, however, January of 2025 arrived, and all sorts of things changed. The so-called Department of Government Efficiency (DOGE) came to power, cutting hundreds of millions of dollars worth of valuable programs. The director of the Office of Management and Budget (OMB) withheld funding from various NASA programs and facilities despite that funding being appropriated by Congress, and the proposed budget put forth in 2025 for the 2026 fiscal year would have brought an end to over 40 separate current and future NASA missions, 11 of which were NASA Astrophysics missions, including AXIS or PRIMA.

And then, the government shutdown of 2025 occurred, furloughing all non-excepted government employees from October 1, 2025, until November 12, 2025: representing a full 12% of the working days available for the year. Finally, in January of 2026, the termination, resignation, or retirement (including early retirement) for a large number of NASA employees was made final, reducing the number of NASA employees from over 17,000 at the start of 2025 to its current number of less than 13,000 at present. On top of all that, there were cuts and closures that affected a variety of NASA facilities, libraries, cafeterias, visitor centers, and more.

Three of these factors played a key role in rendering the AXIS mission ineligible for selection.

1. First, the DRP, or NASA’s Deferred Resignation Program, didn’t just affect 4000 random people; it also affected some key X-ray astronomers who were working on AXIS. Jimmy Marsh, the main Project Manager for AXIS at Goddard Space Flight Center, was lost. So was Will Zhang, the X-ray mirror lead of the project. In total, over 20 key experts working on AXIS were removed from Goddard and from NASA entirely, significantly curtailing the team’s ability to execute.

2. Second, the swift realignment of NASA Goddard’s priorities with the President’s budget request for the 2026 fiscal year — a realignment that occurred when the budget was still only a request — reduced the availability of mission formulation and mission engineering personnel to work on AXIS. This included cost analysts and schedulers, who were forbidden to work on the mission because the President’s budget request eliminated the probe program entirely. Even though that budget didn’t become law, the lost resources were impossible to make up.

3. And third, the core AXIS study team, which was headquartered at Goddard Space Flight Center, was dominated by NASA civil servants who were furloughed during the 6-week-long shutdown in late 2025. The extension that NASA headquarters provided to the Concept Study Report deadline was inadequate to compensate for the disruption and the lost time.

Whenever you have a mission, you have to go through what’s called a “costing process,” where you have to do things like:

• assess the labor requirements associated with the mission,

• conduct a cost-design iteration process to keep the mission on budget and on schedule,

• and have sufficient time to work through the set of cost and schedule savings that could be identified by the team.

But AXIS encountered a number of obstacles that can only be described as a series of self-inflicted wounds by the US Government on itself, and particularly on the NASA Goddard part of itself. There was the loss of many of the discipline engineers who were necessary to assess the labor requirements for the mission. After the mission design was finalized back in April of 2025, the reallocation of resources meant that the initial costing process could only be completed in September. Then the government shutdown occurred during a critical six-week period, and then there was a forced “pens down” declaration in early December that was forced by the Goddard Space Flight Center’s Executive Review process.

Put simply, there was no opportunity to work through the cost savings and schedule savings that the AXIS team had identified, and as a result, they were put in a terrible position: either submit a Concept Study Review that required too long of a schedule and too large of a budget (missing compliance on schedule and cost), or don’t submit one at all.

The AXIS team did the only thing you can do in an impossible situation: they tried their best. They proceeded with the submission, including outlining a path to a cost-compliant and schedule-compliant profile.

NASA Headquarters received the AXIS team’s submission, and ruled that the team’s stance was unacceptable. No technical review was conducted by NASA personnel; no concerns were raised about AXIS’s technology. In fact, the Phase A work that the AXIS concept study conducted indeed met with several major successes in furthering the key technologies, including in fields like:

• Next Generation X-ray Optics, where iridium-coated, stress-compensated mirror segments were shown to achieve sub-arcsecond level performance on the level of an individual segment,

• building the first demonstrator mirror module, learning valuable lessons about alignment, mounting, and bonding,

• and fabricating charge-coupled-devices that achieved AXIS’s required readout rates and spectra resolution.

Similarly, the AXIS science case was also reviewed to be excellent, and only became stronger from there. The AXIS Community Science Book was produced as part of the Phase A work, and will no doubt inform future X-ray mission concepts for years or even decades to come. However, after more than 9 full years of work, AXIS is officially dead, and the community’s great hope is that a small or mid-sized X-ray mission is selected in the future. Graciously, they all wish the PRIMA team incredible success in their endeavor to become NASA’s next Probe program mission.

This is the first time that a Concept Study has been killed due directly to the actions of the government that commissioned the study in the first place. The X-ray mirror lab at Goddard, in particular, was severely affected by shutdowns, illegally reallocated resources, and pressured retirements. By actively implementing the proposed budget cuts to NASA before the budget had actually been decided — the first time in history that NASA management had done so — we will now have to rely on 1999-level technology for many years to come to explore the high-energy Universe.

As one astronomer put it, “We fired everyone who could do this job, and we are punishing you for not doing it.” The truth is that, without AXIS, the soonest that a high-resolution X-ray imaging mission can launch, and can truly succeed Chandra, is now 25 years from the present date, or potentially more. Historically, it takes around 30 years from a space telescope to go from “concept” to a fully-fledged, fully-built, ready-to-fly mission. Without NASA’s Lynx, without ESA’s Athena, and now without NASA’s AXIS, there are no X-ray missions in the pipeline to bring this incredibly important class of astronomy into the 21st century. Importantly, there was recently a big effort to de-fund Chandra, which would leave humanity without a flagship-class X-ray observatory of any type.

The future of astrophysics in space is in enormous peril, and what’s happened to NASA’s AXIS may be a harbinger of many more killed missions to come. The Universe is out there, waiting for us to discover it. But unless we shift our national priorities, we may soon lose our capabilities to conduct X-ray astronomy, and potentially much more, in the years and decades to come.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

One of the great mysteries in the Universe is that, in all the vastness of space, we have yet to detect any sort of life out there beyond our own planet. Whether microbial and simple, multicellular and complex, highly differentiated and intelligent, or technologically advanced, the only form of life we know of here in 2026 is terrestrial life that originated right here on Earth. Despite all of the discoveries and advances that we’ve made in recent years, from the origins and scale of the Universe to thousands of confirmed exoplanets, we still have yet to detect even a single robust signature of a lifeform that originated from anywhere else.

All we can do, at the present time, is to make the best use of the knowledge that we have. Because of all that we’ve learned about our galaxy and Universe, the history of stars and heavy elements, the properties and commonness of exoplanets, we can make very high-quality estimates about the abundance of potentially habitable planets. However, how many of them actually come to be inhabited remains a great unknown, with deeper questions — like how many of them turn into technologically advanced civilizations — requiring us to estimate further unknowns atop them.

Our first attempt at pursuing this logical path was the Drake Equation, which had, as its final term, the lifetime of an average intelligent, technologically advanced civilization. A recent paper has just constrained that lifetime, and concluded that it’s under 5000 years under the most optimistic scenario. With human civilization still struggling to find our way through our technological infancy, does this new study actually predict humanity’s demise? Here’s what we can justifiably say about it on the grounds of scientific merit.

Here on Earth, we recognize that something remarkable happened. Billions of years ago, much closer to the moment of our planet’s formation than to the present day, life on our planet began. Although many of the details remain elusive, we are confident that there were life forms surviving, metabolizing nutrients for energy, and reproducing at least 3.8 billion years ago. By a little over half a billion years ago, some forms of life had become multicellular, had evolved sexual reproduction, and had also become large, complex, and highly differentiated. Intelligent, tool-using species — including our direct ancestors — have been around for millions of years. Finally, humans have become technologically advanced over the past few thousand years, entering the space age in the mid-20th century.

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We also know that there’s an enormous Universe out there, full of stars and galaxies as far as we can see: stretching across billions and billions of light-years. Every one of those stars, and there are hundreds of billions of stars in the Milky Way alone, represents what we might call a “chance.” A chance at having planets. A chance of some of those planets being rocky. A chance for some of those rocky planets to be at the right distance from their star to potentially have liquid water on their surface. A chance for those rocky, water-rich planets to give rise to life. A chance for that life to survive, thrive, and become complex and differentiated, intelligent, or even technologically advanced.

It was by thinking about applying what happened in our Solar System, right here on Earth, to the rest of the stars and star systems in the Universe that led Frank Drake to put forth the Drake equation: the equation that birthed SETI, the search for extraterrestrial intelligence.

Above, you can see the Drake equation as it appeared in its original form. The specific terms themselves are not necessarily of particular importance, what’s important is the concepts behind those terms. Back when Drake proposed his equation, in 1961, we hadn’t yet discovered the cosmic microwave background: the critical piece of evidence that led to our modern picture of the hot Big Bang. We thought there were an estimated 10¹⁸ stars in the observable Universe: a number that’s too small by a factor of thousands. And we thought that we could “educated guess” our way to a reasonable estimate for the number of communicable civilizations out there right now, a perhaps hubristic proposition given the vastness of our ignorance.

What’s still useful about the Drake equation, however, is the following.

• We can use what we know today about stars, stellar populations, and galaxies to detail the number of stars and star systems within a certain distance of us.

• We can use what we know today about stars, metallicity, and exoplanets to estimate the number of stars that have planets.

• We can further use our exoplanet statistics to estimate the fraction of planets that are potentially habitable: planets with the right conditions and raw ingredients for chemical-based life to emerge.

That’s a huge advance over where science was back when the Drake equation was first proposed, and is worth detailing just a little bit before we move on to the latter terms.

We used to estimate the number of stars in our galaxy in a simple fashion: by measuring the nearby stars we can see, measuring the rough size of the Milky Way, and then estimating the stellar density at various distances from the galactic center. Then, we could estimate the total number of stars in the Milky Way. The big problem with that is that it’s the brightest, rarest stars of all that are most easily visible, and the smallest, faintest, most common stars that are the hardest to detect. Over the past 30 years, we’ve discovered that the vast majority of stars in the Universe, perhaps 75-80%, are the small and faint red dwarfs. With observatories like Gaia, we’ve tracked the 3D positions of over a billion stars within the Milky Way. And overall, we now believe there are around 400 billion stars within the Milky Way, although some older astronomers still prefer a smaller estimate of around 200 billion. That’s a huge reduction in uncertainty!

From exoplanet studies, particularly with Kepler and TESS, we know that whether a star has planets or not is highly dependent on the metallicity — or heavy element content — of the star system itself. If you have more than 25% the amount of heavy elements that the Sun has, you’re almost guaranteed to have planets. If you have fewer than 10%, you almost certainly don’t have planets. Fortunately, about 80-90% of the stars we now see are enriched enough to have planets.

And finally, for most of the Sun-like stars, including the K-class, G-class, and F-class stars (but not the lowest-mass red dwarfs or the short-lived blue giants), there are often rocky worlds found at the right distance from their star that, if those worlds had Earth-like atmospheres, they could have liquid water on their surfaces.

Overall, that has led to estimates that there are very likely between 300 million and 10 billion rocky exoplanets that are potentially habitable, and that’s within the Milky Way alone. However, what we still don’t know is profound, and there are terms reflecting that ignorance that are still relevant from Drake’s original equation. Those topics include:

• the fraction of planets deemed “potentially habitable” on which life actually appears,

• the fraction of those planets where intelligent life, at some point in that planet’s history, emerges,

• the fraction of planets with intelligent life where interstellar communication is developed,

• and lastly, the length of time that such civilizations emit potentially detectable signals into space before those signals cease and/or that civilization collapses.

Back in 1961, when this equation was first written down, the estimates were wildly optimistic. Drake and his colleagues assumed, and remember this is with no knowledge of what the actual answers are that life emerges on 100% of potentially habitable worlds, that 100% of those inhabited worlds give rise to intelligent life, and that between 10-20% of those intelligent life-bearing planets will develop interstellar communication, and thus will be releasing signals that are potentially detectable by us: a civilization in its technological infancy, having only recently gained the ability to detect such signals.

The final term in the equation, however, is of paramount importance. Without it, we would be making an implicit set of assumptions:

• that all stars that have potentially habitable planets will have already evolved intelligent life on them,

• that a fraction of those planets will have developed interstellar communication,

• and then that those broadcasts, once they begin, will continue forever.

However, that is very clearly not the case. For the first 4.5 billion years of planet Earth’s history — and remember, it’s been a “potentially habitable” planet this whole time — we didn’t have interstellar communication capabilities.

Above and beyond that, the path to continuing to broadcast interstellar signals indefinitely is fraught with uncertainty: we will have to survive and thrive as a species, and as a collective civilization, if we want to continue those broadcasts. That’s why the last term in the Drake equation, simply parameterized as L, is so important: the longevity, or lifetime, over which an intelligent, technologically advanced civilization continues to make those broadcasts. After all, humanity has only been around for a few hundred thousand years, and with each year that passes, there’s a small but non-zero chance that some event, whether internal or external, will drive our species, and our civilization, to extinction.

Our ignorance on those fronts is profound, of course. We’ve hoped to find signatures of life on a variety of worlds in our Solar System: Venus, Mars, Jovian moons like Europa and Ganymede, Saturnian moons like Titan and Enceladus, Neptune’s moon Triton, and Pluto, among others. While we certainly have a lot of exploring left to do, no robust, reproducible biosignatures have ever been spotted: just flimsy candidate “hints” where non-biological explanations haven’t been ruled out. We’ve just surpassed the 6100 exoplanet mark, but none of them exhibit any hitherto detected signs of life, although new and improved tools to examine them are in the pipeline. And SETI, despite decades of high-quality data and an extremely careful set of methodologies, hasn’t seen anything indicative of intelligent aliens thus far.

To recap:

• There are between 300 million and 10 billion potentially habitable planets, right now, in our Milky Way.

• We have no idea how many of those planets become inhabited, and whether life’s emergence is common, uncommon, or rare.

• Of the ones that become inhabited, we don’t know what fraction have sustained life there for a long time, much less what fraction evolves into complex, differentiated, and intelligent life forms. All we know is that, on Earth, it took billions of years for this to happen, and that it’s only in the most recent few million to tens-of-millions of years that “intelligence,” as we recognize it, has emerged here.

• Of planets where intelligent life arises, we have no idea how many of them become able to communicate across interstellar distances, or of how long (again, that pesky L from the Drake equation) they continue to transmit, on average. All we know is that we gained those capabilities in the 20th century, and we don’t know how long we’ll last.

All we can be certain of, so far, is that if there is one out there, we haven’t found or detected it yet.

But the specter of extinction, either from external or internal means, hovers not just over each of us, but over our whole civilization. Here in our technological infancy, we have a rapidly changing world, one rife with the potential to destroy us all.

• Given the prospect of nuclear war, humanity’s arsenal could indeed wipe out every living human on Earth from detonations and the ensuing fallout.

• With our eradication of the majority of the planet’s natural ecosystems, environmental/ecological collapse could become severe enough to wipe out most or even all of the current human population.

• As humans continue to expand, we come into contact with animal populations that we never have before, and diseases can be transmitted to us through them. Many diseases are more infectious and more lethal than COVID-19 was, and the next such pandemic could prove to be our species’ undoing.

• And finally, externally, impacts from objects passing through our inner Solar System could wipe all of humanity out, just as so much terrestrial life was eradicated 65 million years ago. Comet Swift-Tuttle, the parent body of the Perseid meteor shower, will potentially provide exactly that existential hazard in 4479; if we cannot redirect a large (~26 km) extraterrestrial body by before then, we may indeed meet our demise at that time.

All of these, plus many more scenarios, provide existential risks to our species. And, as you can imagine, once all of us go, so too will our ability to transmit signals that announce our existence.

Of course, there are more optimistic future scenarios for us that are still in play: where we avoid nuclear war, where we learn how to sustainably live in the natural world without destroying its ability to support us, and where we successfully adhere to the best practices for pandemic prevention, containment, and mitigation wherever possible. Asteroid and comet redirection is possible as well, as several missions have demonstrated, and perhaps a long-term future for humanity, as well as whatever humans wind up evolving into, will continue to include that information.

The question is: can we conclude anything about the mean lifetime of technologically advanced civilization, and learn anything meaningful about our own civilization’s likely future, from the non-detection of intelligent aliens thus far?

A new paper accepted in MNRAS Letters, of which a preprint is available here, the team of Sohrab Rahvar and Shahin Rouhani take that question head-on. Assuming the most optimistic scenario for the commonality of intelligent civilizations:

• where every star system with a rocky, potentially habitable planet develops life,

• where every planet that develops life winds up with an intelligent, technologically advanced civilization after ~4 billion years,

• and then those civilizations continue to transmit a detectable signal until they go extinct,

they conclude that the mean lifetime of each such civilization would have to be less than 5000 years. If the fraction of potentially habitable planets that wind up producing technologically advanced aliens is just 1%, instead, the average civilization lifetime could then be raised to 500,000 years and still be consistent with our non-detection of the presence of intelligent aliens.

But that doesn’t really tell us anything about the average lifespan of “the average” technologically advanced, intelligent alien civilization. It only tells us that a more optimistic scenario than the ones considered by the authors are ruled out. If:

• life’s emergence isn’t guaranteed with the right conditions,

• and/or life’s emergence doesn’t guarantee that it will sustain itself and evolve toward intelligence and technological advancement within ~4 billion years,

then the “longevity constraints” are further relaxed. If only (or even, depending on your perspective) 1-in-a-million potentially inhabitable planets becomes as technologically advanced as we are on Earth right now, then the average civilization lifetime could be five billion years and we still wouldn’t have seen one by now.

Of course, the odds of that happening could be still lower than that; without a second example of life in the Universe, we cannot in any meaningful way quantify the number of intelligent, technologically advanced civilizations. We can only place (admittedly, rather weak) upper limits on how many there can be. As for our own civilization’s lifetime, the paper’s authors are sufficiently conservative, and even note, “We emphasize that these results should be interpreted as upper bounds derived from the Fermi paradox, not as predictions of actual lifespans.”

The only thing that we can perhaps count on is that, with no signs of anyone else to talk to, there’s no evidence that anyone is coming to save us from ourselves. It is up to us, here on Earth, to work together to solve the problems afflicting humanity today. The final term in the Drake equation, L, cannot be used to predict humanity’s demise. Instead, it’s up to us, and all of the generations of humans to come, to keep the dream of human civilization alive.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Here on Earth, the very idea of a laser is relatively novel, having only been invented in 1958. The underlying physics is straightforward:

• an electron within a molecule gets excited to a higher-energy state,

• the electron de-transitions back to the lower energy state,

• where it emits light of a very specific wavelength in the process.

Then, pumped or injected energy re-excites an electron within that very same molecule back into that higher-energy state, over and over. This causes light of precisely that same, monochromatic wavelength to get emitted over and over again. So long as you continue stimulating the same transition, you’ll keep getting light of that exact same frequency over and over again, every time.

But out there in the Universe, this exact phenomenon occurs naturally in a number of galaxies at much longer wavelengths than the eye can see: in the microwave portion of the spectrum. Astrophysically, these objects are known as masers, and arise when energy gets injected into large populations of molecules that are only allowed to de-excite in specific ways. Using the MeerKAT array, scientists in 2022 identified the strongest, most distant maser ever seen: an object so strong that it emits more power, just in that one emission line we can observe, than the total sum of all the light emitted from 6000 Suns. Then, just a couple of weeks ago, a new hydroxyl megamaser was discovered at an even greater distance: 11 billion light-years away. This record-breaking megamaser really is a laser from space, with a story that’s billions of years in the making.

In our terrestrial laboratories, the way a laser works is both straightforward and remarkably informative, with applications that not only apply to the technologies that it enables, but to the larger Universe as well. Originally, laser was an acronym for Light Amplification by Stimulated Emission of Radiation, and the way it works is as follows.

1. You start with a system, like an atom, a molecule, or a crystal, that has multiple allowable energy states.

2. The electron moves (or is coaxed) into an excited state, one that has the potential to transition to the same lower-energy state every time.

3. You then stimulate that excited state with a photon of the desired wavelength, causing the excited state to de-excite and emit yet another photon of the desired wavelength.

4. And then you pump energy back into the system, causing the de-excited component to enter back into the excited state.

It’s these oscillations of the electrons — from the lower energy state to the higher one and back down again — where the last step causes the emission of a photon of a very specific wavelength, that leads to the coherent, monochromatic light that’s characteristic of a laser. Ironically, nothing is being amplified; rather, we’ve adopted the modern term laser because of the distaste that would come along with the acronym Light Oscillation by Stimulated Emission of Radiation.

In laboratory experiments, one of the “holy grails” of laser physics is to get to the highest possible intensity of light, where intensity is energy per unit time per unit area. If you want a more intense laser, you have a few options to pursue. You can either:

• increase the total amount of energy in your laser,

• decrease the amount of time over which the energy in a laser “pulse” is emitted,

• or collimate your laser more pristinely, so that the beam intercepts a smaller area.

Laser light can either be continuously emitted or “saved up” and emitted in a single burst. The latter option has a number of important applications in physics, engineering, and technology. In fact, the fundamental technique used to achieve high-powered, short-period pulses without destroying the amplifying material was worth a share of the 2018 Nobel Prize. By creating relatively standard high-powered laser pulses, stretching them in time, amplifying the lower-power but time-stretched pulses, and then time-compressing the pulses again, an ultra-short, high-intensity laser pulse was created for the first time. Four years later, that technology was applied to achieve the vaunted breakeven point in fusion energy research at the National Ignition Facility.

The Nobel-winning research that led to that advance was a 1985 breakthrough, by Gérard Mourou and Donna Strickland, was the development of a technique known as chirped-pulse amplification. Since the arrival of that technique, laser power has increased by a factor of billions.

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In space, nature is incapable of exerting the type of control over a physical system to produce monochromatic, high-powered light like we routinely do on Earth. What does occur in those natural environments, however, is fascinating in its own right. All throughout the Universe, ever since the first generations of stars lived-and-died, the cosmos has not only been populated with hydrogen and helium, but all sorts of atoms. Oxygen, carbon, neon, iron, nitrogen, silicon, magnesium, and sulfur, for example, are the third-through-tenth most common elements in the Universe today.

This means that molecular bound states involving these atoms, particularly if they’re simple bound states, should be found in many locations naturally, including in the interstellar space separating the stars within a galaxy. Some of these bound states lend themselves to the stimulated emission of radiation, even in astrophysical environments, including:

• hydroxyl (OH),

• methylidyne (CH),

• formaldehyde (CH₂O),

• water (H₂O),

• ammonia (NH₃),

• methanol (CH₃OH),

• silicon monoxide (SiO),

• silicon monosulfide (SiS),

• hydrogen cyanide (HCN),

• and carbon monosulfide (CS).

All of these molecules, under the right physical circumstances, have been observed to emit stimulated emission in space.

Unlike in terrestrial laboratory experiments, where masers have been created since 1953 (predating lasers by years, with the latter being known as “optical masers” before the “laser” acronym caught on), there is no possibility of a resonant or oscillatory cavity in space. All you can hope for, at best, is an energy source injected into some sort of “gain medium,” which is effectively just a pile of gas in space, that gets excited to a higher-energy quantum mechanical level. When the molecules de-excite, monochromatic, microwave light is produced, resulting in what we might call a “single-pass laser,” as opposed to the more common laboratory-based oscillator-driven laser.

With oxygen being the third most common element (and hydrogen being the first) in the Universe, it’s no surprise that hydroxyl (OH) masers were the first ones discovered, followed by water, methanol, and silicon monoxide. Astrophysical masers require that these molecules exhibit a significant amount of velocity coherence, meaning that they’re all moving with roughly the same speed, otherwise we would observe radiative couplings between different parts of the gain medium.

The radiation produced by astrophysical masers is generally unpolarized, unlike laboratory-based laser light, with the only exception coming when the gain medium possesses a substantial magnetic field. And the radiation from masers is generally quite weak, as the surrounding unpumped molecules are often capable of absorbing practically all of the emitted maser light from the gain medium.

Based on all of this, you might think that you would need a highly specialized environment for a maser to exist and be detectable, but that turns out not to be the case at all. Masers have been found:

• in the environments of comets, where vaporized volatile molecules create them,

• in the atmospheres of planets, such as when aurorae generate cyclotron masers,

• on the moons of the outer planets, where water masers have been detected in the plumes associated with Hyperion, Titan, Enceladus, and Atlas,

• in the atmospheres of highly evolved stars,

• in supernova remnants that interact with molecular clouds,

• and in star-forming regions, where young stellar objects and compact regions of ionized hydrogen gas produce the majority of astrophysical masers, including some of the rarest ones.

If you know something about star-forming regions, you might realize that the Milky Way itself isn’t remarkably rich in star-formation, but there are other galaxies that are. Therefore, you might think that a galaxy rich in ionized gas, like a merging galaxy, a starburst galaxy, or a galaxy with an actively feeding central black hole might be an excellent candidate for producing a maser of far greater power than we’d ever see within our own galaxy.

As it turns out, these objects do exist, and produce enormous amounts of energy in comparison to the normal classes of masers that we see; they are known as megamasers. The first two types of megamaser ever discovered are also the most common ones:

• water (in 1979)

• and hydroxyl (in 1982).

The first hydroxyl megamaser system ever seen, interestingly enough, occurred in the peculiar galaxy Arp 220 (below). At 250 million light-years away, Arp 220 is remarkable in its own right: it’s the closest ultraluminous infrared galaxy in the known Universe. Ultraluminous infrared galaxies are bright, having more than one trillion times the luminosity of our Sun. They’re named “infrared” because they emit more energy in the infrared than in all other wavelengths combined, with the overwhelming majority of their brightness caused by the stars forming within these galaxies.

Arp 220 itself is the result of two precursor galaxies that collided just a few hundred million years ago, which now find themselves in the process of merging. A massive, recent burst of star-formation has occurred all throughout the galaxy: evidence of a starburst event. Hubble has imaged the central core of this galaxy, revealing hundreds of massive young star clusters, with the most massive containing around ten million solar masses worth of material: about 250 times more massive than the largest star-forming region in our Local Group. Arp 220 also contains two bright masers, with one being a hydroxyl maser and the other being a water maser, and an enormous amount of neutral gas: billions of solar masses worth.

As it turns out, every hydroxyl megamaser that’s ever been discovered also is found within in a luminous infrared galaxy, with the only differences between them being the size, mass, and brightness of both the galaxy and the megamaser. Typically, what’s thought to occur in these megamasers is a little bit different than in conventional masers, as the young starlight in these objects gets absorbed and re-emitted by the surrounding interstellar dust. (This is similar to how Earth absorbs sunlight of all wavelengths, but re-emits radiation in infrared wavelengths alone.) That re-emitted infrared light can then excite the hydroxyl molecules, which leads to the resulting megamaser emission.

Typically, hydroxyl masers within the Milky Way emit about 0.001% of the light of the Sun at those specific microwave maser frequencies. When Arp 220 was discovered, it was termed a “megamaser” because it’s approximately 100 million times as luminous as a typical maser. Today, there are over 100 known hydroxyl megamasers, and they all:

• are very rich in gas,

• have experienced recent galaxy mergers,

• have high molecular densities and high star-formation rates in their centers,

• and have copious amounts of dust that gets heated, causing a very high luminosity in the far-infrared part of the spectrum.

Up until this year, the farthest hydroxyl megamaser was discovered by Arecibo, at a distance of 3.7 billion light-years away.

That record was shattered a few years ago, on April 6, 2022, when the preprint of a paper by the Looking At the Distant Universe with the MeerKAT Array (LADUMA) collaboration appeared. Eventually slated to be part of the Square Kilometer Array, a series of radio telescopes which will form the world’s largest network, the MeerKAT array contains 64 independent antennas that all function together. The LADUMA collaboration targeted a single patch of sky which encompasses the entire extended Chandra Deep Field South, targeting neutral hydrogen emission lines and serving as a complement to Chandra’s X-ray views, NASA’s WISE mission’s data, and the European Southern Observatory’s VISTA observations of the same region.

What they found, coincident with a galaxy at precisely a redshift of 0.5225, was a megamaser: the most distant and one of the most powerful ever seen. The astronomers named it Nkalakatha: after the Zulu word for “big boss”. The megamaser displayed an unambiguous emission line (above) that corresponds to the main hydroxyl emission line, normally seen at 1667 megahertz, redshifted to an observed frequency of 1095 megahertz. That’s exactly what you’d expect if you divided 1667 by (1 + 0.5225): a smoking gun for a (hydroxyl) megamaser signature. Exactly as expected, this galaxy did indeed turn out to be an ultra-luminous infrared galaxy at a distance of 6.6 billion light-years away, with a megamaser luminosity that’s 6300 times as great as the Sun’s total luminosity. Far and away, it’s the most distant megamaser ever seen, being almost twice as far away as the previous record-holder.

In fact, of all the megamasers ever discovered, this one was one of the most luminous both in terms of the brightness of the host galaxy and of the megamaser itself. At the time, it was the most distant laser ever seen in the Universe.

But that record, which stood for nearly four years, was just broken once again! On February 13, 2026, a new preprint of an accepted paper was published that announced the discovery of HATLAS J142935.3-002836: a megamaser located a whopping 11 billion light-years away, at a redshift of z = 1.027, and from a time when the Universe was 8 billion years younger than it is today. It’s the new most distant natural laser ever discovered, and it confirms many of the expectations that arose in the aftermath of the earlier 2022 discovery. We’ve learned that:

• these megamasers really are out there ubiquitously,

• they can be found at higher redshifts and greater distances than they had ever been detected at previously,

• that MeerKAT is capable of finding and discovering them, and breaking many cosmic records,

• and that we can fully expect the number of known hydroxyl megamasers to continue to increase.

When the full Square Kilometer Array is finally completed, commissioned, and fully functional, we can then set a new expectation: that the distance record for farthest megamaser will be broken numerous times, again and again, in the coming years.

We can now be certain that there are megamasers are out there at greater distances than we’ve ever seen before, which motivates us to search farther and deeper, and to continue building next-generation observatories to enable us to find what’s out there. We’re only just learning how galaxies grow and evolve in the Universe, and by adding megamasers into that equation, we’re poised to put together yet another vital piece of the cosmic puzzle.

This article was originally published in April of 2022. It was updated in February of 2026.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Planet Earth is overrun with life. Lakes, rivers, seas, and oceans are teeming with it, from the surfaces all the way down to the bottom, often at depths of miles and miles. The land, both above and below ground, is packed with living organisms of varying size, mass, and complexity, including plants, animals, and fungi. Even the atmosphere houses a wide variety of life forms, from birds and insects to microbes found far above the highest mountain peaks. All told, more than 8 million species of organisms are currently represented on Earth, totaling over half a trillion tonnes of carbon in overall biomass.

We can trace our evolutionary history through time, with notable milestones including:

• the development of mammals and plants,

• the emergence of sexual reproduction and multicellularity,

• the biological creation of oxygen in our atmosphere, and

• the beginnings of photosynthesis.

We have fossil evidence of life existing 3.8 billion years ago, but the start of it all — the origin of life itself on Earth — remains an unsolved puzzle. Although many theories and scenarios abound, one of the least-talked-about may actually be the most likely: a metabolism-first scenario for life’s beginnings.

Here’s why recent research, only conducted in the last few years, may revolutionize the story of life’s emergence on our planet.

The 8+ billion species of organisms found on our world today possess an enormous diversity of properties. Some are large, some are small; some are complex, some are simple; some live only under specific, extreme conditions, while others thrive in a wide variety of terrestrial environments; some complete their life cycle in only a few hours, while others survive for decades, centuries, or even millennia. There doesn’t seem to be a universal set of conditions — at least, among these and many other common metrics — that you can apply to life.

And yet, there are at least five properties that are universal to all modern life-forms:

• All forms of life collect resources of some type from their environment.

• They all possess a metabolism, where energy is extracted from an external source to achieve the organism’s metabolic goals.

• They all have the capacity to reproduce, making offspring that are partially or even wholly identical to the parent organism.

• They all have a separate “inside” and “outside,” delineated by the existence of a cell wall or cell membrane, creating a boundary between the organism and the external environment.

• And they all have, within them, some sort of genetic code that enables proteins to be synthesized and other life processes to occur.

It’s very unlikely that all of these properties arrived simultaneously, in a fully developed fashion. One of them must have come first.

Many look to the raw chemical ingredients at the heart of our understanding of life today, and advocate for their prominence — and even their primacy — in the origin of life. After all, they argue, those raw ingredients, including all of the nucleobases used in terrestrial life, are found extraterrestrially: in asteroids, comets, and other primordial bodies left over from our Solar System’s formation. Even though the odds of a string of those nucleobases randomly forming in a sensible order that encodes a successful protein are astronomically small, they still argue that it only takes one such success to lead to life, no matter how remote the odds.

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Others instead point to the necessity of an inside-outside difference. They argue that the first life-forms required a protective layer to withstand the harshness of early Earth’s environment, complete with:

• solar radiation,

• lightning storms,

• heavy bombardment by objects from space,

• and copious volcanic activity,

to avoid proteins denaturing and their sensitive inner constituents from disassembling.

Those who advocate a cellular structure first often rely on the presence of lipids in an aqueous environment to support their argument. But the question of how the information to create such a structure could arise concurrent with all the other necessary functions that life would need to have remains; a “barrier-first” scenario won’t necessarily lead to life processes being carried out.

So how did life originate? Efforts to answer this question took a huge step forward in 2010, when a landmark paper integrated the evidence across the entirety of life on the planet with modern phylogenetics and rigorous probability theory. Previously unchallenged assumptions — such as the notion that similarity in genetic sequences necessarily implies genetic kinship, or that universal common ancestry was a requirement — were thrown out in favor of assumption-neutral tests. Horizontal gene transfer between barely-related species, including species in different kingdoms or phyla, was considered, along with fusion events. No stone was left unturned.

The results of conducting the formal test were as follows:

• Overwhelmingly, the idea that all extant life shares a universal common ancestor is favored, and all alternative hypotheses are disfavored.

• Horizontal gene transfer absolutely does occur, but it is extremely unlikely to occur between organisms descended from separate incidents of cell formation, as their genes would be converted into non-coding segments.

• The fact that the same 22 amino acids — and those 22, out of more than 80 known to naturally occur — are found in biologically produced protein molecules is additional strong chemical evidence in favor of universal common ancestry.

But even with this insight into life’s development and history on Earth, we still couldn’t draw definitive conclusions about its origins.

That’s why the newest approach isn’t to choose an assumption about what came first, but rather to begin with the conditions that must have been present on primordial Earth and work backward: what types of reactions would have been likely to occur, or even inevitable?

In the beginning, what would become our Solar System was no more than an enriched cloud of primeval gas. Its composition was about 70% hydrogen, 28% helium, and around 1% oxygen, followed by smaller amounts of other elements, including carbon, neon, nitrogen, iron, silicon, sulfur, calcium, phosphorus, potassium, sodium, magnesium, and many others. Some of those atoms were bound up into molecules, including sugars, amino acids, nucleobases, aromatic molecules, and so on. Most of the mass is drawn to the center, where it will eventually form our Sun, but a substantial amount collapses into a rotating disk that surrounds the central protostar: a protoplanetary disk.

While the lightest elements in the inner part of the disk — hydrogen and helium, as well as light species of ice, such as nitrogen, methane, and carbon dioxide ices — boil and/or sublimate away, the heavier elements coalesce, forming longer-chain, more complex molecules. Over millions of years, imperfections arise in that protoplanetary disk, leading first to protoplanets and later, as the Solar System matures, full-fledged planets.

Early Earth was rife with violent events. The most famous is likely the collision with the protoplanet Theia about 4.5 billion years ago, which led to the formation of our Moon, with a subsequent period of heavy bombardment likely persisting for hundreds of millions of years thereafter. A combination of volcanic events and impacts from comets and asteroids led to the creation of oceans and an atmosphere, and precipitation on the planet’s early, highly uneven terrain led to the formation of freshwater stores, including rivers, lakes, and ices.

Although we colloquially use the phrase “boiling the ocean” to describe an overly ambitious, but practically impossible approach to problem solving, there’s a germ of a sound idea in that phrase that’s relevant. Since oceans are made of mostly water, but with many dissolved or suspended other particles and ions within it, “boiling” provides a method for removing the water while leaving the remaining contents behind. If you were to take even a large scoop of ocean water and began to boil it, you’d lower the fraction of water that’s present, removing it step-by-step, while leaving all of the dissolved and undissolved contents behind.

Now consider the various aqueous environments that our planet possessed early on, and you’ll see why the freshwater stores that formed over volcanically active areas — known as hydrothermal fields — are where Earth’s primordial ingredients were most concentrated. As their water evaporated, the density of organics within them — sugars, amino acids, nucleobases, ions, and much more — increased.

How can we be certain about the raw ingredients that were present? The best proxy for that is the composition of asteroids, comets, and meteorites. When we look inside these primitive objects — many of which we can date back to ~4.56 billion years ago — we find:

• they contain over 80 unique amino acids,

• many amino acids come in both left-handed and right-handed, even though all the ones that participate in life processes on Earth are exclusively left-handed,

• carbon-based organic molecules are also present, from the simple (like sugars) to the intermediate (like hexamethylenetetramine) to the complex (like polycyclic aromatic hydrocarbons),

• and, quite profoundly, all five of the nucleobases that encode genetic information on Earth are present in meteorites as well.

If you create a nutrient and organic-rich environment in the lab, you can do things like make an early-Earth analogue. You can apply energy to it, enable phase changes, and allow long-term chemical synthesis to occur. Large, complex molecules easily emerge, including full-fledged nucleotides, complex proteins, and enzymes. You’ll synthesize not just sugars but polysaccharides and even starches, as well as molecules bearing many similarities to modern cholesterols, alcohols, and lipids.

Lots of complex, long-chained molecules are going to form in an environment such as this. Amino acids will assemble, link up, and form proteins. Most of those proteins will be completely inactive; they won’t perform any biologically useful functions. However, if you replace the neutral atom at the end of one of those proteins with an ion — particularly with a heavy element ion, such as magnesium — then that protein becomes an enzyme. Suddenly, your previously useless protein gains the ability to do things like:

• cleave molecules in two,

• catalyze energy-releasing chemical reactions,

• and turn previously “useless” molecules into a source of food and/or nutrients.

This isn’t a scientifically validated scenario depicting how life on Earth must have gotten its start, but rather a plausible scenario for how, before there was anything else (a cell membrane, a string of nucleic acids that encoded information, or even the ability to reproduce), there could have been molecules conducting metabolic activity.

As was first shown in a groundbreaking paper in 2013, this idea of a metabolism-first development of life should be considered, if not the default story of how biological life processes developed here on Earth, then at least a default component of the story.

This conversion from a useless protein to a useful enzyme can occur not only in hydrothermal field situations, but in tidepools, around hydrothermal vents at the bottom of the ocean, at the sea/air interface, or at other locations where non-equilibrium conditions persist. Amino acids interact and smack into one another, spontaneously forming and breaking bonds. Ions come along and bind to these primitive peptides, creating enzymes. Although these molecules are fragile and easy to destroy or denature, they’re very numerous and were found in high concentrations in these early environments, creating copious possibilities — set by the so-large-it’s-barely-fathomable mathematics of combinatorics — that truly boggle the mind.

Some of the proteins that formed likely gained the ability to perform specific functions merely by chance. These functions might have included the ability to:

• hoard resources, including specific peptides that can serve as food,

• split/recombine other molecules in a way that liberates usable energy in the process,

• “bite” or cleave other useful molecules, releasing energy while remaining intact themselves.

Whatever the case, the 2013 paper showed that the spontaneous creation of these metabolic peptides is all but inevitable. Then, less than 10 years ago, another incredible biological breakthrough was made into origin-of-life research: the discovery of RNA-peptide coevolution.

In an aqueous environment, nucleobases — the genetic “letters” of structures like RNA, DNA, or even PNA (peptide nucleic acids) — may line up along the various amino acids in a peptide chain. If each amino acid can pair up with its corresponding three-nucleobase codon, which can then “peel off” and draw additional amino acids onto that genetic strand, they can effectively reproduce, to a high degree of accuracy, the original peptide chain.

The RNA-peptide coevolution scenario, although new on the scene, has rapidly gained a following and is considered by many to be a leading theory for the origin of life not only here on Earth, but possibly anywhere the conditions for life’s emergence exist. The border between chemical and biological processes is blurry, but the idea of a primitive molecule that can metabolize nutrients found ubiquitously in its environment is highly attractive. If you then have an abundance of nucleic acids, and those nucleic acids can spontaneously align along the amino acid sequences that DNA, RNA, or even PNA can encode, you get a mechanism for another key component of life: replication.

If you have a metabolizing replicator that can successfully reproduce before its environment runs out of resources, denatures the molecule, or otherwise drives it to extinction, then the next steps can begin to fall into place, with the development of cell walls or membranes that delineate an “inside” of an organism from an “outside” chief among them.

We still have a long way to go in determining whether life is common, uncommon, rare, or even unique in the Universe; life here on Earth remains the only example we’re aware of. However, the clues to our origins aren’t just written into Earth’s history, but also into the laws and conditions found throughout the Universe. If life happened here, it could happen elsewhere. Perhaps our first detection of life on other worlds will happen in the very near future. Perhaps that discovery, coupled with some key insights into how life first arose on Earth, will allow us to finally understand if metabolism, rather than cellular structure or an underlying genetic code, is the key that unlocks life’s emergence in general.

This article is part of Big Think’s monthly issue Biology’s New Era.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

One of the most important lessons we learn from studying the Universe is that none of the sources of light that we see — none of the stars, galaxies, stellar remnants, quasars, or heated matter — will continue to shine forever. After a finite amount of time, anything powered by nuclear fusion or infalling matter will run out of fuel. Anything that emits light because it’s hot will cool, and once it’s cooled enough, it won’t emit detectable light signatures any longer: not only ultraviolet and visible light, but infrared, microwave, and even radio emissions will eventually cease. Every point-like and every extended light source, even though they shine brilliantly and ubiquitously today, will someday be snuffed out.

For stars, there are three main fates that a star can have, all of which are heavily dependent on their mass at birth.

• The most massive stars will burn through their fuel and undergo collapse: either direct collapse to a black hole or core-collapse, leading to a supernova. These stars can leave black holes, neutron stars, or nothing at all behind when they die.

• The least massive stars take an enormous amount of time to burn through their fuel, living as red dwarfs and dying as white dwarfs, before fading away to black after roughly a quadrillion years pass.

• But the intermediate mass stars, including stars like our Sun, most commonly die by becoming red giants, then entering the asymptotic giant branch (AGB) phase, eventually transforming into planetary nebulae alongside a white dwarf, where the planetary nebula fades away after a few ten-thousand years.

However, for intermediate mass stars, there’s an in-between stage: after the AGB phase but before the planetary nebula phase. Known as a preplanetary nebula, we’ve got a spectacular example nearby: the Egg Nebula. Using Hubble, we’ve just imaged it as never before, revealing details not only about this one system, but about all dying Sun-like stars, giving us a novel glimpse of our own future.

Originally mis-catalogued as a pair of galaxies by Fritz Zwicky, this unusual object is actually located within our Milky Way: just 3000 light-years away. As Sun-like stars — a proxy for all intermediate mass stars, or stars with between about 0.4 and 8 times the mass of the Sun — age, they burn through their fuel and evolve. They’ll then encounter the following evolutionary stages.

• First, the core will run out of hydrogen fuel, causing the inner core of the star to become inert while hydrogen fusion continues in a shell surrounding it.

• This causes the outer layers of the star to expand and cool, evolving into a red giant, while the interior of the core heats up, intensifying the rate of fusion within it.

• Eventually, the internal temperatures rise above a critical threshold (somewhere around 25-30 million K) that initiates the fusion of helium into carbon: the helium flash of the red giant phase.

• When the helium is exhausted, the star enters the asymptotic giant branch (AGB) phase of its life, where the inner core, now primarily composed of carbon and oxygen, undergoes no further fusion reactions, but helium fuses in a shell around it while hydrogen fuses in a shell surrounding the helium fusion layer.

• Later in the AGB phase, the star begins thermally pulsing as the helium shell runs out of fuel, only igniting periodically in 10,000+ year intervals, leading to regular ejections of material.

• Then the star enters what’s known at the post-AGB phase: where the outer layers of the star are shed, while the remaining stellar material contracts and heats up. The more massive the original star is, the shorter the post-AGB lifetime, down to a minimum of just over 1000 years.

• The heating up of the star, as it transitions into a stellar remnant, first illuminates and later ionizes the prior ejecta, creating a preplanetary nebula (renamed from protoplanetary nebula, to avoid confusion with the protoplanetary disks that form around protostars and newly ignited stars) during the illumination stage and becoming a full-fledged planetary nebula once ionization commences.

• Finally, the nebula fades away and only a white dwarf is left behind.

Those are the stages of how a Sun-like star evolves and dies. Even though the details vary, this story applies to practically all intermediate mass stars that are born all throughout the cosmos.

From their earliest beginnings to their final extent before fading away, Sun-like stars will grow from their present size to the size of a red giant (~the Earth’s orbit) to up to ~5 light-years in diameter, typically. The largest known planetary nebulae can reach approximately double that size, up to ~10 light-years across, but none of this necessarily means that the Sun is a typical, average star. Credit: Ivan Bojičić, Quentin Parker, and David Frew, Laboratory for Space Research, HKU

Because of how bright they are, stars in the red giant phase are easy to identify and are found copiously: both within the Milky Way and beyond. Similarly, AGB stars are easy to find; although they’re rarer and shorter-lived than red giants, they’re also very bright, and the combinations of their brightness and color makes them easy to pick out against the rest of the stars. Later on, as some of the brightest extended objects visible with even the most primitive of telescopes, planetary nebulae are numerous and prominent in the catalogues of astronomers, with the first ones spotted way back in the 1700s.

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But that short-lived, in-between phase — the post-AGB phase with a preplanetary nebula around it — is kind of a rarity. The Egg Nebula, in fact, is:

• the first preplanetary nebula ever discovered and identified as such (with its properties first correctly measured in the 1970s),

• the youngest preplanetary nebula ever found, meaning that the least amount of time has passed since it entered its post-AGB phase,

• and is the closest preplanetary nebula ever identified,

with other preplanetary nebulae like the Westbrook Nebula, IRAS 13208-6020, IRAS 20068+4051, and Roberts 22 all being more distant and farther into their evolutionary stages. The central star that illuminates the Egg Nebula is cooler than all the other central stars powering all other known preplanetary nebulae.

This short-lived stage — of a preplanetary nebula — is one of the most interesting in terms of the evolution of a Sun-like star. An AGB star is a giant ball of plasma and gas, where the star can swell to even larger sizes than the size of Earth’s orbit around the Sun. Slow pulsations near the end of the AGB phase lead to ejecta, but these ejecta are rarely visible directly. Meanwhile, if you look at a planetary nebula (the fate of late AGB stars after just a few thousand years), you’ll see a wide range of shapes, sizes, and ionization levels to the complicated structures surrounding them. But the step from “late AGB star” to “planetary nebula” is only poorly understood, making the few examples of preplanetary nebulae that we know of incredibly important laboratories for filling in the blanks.

The most famous picture of the Egg Nebula, as shown below, showcases a rainbow-like appearance. This isn’t because the Egg Nebula actually has rainbow-like colors to it, but rather because this famous Hubble image was color-coded by the polarization of its emitted (or reflected) light: something observable to our telescopes and instruments, but not directly to human eyes. At this stage in its evolution, the central star, the one that’s contracting down on its way to becoming a white dwarf, is only slightly hotter than the Sun is. None of the illuminated material is ionized; it’s merely lit up by the central star’s light that gets reflected off of all of the ejecta.

There are three clearly identifiable components to this nebula, which you can see for yourself even with a visual inspection.

1. First, there are the concentric circle features, which look like wispy shells of ejecta blown off of the star. This makes sense if we think of the late AGB phase as being filled with regular thermal pulses: pulses that blow off material from the outermost layers of the progenitor star.

2. Second, there are the dual searchlight-like rays emerging from both sides of this nebula. It’s a little difficult to see, but there are bright, white-colored components closest to the central star, where as you move farther away from the center, the rays extend much farther than the full extent of the optically-revealed concentric rings.

3. And third, the center of the nebula is obscure, with what looks like a wispy cloud in the foreground blocking us from seeing the central star itself.

These features both are and aren’t what they seem. There are concentric rings, sure, but they don’t arise from stellar pulsations; their shapes are too regular for that. The searchlight-like rays aren’t intrinsic features of the nebula itself, but rather represent where starlight can escape from the central, dusty region; they are “holes” or “gaps” in the dust. And the center of the nebula doesn’t have a cloud in the foreground; it’s almost certainly a dusty torus.

The polarized-light view, above, is why the nebula is so often seen with a rainbow-like colorization. But that’s not what the nebula would look like to human eyes, as the non-polarized version below clearly shows.

It’s natural to look at an object like this, note that this is the eventual fate (a preplanetary nebula) of most Sun-like stars, and wonder whether this is what our Sun will look like several billion years into the future. After all, if this system, powered by a star known as V1610 Cygni, represents the final stages of evolution for a Sun-like star, shouldn’t we expect the Sun to look like this when the end of its life arrives?

The answer, quite straightforwardly, is no. The Sun will not look like this nebula, nor like any of the preplanetary nebulae that have been discovered and imaged.

It isn’t immediately obvious, but the reason for this is carved into the ejecta that looks like concentric rings. These features cannot be made by a single star that pulses and ejects material periodically; that would simply lead to a smooth distribution of matter. Instead, these repeating patterns are always the result of an unseen binary companion star: a second mass that orbits the large, dying, post-AGB star. We see this in many systems: the Red Rectangle Nebula, the Wolf-Rayet star WR 140, and the evolved star R Sculptoris, the last of which is shown below.

It’s kind of remarkable, when you think about it, that we can infer so much from just a few images of a dying star in a very short-lived evolutionary phase. But that’s exactly what we’re capable of doing for one main reason: astrophysics is a mature, well-developed scientific field, and the lessons we learn from the full suite of objects we’ve observed can be applied to systems, under the right conditions, that are quite different in detail.

The “concentric ring” features seen are a giveaway of a binary companion. The temperatures we see tell us that this star has only finished the AGB phase (and entered the post-AGB phase) very recently. The dusty ejecta, however, can be best studied in young systems like this. In more mature planetary nebulae — or even in preplanetary nebulae that are closer to the end of their preplanetary stage (with hotter stars powering them) — the ejection process has been muddied by a thousand years or more of earlier ejecta being overtaken by faster-moving, more recent ejecta.

But in the case of the Egg Nebula, the youngest known preplanetary nebula, we’re seeing the earliest stages of dust ejection coming from a post-AGB star even seen in the history of astronomy.

Among all the telescopes that we have, only Hubble:

• is located in space, where atmospheric absorption doesn’t exist,

• can fit a large enough number of wavelengths across its primary mirror to view the intrinsic features of the nebula this sharply,

• and has the right range of wavelength sensitivities to reveal the features shown here.

They paint a picture where thousands of years of ejecta, still moving away from the central star at speeds of around 18 km/s, are propagating outward. More recently, there’s a high-velocity wind inside of that older AGB wind: ejecta moving at more like 100 km/s. There’s a dusty cloud surrounding the central, contracting star, and that cloud is strongly suspected to be disk-like, with outflows and gaps in the dust in the two directions perpendicular to the disk. Based on the size of the bright outflows and the speed at which they’re moving, it’s plausible that it’s only been a few centuries, perhaps as little as about 400 years, since the star V1610 Cygni left the AGB phase and entered the preplanetary nebula phase.

Interestingly, the star V1610 Cygni has been viewed very well, and nearly continuously, over the past 30 years or so. What those who’ve observed it have noted shows a small but significant amount of variability, with a gradual increase in overall brightness occurring since the mid-1990s as well.

The variability is expected; there’s a dusty environment and the dust distribution is constantly changing. Therefore, it shouldn’t be a surprise that the brightness can increase or decrease by a few hundredths of an astronomical magnitude even over a short period of time.

What is, perhaps, a surprise is seeing a long-term trend, particularly apparent on decadal timescales, that tends toward a brighter (more negative) visual magnitude. This is precisely what you’d expect for a:

• giant, post-AGB star,

• that’s contracting,

• because contraction is an adiabatic process that traps heat,

• meaning that as your star’s volume decreases, the temperature goes up,

• and so, over the next few centuries and millennia, we can expect the star’s temperature to rise and its spectral class to change: from F to A to B, and possibly even to hotter temperatures after that.

Whereas the Egg Nebula is powered by a mere “F5” star, or a star whose temperature at the edge of the photosphere is only a few hundred K hotter than the photosphere of the Sun, a G2 star, the Westbrook Nebula, a later-stage preplanetary nebula where ionization is just beginning to occur, is powered by a B0 star, or a star that’s several thousands of degrees hotter than our Sun is.

And, at last, with the latest Hubble imagery, our best model for the central dust has been validated: it looks like there really is not only a central dust “cloud,” but that cloud has a disk-like configuration, with gaps in the disk allowing starlight to poke out in rays, just as sunbeams poke through clouds under the right weather conditions here on Earth.

It’s true that someday, the Sun will die. It will become a red giant, begin fusing helium in its core, run out of helium and transition into an AGB star, and when helium shell fusion ceases, it will begin to pulse, igniting helium shell fusion with those pulses, and causing the regular, slow ejection of material. Then, in the post-AGB phase, the once-giant Sun will contract and heat up, illuminating the ejecta and forming a preplanetary nebula, before heating so significantly that it ionizes the ejecta, creating a true planetary nebula. The stellar remnant will contract to form a white dwarf, and while the planetary nebula will fade after perhaps 20,000 years, the white dwarf will continue to shine for many trillions of years before fading out.

But no, none of the preplanetary nebulae that are known, including the spectacular Egg Nebula, are representative of what the Sun’s final stages will look like. The Sun is a singlet star, and so the:

• shell-like structure of the ejecta seen in the Egg Nebula,

• bipolar structure that the Egg Nebula exhibits, and

• dusty disk that allows for searchlight-like rays to emerge from the Egg Nebula’s center,

are features that all indicate “this is a binary system,” and that won’t apply to our Sun. We will most likely turn into a more spherical, fainter nebula, first a preplanetary nebula and then full-on planetary nebula, when our parent star’s time as a living star expires. It’s amazing that, after all these years, Hubble can still conduct world-class science, unmatched by any other observatory. We now have the data to fully 3D model the Egg Nebula, and while it’s the most informative preplanetary nebula in astronomical history, it’s the differences from our own Solar System, not the similarities, that hold perhaps the greatest lessons for our far future.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Since the dawn of the quantum era, perhaps no question has loomed larger in the minds of theoretical physicists than just what, exactly, the nature of reality is. Are quantum objects real, with well-defined positions and momenta, even in the absence of an observation or measurement to determine them? Out of all the ways to interpret quantum mechanics — from parallel universes to a collapsing wavefunction to theories of hidden variables — we still don’t have any evidence that favors one interpretation over another. All we’ve been able to do, even as of 2026, is rule out certain deterministic interpretations that cannot be consistent with the experiments we’ve actually performed.

Nevertheless, despite how slow progress has been in uncovering the full nature of our quantum reality, humanity has taken many important steps since the founding of quantum mechanics. We’ve uncovered the deeper science of quantum field theory, understanding that not just the particles that compose reality but that even the underlying fields have a quantum nature. Bell’s theorem and Bell’s inequality have opened up whole new classes of quantum experiments to probe the behavior of our quantum Universe. The cloning of quantum states has been disproven. Local, real, hidden variable interpretations have been ruled out, and Nobel Prizes have been awarded for advances in quantum foundations, for quantum tunneling, and for improving our understanding of entanglement.

But here in the 21st century, one advance stands out above all the rest: the Pusey-Barrett-Rudolph (PBR) theorem. Although it hasn’t received a lot of fanfare, it was proven back in 2012 and remains the most important quantum advance of this new century. Here’s what it teaches us about reality.

In quantum physics — at least, to the best of our current knowledge — there are a great many physical scenarios you can set up where even if you know absolutely everything that’s knowable about:

• how the system is set up, including the initial conditions of every particle and the boundary conditions of the system itself,

• and what the laws of physics are that govern the system, including all of the fundamental laws and interactions between every particle and field that exists in the system,

you still won’t be able to determine what’s going to happen and when. The best you’ll be able to do is come up with a set of probabilities for each possible outcome, which is extremely limited in its predictive power.

Have a radioactive atom? You can know what its half-life is, but whether this particular atom decays after a tenth of a half-life, two-thirds of a half-life, one half-life, 2.5 half-lives, six half-lives, or 23 half-lives is something you can’t know in advance; you can only watch the atom and determine when it does actually decay.

The situation is similar if you want to know where an electron passed through a double slit will arrive on a screen behind it, or which direction a spin-½ particle will deflect in an upward-directed magnetic field. You can calculate the probability of a set of outcomes, but quantum physics gives us no way to determine what the outcome of any one particular quantum system will actually be, no matter how much you know about it.

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This fact about the Universe has spawned much outrage among physicists and philosophers alike since it was first noticed, which in turn has led to many proposed scenarios to attempt to resolve the feeling of discomfort that we feel when we encounter and ponder these properties.

• One idea, put forth by Bohr and known as the Copenhagen interpretation, is that reality is fundamentally indeterminate until a measurement or observation is made, where a measurement or observation is defined as an energetic enough interaction with another particle that either reveals knowledge about the system’s state or compels the system into a different state.

• Another idea, the ensemble interpretation, is that reality is fundamentally indeterminate and exists as an ensemble of all possible solution states, and that quantum physics can tell you the probability of outcomes for an infinite ensemble of identically prepared systems. It’s the measurement or observation that then “picks out” which state it’s actually in.

• Still another idea is the many-worlds interpretation, which asserts that the wavefunction, each time it’s not in a single, well-determined quantum state, creates and expands into a series of parallel universes, and that only when a measurement occurs is “our Universe’s path” chosen, leaving the others behind as alternative timelines in the multiverse which don’t interact with our own.

• And finally, there are collections of interpretations that insist that reality, as a fundamental level, truly is deterministic, even if we cannot identify or measure the variables that determine those outcomes, and that there are instead “hidden variables” that underlie the system and do 100% determine what outcomes will occur.

This last idea is part of a wide collection of “hidden variable theories” or interpretations of quantum mechanics where even if we ourselves can’t determine what’s going to occur, all of the events that ever have occurred or ever will occur are preordained. It’s sort of like imagining you have a vibrating plate with grains of sand atop it, and that you can only measure the individual grains of sand but you can’t observe or measure the plate or its vibrations. To you, it would appear that the grains of sand fluctuate wildly and at random, but it’s merely a manifestation of classical chaos; if you could see, measure, and understand the physics of the plate, you would find that this was in fact just a complex, deterministic system.

Einstein favored the notion of hidden variables, arguing in his famous 1935 paper (with Nathan Rosen and Boris Podolsky) that quantum mechanics is an incomplete theory, and that finding these hidden variables would complete it. Modern adherents to determinism include pilot wave theory (often known as Bohmian mechanics or de Broglie-Bohm theory), the aforementioned many-worlds interpretation, or the more recent suggestion of superdeterminism.

However, issues about the nature of reality are never, ever decided by our preferences, our ideologies, or our intuition. They are decided by what’s provable and predictable from a theoretical perspective, and then what’s determined to be consistent (or inconsistent) with reality from an experimental, measurable, observable perspective.

The first meaningful step toward this came in 1964, when physicist John Stewart Bell came up with a collection of theorems and inequalities that allowed the first meaningful, experimental constraints on local (where no signals travel faster-than-light) hidden variable theories. The early tests of Bell’s theorem, as well as extensions to it, were sufficient to rule out or constrain a number of local hidden variable interpretations of quantum mechanics, and that was what the 2022 Nobel Prize in physics — awarded to Alain Aspect, John Clauser, and Anton Zeilinger — was all about.

However, there are always loopholes in any theorem, and one potential way to “get away with” having hidden variables is to have them be non-local: where things happening in one location can affect things happening in another region not via a signal exchanged at or below the speed of light, but at speeds exceeding it, even up to the maximum speed of instantaneously. Contained in these non-local hidden variable theories are the hopes of everyone who seeks to make deterministic sense of the quantum Universe, and the hope that somewhere, somehow, there’s a way to extract more information about reality and what outcomes will occur than standard, conventional quantum mechanics permits.

In classical mechanics, after all, there’s no role that the observer plays in determining the outcome of an observation. What happens would have happened the exact same way whether you were there to see it or not.

But in quantum mechanics, we know this isn’t the case in a variety of ways. For example, there’s the double slit experiment. If you pass a series of particles (like electrons) or waves (like light) through a double slit, you won’t get, as an outcome, two piles that correspond to things that went through slit #1 and things that went through slit #2. Instead, you’ll get an interference pattern: a hallmark of the quantum wavefunction.

If you reduce the series of particles or waves to individual quanta — electrons sent one at a time or photons sent one at a time through the double slit — you still get the interference pattern. However, if you measure “which slit does each quantum go through,” you can indeed make that measurement, but the act of making it destroys the interference pattern and leads to you seeing the “two piles” option instead. It seems as though nature really does behave differently whether you watch it or not, where “watching” or “making an observation” involves an interaction with another, sufficiently energetic particle to determine the quantum state of the particle passing through the slits.

That’s what physicists often wonder: is there a way to extract more information about the quantum state of any system — and hence, to know more about the potential outcome that will occur — than standard quantum mechanics provides? Are there, at some level, even if we can’t see or measure them, hidden variables? The argument still rages today. Fifteen years ago, a paper came out proving that no extension of quantum theory can have improved predictive power over the predictions of standard quantum mechanics, but then another paper came out arguing that they indeed found loopholes, and potentially testable counterexamples, to that proof.

At the core of the argument is whether quantum states are “ontic” or “epistemic” in nature. These aren’t common terms that people use (even most physicists rarely use them), with the difference being as follows.

• For ontic quantum states, those states would correspond directly to states of reality, with no room for additional knowledge about reality existing in some hidden, but unknown to humans, set of information-carrying variables.

• Meanwhile, for epistemic quantum states, those states may correspond only to probabilistic states of knowledge about reality, but those states are allowed to be incomplete, where additional knowledge could exist in some type of hidden, information carrying variables.

With this background in mind, we come to the Pusey-Barrett-Randolph (PBR) theorem, put forth in a paper in 2012.

So, what does a quantum state represent? Is the state a physical property of a quantum system, the same way “position” or “momentum” is a physical property of a classical (non-quantum) system? Or is the quantum state only a statistical/probabilistic property, the same way that the temperature of a gas is a statistical property of a classical system, where the physics can tell you the mean speed of all of the particles and how those speeds are distributed, but cannot tell you anything meaningful about the actual speed of any one individual particle?

That’s what’s at stake if we can determine whether the quantum state is ontic, in which case there is no “hidden” information that could be gleaned if you somehow knew more than what quantum mechanics provides, or epistemic, where just as (classical) particles do have well-defined speeds, the actual quantum state could have underlying properties that are indeed better-informed than we are from quantum mechanics.

The theorem begins, as all good theorems do, from a set of assumptions.

1. Quantum systems (like atoms, electrons, and photons) exist, and have at least some physical properties.

2. And that either the quantum state encodes everything there is that’s knowable about the system (the ontic approach) or that it merely encodes what an experimenter can know about a system’s properties (the epistemic approach).

These assumptions are generally not seriously debated, as observations and experiments are extraordinarily consistent with all of them.

They then proceed by imagining there are two different methods of preparing a quantum system: method 1, which leads to one quantum state, and method 2, which leads to a second quantum state that is not identical to the first quantum state. Because of the first assumption, after state preparation, these two quantum systems then have some physical properties. Whether those properties are completely described by quantum physics, or whether they’re only completely described by some other, yet undiscovered theory is irrelevant; only that they exist. Whatever those physical properties are, it’s possible to make a complete list of them, at least from a mathematical point of view. Then depending on how reality works, two options arise.

• If the quantum state is ontic, then if you specify the full list of physical properties, you uniquely determine the quantum state.

• On the other hand, if the quantum state is epistemic, then fully specifying the full list of physical properties might not uniquely determine the quantum state; there may be multiple compatible options.

Then imagine that you prepare two quantum systems independently, and that each one’s quantum state is determined by the preparation method. The outcome of the measurement may then only depend on the physical properties of the two systems at the time they are measured.

What Pusey, Barrett, and Rudolph then showed is that unless the quantum state is ontic in nature, the measuring device is sometimes uncertain which preparation method was used. But because the quantum state is determined by the preparation method, it runs the risk of giving an outcome that quantum theory demands must have 0 probability: it cannot occur. And therefore, there is no physical state that can be compatible with both “method 1” and “method 2” simultaneously.

If you then apply this same argument to any pairs of quantum states that arise from preparing two quantum systems made of identical particles in two different ways, then you can uniquely infer the quantum state from the observed physical states. The quantum state is therefore a physical property of the system, and the statistical/epistemic view is demonstrated to be false.

This has enormously profound consequences. It implies that the phenomenon known as “quantum collapse” or “wavefunction collapse” must correspond to a real, physical process, even one that occurs simultaneously between entangled particles separated by large distances: in a non-local fashion. However, if there is no collapse of the quantum state (as in many deterministic interpretations), then each measurable component of the quantum state must have a direct counterpart in reality: undermining the very idea of “hidden variables that determine reality” inherent to the epistemic point of view!

What’s remarkable about this theorem is that it relies solely on three base assumptions made by the authors:

1. That if an isolated (non-entangled) quantum system can be prepared with a pure state, there will be a well-defined set of physical properties resulting from that preparation.

2. That multiple quantum systems can be independently prepared such that their physical properties are uncorrelated with one another.

3. And that measuring apparatuses respond solely to the physical properties of the systems that they are measuring, whether or not those properties are well-determined or obey a probability distribution.

If any of these assumptions are violated or invalidated, then there’s still wiggle room to argue that the quantum state is not a real object, or that quantum systems don’t have any physical properties at all. However, if all three of these assumptions are accepted, then the epistemic interpretation of reality is ruled out, leaving us with no alternative but to accept the “weirdness” of quantum mechanics as inherent to, and fundamental to, the nature of reality. That truly is profound, and why the PBR theorem stands tall as the most important development in quantum foundations of the 21st century so far!

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

For as long as we’ve been human, we’ve turned our gaze skyward and marveled at all that there is to view beyond planet Earth. Even the recognition that Earth itself is merely one of many planets orbiting the Sun is profound, where the stars glittering up in the canopy of the night sky are just very distant analogues of our own Sun: with many of them likely having their own planets, and where some of those planets might even have life on them. However, arguably the biggest changes that result from viewing the Universe don’t come from merely the scientific knowledge we gain from those astronomical endeavors, but rather how they shift our perception of what reality is, and how we, as humans on Earth, fit into the grand cosmic story.

The images we’ve taken of the Universe — originally merely in the forms of sketches, but later, with the advent of photography, by direct imaging — have quite profoundly shifted how we make sense of ourselves, and our very existence. At every stage, we’ve learned that the Universe is:

• grander and vaster than we imagined,

• within reach of human understanding,

• where our Earth and Sun are both very special (to us) and not very special at all (cosmically),

• and, despite all that we may have assumed previously, neither designed for nor centered around us.

These images, carefully selected from both ancient and modern history, serve as examples to show off how just a single image can change humanity’s perspective in a variety of ways: shifting how we view ourselves, as well as the Universe we inhabit.

Galileo’s telescopic glimpse of moons around planet Jupiter.

Back in the early 1600s, our view of the world — and our place in it — was dominated by a narrative that had held sway for thousands of years: the Earth was unchanging and unmoving, and all of the heavenly bodies, including the Sun, revolved around it. Although Copernicus had put forth a heliocentric alternative back in 1543, its matches to the observed orbits for the planets were even worse than Ptolemy’s. (That remained true even when Copernicus attempted to add epicycles to his heliocentric system’s circular orbits.) Meanwhile, in Italy, the Catholic Church flexed its power in executing Giordano Bruno for his cosmological theories: simply for daring to suggest a non-Earth-centric view of reality.

Yet armed with a modified spyglass, serving as the world’s first telescope, Galileo Galilei began observing the heavens in 1609 as no human had done before, and immediately noticed some spectacular features.

• Saturn had “ears” to it, now known to be its massive and bright system of rings.

• Venus exhibited the full suite of phases, appearing small and “full” when farthest from us, but large and “crescent-like” at closest approach.

• And Jupiter, perhaps most spectacularly (as recorded in Galileo’s sketchbook), was found to have four major moons, all of which orbited around Jupiter and definitively not the Earth or Sun.

Contemporaneously, Kepler was working at deriving a heliocentric Solar System with elliptical orbits for the planets, and by century’s end, Newton had concocted the law of universal gravitation. In many ways, the observations of Jupiter were a key turning point in bringing about that tremendous revolution in physics, astronomy, and humanity’s view of our cosmos.

Lord Rosse’s view of the first spirals ever spotted in the sky.

While today, when we hear of a spiral in the night sky, we immediately default to thinking “galaxy,” the reality is that humanity was very slow to understand the nature of these objects. In fact, it was only in the mid-1800s that our first telescope capable of gathering enough light to reveal the faint, spiral structure of even the closest, brightest galaxies arrived: the 72-inch Leviathan of Parsonstown. Built by William Parsons, also known as Lord Rosse, in Ireland and completed in 1845, it was only with this enormous telescope that spiral structures in these distant nebulae could be revealed.

Rosse recorded these sketches in his sketchbook, and discovered that spirals were actually common. Although there were a few astronomers who asserted that these spirals might be “Island Universes,” or galaxies like the Milky Way, all unto themselves, that was a minority opinion for nearly 80 years. The leading thought of the day was that these were newly-forming protostars: precursors to evolved stellar and planetary systems like our own Solar System. However, even though the question of the nature of these spirals didn’t get resolved until Edwin Hubble’s observations of 1923 settled the issue, the very existence of these spirals represented an enormous leap forward for science, and for humanity.

Isaac Roberts’ photo of Andromeda: the first photo of a galaxy beyond the Milky Way.

Even though Andromeda is the closest, brightest large galaxy outside of the Milky Way, it was only with the advent of astrophotography — or the combination of using telescopes and photography together — that its spiral structure was revealed. Unlike all of the other known spirals at the time, Andromeda isn’t face-on or even nearly face-on to our perspective, but rather is closer to edge-on, being only inclined by about 30 degrees. And yet, because of its proximity to us, it just happened to be the subject of one of the earliest astrophotos ever, as astrophotography only began as an amateur hobby in the 1880s, by Isaac Roberts in 1887/8.

The photo you see, above, actually represented our deepest, most detailed view of any astronomical object at the time, and showed that spirals were very common, and may even be lurking in places where we didn’t expect them. Here in 2026, nearly 140 years later, every star you can see in this image that’s a part of Andromeda (or one of its satellites) is still in the exact same position if you observe it today; Andromeda, at its distance of 2.5 million light-years, hasn’t experienced hardly any perceptual changes in more than a century. This iconic image helped reveal the distant Universe and the Universe beyond our own cosmic backyard, teaching us both that spirals were common, and that they could in fact appear in any orientation relative to us.

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The first photograph, taken from a rocket, to directly show the curvature of the Earth.

While scientifically, the roundness of the Earth was established more than 2000 years ago, there’s nothing quite like seeing it for yourself: with your own eyes. Conventional airplanes don’t fly high enough for that curvature to be visible, nor even do Earth’s highest mountain peaks. While evidence for the Earth’s curvature had been obtained photographically since the 1930s, those effects were subtle and required special tools (and a lot of math) to detect it.

However, in the post-World War II years, several German V-2 ballistic missiles were captured by various Allied powers, and on October 24, 1946, a 35 mm motion picture camera was attached to one that was launched at the White Sands Missile Range in New Mexico. The result can be seen above, at left: where the curvature of the Earth is clearly visible from a single photograph near the rocket’s maximum altitude of 65 miles. Six months later, a second rocket was launched to an altitude of 101 miles, clearly above the Kármán line that delineates the boundary between Earth and space, and saw the curvature even more severely. This was more than a full decade before the launch of Sputnik 1 and the dawn of the space age, but it clearly showed us Earth as it hadn’t been seen before: without national or geographic borders, where only land masses, water features, mountains, cities, farmland, and clouds divided one part of Earth from another.

Earthrise: the first photo of Earth seen rising over the limb of the Moon.

What is it like to see your own planet from so far away? Where you can see the whole sphere of our world, or at least the portion that faces you and is illuminated by the Sun, from a great distance? It’s many things: awe-inspiring, humbling, and makes you feel a sense of care for the fragility of our planet and the life on it. Out of all of the worlds that we’ve ever glimpsed, up close or from afar, Earth is the only known one with life on it, containing more than 8 billion human beings at present.

And yet, from far away, all of the wars we wage, all of the divisions we impose on one another, all of the ways that we contrast ourselves with one another fall by the wayside. All that you feel is a unity, and a need to protect this tiny little blue dot in space: the only hospitable place, and the only home, that we’ve ever known. There’s a name for the feeling that astronauts get when they gaze upon planet Earth from afar: the Overview Effect. In fact, two of the three astronauts aboard that Apollo 8 mission, Frank Borman and Bill Anders (the latter of whom took this now-iconic photo), had extremely memorable quotes about this experience:

“We came all this way to explore the moon, and the most important thing is that we discovered the Earth.” –Bill Anders

“When you’re finally up at the moon looking back on earth, all those differences and nationalistic traits are pretty well going to blend, and you’re going to get a concept that maybe this really is one world and why the hell can’t we learn to live together like decent people.” –Frank Borman

This image is, even today, arguably the most environmentally and existentially impactful photo ever taken.

The first photo of an untethered spacewalk: of Bruce McCandless hovering in the distance.

Up until 1984, humans in space had never been free-floating; they had only ever performed tethered spacewalks. That changed with the development and invention of a jetpack, the Manned Maneuvering Unit, suitable for space: a device that would allow astronauts to travel arbitrarily far away from their home station, with the ability to return so long as they didn’t use too much of their nitrogen propellant in the process. The first such untethered spacewalks came during the Space Shuttle mission STS-41B, which used the Challenger shuttle two years before its ill-fated flight. As Bruce McCandless, the astronaut in the photo, later said, “It may have been one small step for Neil, but it’s a heck of a big leap for me.”

Although this image is now more than 40 years old, its impact still resonates today. Up above the Earth, recognizable with its thin atmosphere, wispy clouds, and deep blue oceans, the black abyss of space is joined by a distant human figure: astronaut Bruce McCandless. Floating freely for about an hour and 22 minutes, McCandless’s untethered spacewalk, the very first one, brought him more than 300 feet (91 meters) away from the space shuttle that launched him. Planet Earth, although it looks like it’s just below him, is actually a full 274 kilometers “down” from his perspective. Again, it’s our perspective of humanity, and our place in the Universe, that evolved further with the advent of this photo.

The Hubble Space Telescope’s first-ever deep-field, long-exposure image of… nothing.

Back in the 1990s, after it was serviced and its optics were corrected, the Hubble Space Telescope began taking the deepest, sharpest images of the distant Universe that had ever been obtained. The December 1993 servicing mission saw the corrective optics of COSTAR installed, and also the Wide Field Planetary Camera 2, an upgraded, wide-field camera that enabled superior imaging of the Universe in a variety of ways. But it was the courage of the director of the Space Telescope Science Institute, Bob Williams, that would lead to the most iconic space telescope image in history: the original Hubble Deep Field.

Williams put his name, his reputation, and arguably, even his job on the line by devoting his “director’s discretionary time” to a radical idea: that the Hubble Space Telescope would point its eyes at nothing, over and over, across four separate wavelengths, and see what appeared. Many detractors thought it was a waste of the most valuable telescope time ever, but Williams devoted 342 Hubble orbits to the endeavor, all in a patch of sky smaller than a ten-millionth of a square degree.

The result?

Thousands and thousands of galaxies, including many of (at the time) the most distant ones ever detected. It showed us, for the first time, what the faint, ultra-distant universe looked like, leading to our modern picture of the cosmos, where the total number of observable galaxies rises into the trillions.

Ever-improving views of the Pillars of Creation: a hotbed of star birth right here in the Milky Way.

It may seem hard to believe, but when the original, iconic Pillars of Creation image was snapped by the Hubble Space Telescope back in 1995, it represented a generational leap in showing us the Universe. It showed, for the first time, the precise location, within a star-forming nebula, where new protostars were still assembling and forming, while the outsides of these dust-rich pillars are being slowly photoevaporated by the radiation from the young stars surrounding them.

Then the region was re-observed again: by Hubble with a new camera, by the Chandra X-ray observatory (which found many young stars inside the pillars, as well as more distant background sources), and eventually, by JWST. The pillars aren’t being destroyed by supernova blasts, but rather are only evaporating away extremely slowly: on timescales of 100,000 years or more. The deeper we look:

• with higher resolution,

• with greater wavelength coverage,

• and with the ability to focus on stars, gas, dust, or other unique features,

the better we come to understand where Sun-like stars — and opportunities for living planets — come from. These profound lessons apply not just here in our own backyard, mind you, but all throughout the Universe and all across cosmic history.

A shocking find from JWST: its view of disks and rings around Fomalhaut, upending our view of our Solar System as typical.

It was only a few years ago, back in 2022, that we had no direct, high-resolution images of debris disks, or asteroid/Kuiper belt analogues, in any stellar system other than our own. We assumed, as would be natural, that the way things unfolded in our Solar System would be the norm. We expected:

• inner planets,

• an asteroid-like belt, where ices formed/didn’t form early on in the system’s history,

• outer planets,

• and then a Kuiper-like belt, out beyond the last massive planet.

With the advent of JWST, we finally progressed to the point, technologically, where we could test our expectations directly, by imaging known debris disk systems around nearby stars, like Fomalhaut and Vega.

Fomalhaut went first, and to everyone’s surprise, there weren’t two belts in the system, but three. In addition to an asteroid-like belt and a Kuiper-like belt, there was also an intermediate belt: a property that no one anticipated, and that astronomers are still struggling to explain. But yet, it’s real. Meanwhile, Vega, the second such system to be imaged, shows no evidence for belts of any type, and instead shows a smooth, relatively featureless disk, with no planets as massive as even half of Neptune or Uranus. We used to think that two belts were normal; now we don’t even know if that’s the most common or typical configuration out there.

There’s no doubt that, at some point in the future, there will be another image of space that again upends and enhances our knowledge of the Universe — or our view of our place in it — so profoundly that there will be yet another revolution in how we perceive reality. The key, as always, is to keep looking outward, and based on what we find, to then look inward as well.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

One of the most remarkable facts about the Universe is simply that, over the past couple of centuries, humanity has actually been able to make sense of much of it at a basic, fundamental level. We’ve determined what all of the luminous and light-blocking material, plus radiation, is made of: the normal matter and energy in our Universe that consists of particles within the Standard Model. We’ve discovered black holes and have come to understand how gravity and the expanding Universe works: governed by the laws of Einstein’s general relativity. And we understand the rules governing how particles interact: through the strong nuclear, weak nuclear, and electromagnetic forces, as dictated by quantum field theory.

While these developments occurred both theoretically as well as observationally and experimentally, this picture has truly been cemented over the last 50 years by a large suite of data: collected from precise particle collider and detector experiments here on Earth, in space, as well as astrophysical and cosmological observations of the grand Universe. The Standard Model of particle physics, as well as the Standard Model of cosmology — rooted in quantum field theory and general relativity — form the grand foundation of our modern scientific enterprise.

Many have devoted their lives to exploring extensions and alternatives to these standard scenarios, and are (unfortunately) often ridiculed as doing “useless” research. But this research, when done correctly, is anything but useless; it represents the most underappreciated achievements in theoretical physics over the past several decades. Here’s what we’ve learned, and why it’s so valuable.

When it comes to making sense of the Universe, it’s important to focus our attention on the big picture: the full suite of data and what it both does and doesn’t indicate. Whenever you have a scientific theory, model, or framework that you work within, one of the biggest challenges is to extract testable predictions from it: predictions that are quantitative (i.e., that answer the question of “how much” of an effect occurs), predictions that lead to potentially observable and/or measurable signals, and predictions that differ from the predictions of the other theories, models, or frameworks that are used to describe and predict reality.

We can immediately recognize, historically, that there are many such examples where this is exactly how science advanced. Some notable revolutions included:

• Kepler’s theory of elliptical orbits in a heliocentric framework, which matched the observed orbits of the planets, especially that of Mars, better than any other model, including the models of Ptolemy and Copernicus.

• Dirac’s theory of the relativistic electron, leading to the successful prediction of positrons (and other forms of antimatter), of the behavior of light and charged particles near the speed of light, the understanding of electron spin, and to an understanding of the fine structure of the hydrogen atom.

• And Einstein’s theory of gravitation — general relativity — which successfully explained aspects of gravity (like Mercury’s perihelion precession) that Newton’s theory could not, and made several new predictions like the deflection of starlight that passed near a massive object (like the Sun during a solar eclipse) that agreed spectacularly with experiment and observation.

These examples stick out as memorable in many ways for a simple but uncommon reason: because these theoretical developments, which can either be viewed as “alternatives to the then-accepted mainstream” thought of the time or as “extensions to what was generally known at the time,” turned out to lead to predictions that better agreed with our observable, measurable reality than the pre-existing frameworks that they eventually superseded. There were also hints that something was a bit “off” about our pre-existing theories, as:

• there were notable discrepancies between the orbits of the planets (especially the ones that we now know have the greatest eccentricities) as observed by Tycho Brahe as compared with the predictions of Ptolemy and Copernicus during the time of Kepler,

• there was a lack of agreement between the predictions of the Schrodinger equation under relativistic conditions and observations, and a lack of agreement between the observed fine-structure of the hydrogen atom and the predictions of the Bohr model of the atom in Dirac’s time,

• and there was an observed disagreement in Mercury’s orbit as compared with the predictions of Newtonian gravity, while alternative explanations (like the existence of a hypothetical “Planet Vulcan” interior to Mercury) lacked direct observational support.

These already-apparent inconsistencies — or from a different perspective, a lack of a perfect agreement with the prevailing theory’s predictions — served as both motivations for the development of a novel, competing, alternative theory to the mainstream, but also served as an immediate sanity check: could the newly proposed theory explain what those longstanding theories couldn’t?

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That’s very analogous to the situation we find ourselves in today, as we seek to understand the nature of reality to the most extreme degree possible. Sure, we have an incredibly successful model of reality right now: the best of all-time. We have a set of force laws that describe all of the interactions we can detect and measure, and a list of cosmic ingredients that describe the contents of our reality exquisitely. We have all sorts of predictions — from both general relativity and quantum field theory — that are extraordinarily consistent with everything we’ve observed and measured.

And yet, there are all sorts of observed facts about the Universe that our current theories cannot and do not explain. Some of them could simply be, “that’s just how the Universe was born,” and we can foist these mysteries off onto “initial conditions” and ignore them. But there are a great many of them, and they sure are piling up the deeper we look. They include:

• the existence of dark matter,

• the existence of dark energy,

• the vastly disparate values of the fundamental constants, including the disparate strengths of the fundamental forces and the disparate masses of the known particles,

• the origin of neutrino masses and why there are only left-chiral neutrinos and right-chiral antineutrinos,

• the unexplained origin of the matter-antimatter asymmetry,

• the Hubble tension, or how different methods of measuring the expanding Universe yield different values,

and a whole lot more. These mysteries, and attempts to resolve them, have led to a great many proposed extensions to the Standard Model.

Of course, none of them have cleared the three major hurdles that all proposed new theories must conquer if they are to supersede and replace the prevailing theory as part of our standard, consensus picture of reality. Those three hurdles are as follows.

1. The new theory must reproduce all of the (often prodigious) successes of the pre-existing, prevailing theory of the day. That means that every one of the changes or extensions that the new theory makes in comparison to the old theory must not conflict with any aspect of the full suite of data that already doesn’t conflict with the prevailing, leading theory that the alternative is seeking to replace.

2. The new theory must also — and this is often the very reason it was proposed — successfully explain at least one already-observed phenomenon or solve at least one still-unsolved problem that the old theory cannot successfully account for.

3. And then, assuming both of those earlier hurdles have been cleared, the next step is for the new theory to make novel, testable predictions that differ in some measurable, observable, and testable way from the predictions of the prevailing theory.

Only once all three of these hurdles have been cleared can a new theory be said to supplant, or supersede, the older, prevailing theory. That’s how the foundational picture of reality, often called our current “consensus” position, evolves over time.

This is how the Big Bang came to be our leading picture of reality, and later, how cosmic inflation altered our view of the beginning of the Universe and changed how we viewed the Big Bang: not as the origin of our Universe, but rather as a hot, dense state that emerged in the aftermath of inflation’s end. This is how we came to accept the existence of dark energy, and how we came to model it successfully as a cosmological constant. It’s how we came to understand that our Universe is filled with a species of matter that neither absorbs nor emits light, and that doesn’t interact with normal matter in any directly detectable way except gravitationally: what we presently call dark matter.

It’s only by matching the existing observations and being consistent with the full suite of data that we already have, reproducing every one of the successes of the current theory, succeeding in at least one instance where the current framework does not succeed, and in making robust, testable predictions that differ from the currently accepted theory in a measurable way, that science truly advances.

In the realms of particle physics and cosmology, however, revolutionary successes are exceedingly rare. Far more common are attempts at replacing the existing theory that fail to be borne out by reality: an all-important endeavor that, unfortunately, is largely met with scorn among many influential physicists.

Such scorn is certainly ill-deserved, of course, as it’s the ability to put a novel, promising theory or idea to the test that is the hallmark of good theoretical science. Most people have an incorrect and even a harmful view of science, where it goes something like,

“The first person to stumble upon an idea that ultimately turns out to be correct, even if they wind up getting the right answer due to incorrect reasoning, is the person whose genius should be lauded.”

This thought, despite how common it is, is entirely wrongheaded. The hallmark of a good idea is one that:

• doesn’t conflict with already-existing data (because if it does, it’s dead-on-arrival),

• can explain at least one puzzle that the current theory doesn’t explain (because puzzles are there to be solved, and an idea that solves more puzzles has more predictive power),

• and can, in some novel way, be tested and measured against the old theory in a definitive fashion to determine whether the old theory, the new theory, or neither theory is correct.

Attempts to solve the puzzle of the matter-antimatter symmetry, for example, was one of the prime motivations for considering Grand Unified Theories as scenarios worth exploring: where it isn’t just the electromagnetic and weak nuclear forces that unify at higher energies, but where the strong nuclear force also joins them at still-greater energies.

This was a great idea, not because it’s true, but because the idea could be tested. One consequence of grand unification is that — if it’s correct — it mandates the existence of new, super-heavy particles: particles that couple quarks to leptons in a way that goes beyond the normal Standard Model coupling. This enables baryon-number and lepton-number violating interactions, which is wonderful for clearing the second hurdle that a new theory must hurdle: it provides a potential explanation for a hitherto unexplained phenomenon: baryogenesis. However, the true test comes with the third step: a novel prediction that differs from the predictions of the old (Standard Model without grand unification) theory.

That prediction was simple, straightforward, and profound: the proton itself must be fundamentally unstable, with its lifetime dependent on the exact particle content and energy scale of the grand unification scenario under consideration. For example if the straightforward Georgi-Glashow SU(5) unification scenario were correct, the proton should decay with a mean lifetime of around 10³⁰ years.

At present, based on all of the large-scale detectors we’ve built with large amounts of liquid inside (including large numbers of protons), we have yet to observe proton decay, enabling us to set lower limits on the proton’s lifetime that are around a factor of 10,000 longer than that expected decay time; the proton’s lifetime is at least ~10³⁴ years. As a result, we can rule out the original Georgi-Glashow grand unification model, and can place meaningful constraints on other GUT scenarios as well.

Many other ideas, similarly, can have measurable, testable predictions teased out of them. In electroweak-scale supersymmetry, for example, predictions include:

• the existence of flavor-changing neutral currents (which are forbidden in the Standard Model),

• the existence of a lightest supersymmetric particle that’s within reach of the LHC (which has not shown up),

• and the existence of a spectrum of Higgs bosons, including charged Higgs particles, rather than a single neutral Higgs (whereas the LHC has only revealed one).

This doesn’t eliminate supersymmetry from the realm of possibility, just as the non-observation of proton decay doesn’t eliminate the possibility of grand unification, but it does rule out and constrain a variety of supersymmetric scenarios.

Similarly, many alternatives to or extensions of general relativity have been proposed, with some key observable differences including the existence of scalar and/or vector contributions to gravitational effects in addition to the standard tensor-only contributions put forth by Einstein, as in the case of Brans-Dicke gravity or TeVeS, or with a speed of gravity that differs from the speed of light, as in Gauss-Bonnet gravity. The testability of these theories and ideas is of paramount importance, as the observations of the kilonova event GW170817, which saw an electromagnetic signal and a gravitational wave signal arrive within 1.7 seconds of one another from 130-140 million light-years away, have either ruled out or severely constrained these alternatives to Einstein’s general relativity.

It’s all too easy to point one’s finger at any number of theories (and theorists) and claim that they’re a waste of time, energy, and brain cells, and many do exactly that. But that’s completely unjustified. You can lambast people for clinging to an idea once it’s already been ruled out by the data, but to assert that the point of theoretical physics is to derive how the Universe is misses the point of what physics actually is at its core: an experimental science, not a theoretical endeavor. One of the most important things a theorist can do is to transform the general framework of a novel idea or extension into measurable, testable predictions that can be searched for experimentally or observationally: the science of phenomenology.

Why should a new discovery be guaranteed? Why should one or more favored or fashionable ideas be correct?

The truth is we don’t know until we look, and knowing what we should be looking for, as well as where to look and how to look for it, is an incredible part of the story. From LIGO to the LHC, from neutrino detection experiments to cosmic ray observatories, and from the Event Horizon Telescope to the JWST, we are living in a golden age of high-quality data, capable of testing alternative theories and possible extensions to the Standard Model as never before. Each step we take forward can help us determine not only how nature behaves, but how it doesn’t behave. That’s a tremendous success! Not everyone appreciates it, of course, but it’s probably the most important class of achievement — short of a revolutionary discovery that corroborates a novel theory — that occurs in fundamental physics today.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

For nearly 100 years, the United States has been the world’s leader in a wide variety of scientific fields. No other country has:

• invested as much in fundamental scientific research,

• has made more scientific breakthroughs and scientific advances,

• has attracted more scientific researchers to move there to conduct their research,

• or has conducted more projects and been home to more scientists that have won Nobel Prizes.

From public health to food safety to clean air and water to vaccines to dental health to disease eradication and pandemic prevention, the United States was the world leader. From rocketry to space exploration to planetary science to astrophysics, heliophysics, and Earth monitoring, the United States’s standing in the world was unparalleled. From education to energy, from chemistry to biology, and from environmental science to medical breakthroughs, no other country came close.

And yet, this past year — 2025 — has seen much of that scientific legacy dismantled. Facilities and libraries have been closed. Thousands of our best and brightest scientists have had their government positions eliminated. Projects have been canceled and/or defunded. Even entire governmental departments and organizations have been terminated, including the Department of Education and HEPAP: the high-energy physics advisory panel.

Scientists and non-scientists alike have fought back with great vigor against these cuts, and a newly agreed-upon congressional budget promises to restore key levels of funding to several organizations, including NASA’s science mission directorate and the National Science Foundation. It’s vital that we keep fighting for a solid scientific foundation (and future) within the United States, but that alone won’t lead us to the future we need. Here are the four key paths forward for US science and scientists: not only here in 2026, but beyond as well.

1.) Keep fighting for the continued funding of our current-and-future projects and scientists.

This is not a drill: in 2025, an enormous number of projects, agencies, and scientists had their funding and positions cut, withdrawn, or eliminated entirely. At the start of 2025, there were over 17,000 people employed by NASA; as of today, in mid-January of 2026, that number is down below 13,000: a reduction of more than 4000 of the most skilled and qualified members of the US workforce. NASA’s Science Mission Directorate saw the largest funding cut in its history proposed in 2025, threatening to close out, cancel, or otherwise eliminate many of the most ambitious opportunities for 21st century science in the entire world.

The threat to science, in many ways, has been existential and across-the-board, and astronomical/space science at NASA and the NSF have been no exception. The Nancy Grace Roman Space Telescope — NASA Astrophysics’s next flagship mission after JWST — was completed ahead of schedule and under budget, and its reward was being denied sufficient funding to launch it. Mission control at NASA Goddard’s Space Telescope Operations Control Center (STOCC) was slashed, threatening the necessary resources to operate our already-existing space telescopes, not to mention new ones. The Thirty Meter Telescope was cancelled by the National Science Foundation; NASA’s Mars Sample Return mission was eliminated; thousands of NASA employees were compelled to face early termination; the world’s flagship solar observatory (DKIST) was forced to reduce operations; many of the most prestigious fellowships and opportunities for early career scientists were taken away by the hundreds.

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Importantly, a lot of people fought back. Organizations, including:

• the American Astronomical Society,

• the National Academies of Science, Engineering, and Medicine,

• the American Physical Society,

• the American Association for the Advancement of Science,

as well as thousands of professional scientists at all stages in their careers, sounded the alarm and refused to remain silent. Warnings were issued, letters were written, even whistleblowers at various federal agencies, despite orders not to talk about what was happening, spoke up. Campaigns were waged on Capitol Hill, congresspersons and senators were contacted, and much, much more. In the first week of 2026, at last, a new congressional budget agreement was reached, setting the stage to restore much (but not all) of the needed funding to NASA and NSF, among others.

This path — which one can think of as “plan A” — is a path that has worked many times throughout history, as new NASA Astrophysics Division Director head Shawn Domagal-Goldman reminded listeners at the recently-completed 247th meeting of the American Astronomical Society. However, this path isn’t working for everyone, including:

• the thousands of US scientists who’ve lost their jobs,

• US projects that have had their US contributions cancelled, such as Mars Sample Return mission and the Thirty Meter Telescope,

• or the facilities and sets of infrastructure that have already been closed or repurposed.

The “plan A” path might work for some missions, and possibly even the future Habitable Worlds Observatory, but there are three other paths that everyone, from mission directors to project scientists to professors to researchers to students, should strongly consider investing in as well.

2.) Plan a contingency in the event that the US Government suddenly rips out the rug from under you.

As 2025 definitively proved, the US government might, at any moment:

• cut off already-appropriated funding for your project,

• reorganize and eliminate the overarching, governing structure for your project or career,

• revoke the visas of any international scholars participating in your project,

or much, much worse. The track record from 2025 shows that everyone from agency heads down to early career scientists faced termination, irrespective of the merits of either their project or their own actual work. While this came as an unprecedented surprise to many in 2025, it will not be a surprise moving forward; projects, agencies, organizations, and individuals know to expect this now.

And that’s why everyone who has either a job working in science for the US government or has a project that relies on US government funding should be working on a “plan B” in the event that their project, career, or contract suddenly has the rug pulled out from under it. Contingency plans are varied, but will include:

• looking for private investors or other alternate funding streams to replace any funding that the government might yank,

• relying on international partners to fill in the gaps that will be left by the US government abandoning your project,

• seeking the cooperation of academic societies from around the world to provide facilities and infrastructure for any displaced scientists,

and much more.

The fact is that the entire world has taken notice of the US government’s unreliability to be a stable, long-term funding partner in any long-term ventures. (And yes, fundamental scientific research is always a long-term venture.) Despite the recent victory for science funding to open the new year, many are anticipating renewed attacks on the scientific foundation of the United States across the board. The European Space Agency, for example, has expressed reservations about its EnVision probe, and in particular about the NASA-made instrument VenSAR that’s supposed to fly aboard it.

The space-based gravitational wave flagship mission, LISA, is considering abandoning their NASA partnership, with Max Planck Institute professor Guido Mueller stating, “My biggest concern is if they say that they will do it and then pull out five years later. For LISA, that would be a huge disaster because at that point, everything would be going into flight hardware, everybody here has industrial contracts that we would have to continue to pay while trying to catch up on the hardware development that NASA should have provided.” All told, 19 joint space science missions, plus the ExoMars rover, would’ve faced funding shortfalls or even outright cancellation under the Trump administration’s earlier proposed budget.

As David Spergel of the Flatiron Institute and the Simons Foundation put it, “There are two possible futures for American science. One is that this proposed budget is a bump in the road and funding will stay at close to the same level. A second version is where this represents a real sea change in the relationship between government and science funding. I don’t think we know what world we’re in yet.” To not make contingency plans in today’s world is an extraordinary gamble: one that might lead to a trap door opening beneath your project’s — and your career’s — feet.

3.) Attempt to salvage-and-save the projects, careers, and opportunities that have already been terminated here in the US.

Will there be a Mars Sample Return mission happening in the 21st century? Perhaps, but it will likely be a nation like China or Japan that does it, not the United States, as it’s been eliminated from NASA’s plans. Will there be a Thirty Meter Telescope? Perhaps, but it won’t be part of the National Science Foundation’s arsenal of telescopes, and at this point it’s looking increasingly unlikely that it will be built on US soil at all. Over 8000 student visas have been revoked, along with around 2500 specialized worker visas: an absolutely essential part of an internationally competitive workforce.

Overall, a record-high number of US-based scientists are leaving or considering leaving the United States entirely, accelerating the already-in-progress brain drain that many have been warning about in scientific circles: a brain drain comparable to the one that occurred in Germany during the 1930s. Fortunately, science is not restricted to one country, but instead is a global endeavor.

• Europe is investing nearly a billion dollars in attracting top (formerly-)US talent, offering special additional funding for researchers to move their labs overseas.

• Japan has built a fund of ¥100 billion to attract researchers from abroad, including offering special positions for displaced scientists.

• Canada and Australia are also offering funding for early career researchers, attempting to close the gap left by the US’s abandonment of their careers.

Whether funded by private donors, philanthropists, research consortiums, or other nation-states — all of which are options for funding that should seriously be considered — just because the United States has pulled the plug on many projects of high scientific merit and high value to humanity doesn’t mean that they need to be lost entirely. Research on a wide variety of topics, including:

• on sustainable energy solutions,

• on high-energy physics,

• on pandemic prevention and mitigation,

• on climate change, natural disasters, and extreme weather events,

• on environmental sustainability,

• and in space science and exploration,

is not confined to any one nation, and perhaps never should have been.

Science benefits all of humanity, and rather than simply focusing on what can be saved within the United States, there should be extraordinary efforts made to save all of the research and all of the researchers whose careers and funding have been (and that will be) canceled and/or defunded in the United States for the good of the world. While the US diverts the resources that should be protected for fundamental research toward dubious but expensive endeavors like large AI datacenters and nuclear reactors on the Moon, basic science will largely have to continue elsewhere in the world.

The scientific foundation of modern society is a bit like an intricately constructed sand castle: it takes a great many people a great deal of effort to build it, but it can be destroyed in mere seconds not even by great deliberate force, but merely through the carelessness of a briefly unattended toddler. That is why it’s critical to save the science that’s already suffered under the chopping block of American politics: not just for the country or for the individual researchers, but for the good of the world.

4.) Work on a long-term plan to rebuild American science after the destruction has ceased, and to rebuild it in a way that will be resilient against the whims of any one political administration or party.

Someday, no matter how impossible it may seem at present, the current storm will be over. There will be no more worries of the unilateral destruction of science or scientific projects — the foundation of our modern, civilized world — coming seemingly out of the blue from the governments designed to support and steward them. Whatever remains of American science, whether it’s something comparable to what still exists today or something much less, will be ready to be rebuilt.

Ideally, we’ll want to build a new set of scientific infrastructure that won’t be subject to the whims of our political winds or the personalities that advantageously manipulate them. We’d want to build a world where foundational science remains our bedrock: where funding for our scientists and our scientific endeavors is guaranteed to persist. There are many different ways to do that, including:

• setting aside the entirety of a budget for a project right from the outset, where it cannot be touched or diminished by outside forces,

• for guaranteeing funding for projects and their personnel outside of the scope of “discretionary budgeting” that can vary or cease from year-to-year,

• or by explicitly writing it into a new constitutional amendment (or a new constitution entirely) that cannot be violated by any governing body, including the country’s leader.

It’s easy, particularly here at the start of 2026 when there are reasons for optimism for at least some aspects of science funding in the United States, for people to gravitate only to the first option: to what many have called “plan A” in the fight for a continued investment in fundamental science in the United States. It’s easy to look at some of the private and philanthropic efforts that have gone into funding science, which are of extreme value and importance, and wonder if foundations such as the Kavli Foundation, the Sloan Foundation, the Moore Foundation, the Simons Foundation, and the newly announced Schmidt Observatories, will make up for whatever shortfalls arise from the US government.

Both of these routes are of course tremendously important. It’s important to fight for science in the United States, and for all of us to work together to save as much of it as is humanly possible. Science and sustained investment in it, as has been well-documented, is essential to a modern, functioning society. According to UNESCO, of everything that humanity has ever done or created, “science is the greatest collective endeavor.” And that’s precisely why we must not merely rely on any single institution for its survival. Not on the US government, not on private funders or the philanthropy of the super-rich, and not even on the full suite of plans that we can concoct.

We will need to build a new system: one that’s resilient against the exact types of cuts that we’ve seen afflict science funding over the past year, and is resilient against whatever attempted cuts will take place over the coming years.

To those of you who are working on the first part of the plan, of saving as much of the science, and as many of the science careers, as can possibly be saved here in the United States: please, keep pursuing those opportunities, anywhere and everywhere you can find them. To those of you who are making contingency plans for how to save your scientific projects and personnel if-and-when your promised funding falls through: absolutely, keep doing this; there will very likely come a day, perhaps even soon, when the groundwork that you’re laying today can save the projects and careers that would otherwise simply be terminated.

And to those of you working on the latter two plans:

• working on saving science and scientists for humanity,

• including by taking projects and people overseas,

• or working on a long-term plan to rebuild American science in the aftermath of whatever’s going to happen in the short-to-long-term future,

rest assured that you are working on creating and preserving opportunities in science that the entire world will someday benefit from, perhaps more greatly than by any other set of endeavors on this list. There are a lot of tough choices that many of us are having to make here in 2026, including among scientists across the nation and world. Whichever of these plans is the right one for you to pursue, rest assured that there will be others working alongside you, while still others will choose to work on the other three. It will take all of us, working on all of these plans, to build the stable, resilient scientific future that the world truly needs: now more than ever.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Out there, in the vast Universe, are clumps of matter that come in many different sizes and masses. We might be most familiar with galaxies like our Milky Way: with hundreds of billions of solar masses worth of stars, even more gas and plasma, and more than a trillion solar masses worth of dark matter. At smaller masses, however, it takes longer, and becomes more and more difficult, for clouds of normal matter to collapse. If these low-mass clumps of matter do collapse, they’ll form stars, with the radiation from those stars often blowing the remaining gaseous matter away, but it’s also possible that — with enough externally injected energy — those clumps will never collapse.

That’s the idea behind what astronomers call a RELHIC: a Reionization-Limited HI Cloud, or a cloud of neutral hydrogen found within a more massive dark matter halo that’s in thermal equilibrium with the cosmic ultraviolet background. Although starless gas clouds have been found before, they:

• are usually rotating, indicating the presence of a collapsed disk,

• are typically high in mass, of around 100 million solar masses or more,

• and have fairly weak limits on how few stars could be located inside, usually not dropping below 100,000 solar masses worth of stars.

But with the discovery of a new starless gas cloud known as “Cloud 9,” coupled with deep Hubble imaging of that same field-of-view, we just might have stumbled upon the first RELHIC known to humanity. Here’s what we’ve found, plus the story of what it all means.

First, let’s start with “Cloud 9” itself, and what makes it such an interesting find. Above, you can see the galaxy Messier 94: a close, bright, massive galaxy located just 16 million light-years away. Although it’s marginally smaller than the Milky Way, there are around 20 known satellite galaxies around it, all of which contain significant quantities of stars and which appear to be gravitationally bound to Messier 94 itself.

That’s probably only the tip of the proverbial iceberg, however. It’s very likely that there are more satellite galaxies around Messier 94, as the nearest galaxies with the highest-resolution imagery show evidence for there being anywhere from 30-100 small satellites for every Milky Way-like galaxy that’s out there.

To search for these suspected additional satellites, researchers in China took the world’s largest ground-based radio telescope — FAST, or the Five-hundred-meter Aperture Spherical radio Telescope — and surveyed the area around Messier 94 in a comprehensive fashion back in 2023. In addition to confirming the presence of the already-known satellites around it, it discovered several new satellite candidates by detecting the hydrogen gas within them.

Most of those detected gas clouds corresponded to large masses of gas: tens of millions (or more) of solar masses worth of neutral hydrogen. But one of them — the ninth one found by FAST — had a large, extended distribution of neutral hydrogen gas, but only a small amount of it. Spanning over 4000 light-years in extent, it contained “only” around one million solar masses of neutral hydrogen. And, perhaps even more remarkably, the other imaging we had conducted of that region, including with a variety of optical, infrared, and radio telescopes, showed no conclusive evidence for the presence of stars within that gas cloud at all.

That caught the interest of a wide variety of astronomers, and led to an interesting follow-up project: to deeply view this region of space with not just other radio telescopes, but to acquire deep imaging of it with the Hubble Space Telescope. This would enable us to see if “Cloud 9” was truly a starless gas cloud, to the limits of our ability to observe small numbers of stars at a distance of 16 million light-years, or whether there were actually stars inside of it after all, and whether this was only initially designated as a potentially starless cloud because our observations simply weren’t of sufficiently high quality.

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It’s not as straightforward a task as it might seem. You might think, “oh, all I’ll have to do is take my high-quality observations, see if there are any individual stars revealed by those observations, and then, if there are, to infer what the total mass of stars is given the distance that this object is at.”

But the reality is much more complicated. At a distance of 16 million light-years, this could easily be a faint dwarf galaxy, particularly a low-surface-brightness galaxy, whose stars are simply beyond the limits of telescopes to resolve. When a ground-based telescope can’t detect stars above the overall background sky brightness, it’s enough to limit the total mass in stars to a few hundred thousand solar masses for an object like Cloud 9.

By going to space, however, there’s no longer any atmospheric/all-sky brightness to contend with. Hubble imagery can reveal, in the same location where the radio data indicates the greatest densities of hydrogen gas, whether there are any detectable stars or not. If there are stars that appear in that data, then we can use a mix of direct observations and simulations to infer what the overall stellar mass of that region is.

But if no stars appear, we can use what we learn from those observations and simulations to place an upper limit on how many stars could possibly be contained within that region. As you can see, from the VLA data overlaid atop the raw Hubble image of the field, there doesn’t appear to be any evidence for the presence of stars in the region of interest highlighted here.

In fact, there are several features that make Cloud 9 such a compelling candidate for being a reservoir of hydrogen gas that has never formed stars throughout its history. They include:

• the fact that it recedes from us at a speed of just over 300 km/s, the precise same speed that Messier 94 recedes from us,

• the fact that it’s located around 250,000 light-years from Messier 94,

• the fact that it exhibits such a strong signature of neutral hydrogen without any stars,

• and the fact that, to have all of this hydrogen present with no stars, a significant “halo” of dark matter, in excess of a billion solar masses, must be present.

You see, if you have a dark matter halo with neutral hydrogen gas within it, if it were in total isolation, you’d expect to form stars so long as you had enough mass gathered together in one place. When the astronomers who researched this gas cloud brought the Hubble imagery data together with the rest of the observations that had been acquired, however, they were able to place very tight constraints on the stellar populations that were allowed inside.

With ground-based imaging, we could only say, “well, there aren’t many hundreds of thousands of stars in there,” but we weren’t able to place any tighter constraints than that. With Hubble imaging, however, they were able to rule out stellar masses of 100,000, of 30,000, of 10,000, and even of 3,000 solar masses. All told, it looked like no more than about 1000 solar masses worth of stars could exist inside of this object: a remarkably small figure given a million solar masses of neutral hydrogen gas present within this gas cloud.

The way the researchers went about determining constraints on the allowable masses for the stars inside was to simulate the following:

• if we had this number of total stars inside of it,

• then what would the range of the number of individual stars be that appeared in the Hubble data,

• and, given that we have seen at most 1 excess star (above the normal background) in the Hubble data and given what we know about the color/magnitude of stars, how significantly can we rule out the higher stellar mass values?

At greater than 5-σ significance (the gold standard for astrophysics and particle physics science), stellar masses above 10,000 solar masses are ruled out entirely. Stellar masses of 10,000 solar masses are ruled out at the 99.5% confidence level (around 3-σ), and stellar masses around 3000 solar masses are ruled out at about the 95% (or 2-σ) confidence level.

With this detailed analysis, supported by a visual inspection of the Hubble data, the researchers rightfully conclude that this object, Cloud 9, observed (with the Green Bank Telescope) to have 1.4 million solar masses worth of neutral hydrogen, can have no more than about 3000 solar masses worth of stars. That’s a ratio of more than 400:1 in terms of gas-to-stars, which is a tremendous outlier among dwarf galaxies.

Typically, what happens is:

• a large, massive dark matter halo,

• also draws in large amounts of gas,

• then when enough of that gas accumulates in one place, it cools and collapses,

• and that collapse leads first to a disk and then to the formation of stars,

• and those stars, if they’re massive and energetic enough, blow the rest of the gas out of the galaxy,

• leaving only a series of stars embedded in a deep dark matter halo.

We see this for all sorts of dwarf galaxies in the nearby Universe, including for galaxies like Segue 1 and Segue 3, which are nearby galaxies with the lowest stellar mass contents — of only a few hundred (or perhaps up to 1000) solar masses — while displaying relative speeds (or velocity dispersions) that indicate hundreds of thousands of solar masses worth of dark matter. It’s by contrasting this new object, Cloud 9, with the previously known (and more common) dwarf galaxies that we find in the Universe, that we can truly see how remarkable Cloud 9 is.

That’s because this standard “story” we tell about dwarf galaxies corresponds to the overwhelming majority of dwarf galaxies that we’ve found. However, there’s another possible story to their evolution as well. In this scenario:

• you begin with a dark matter halo,

• and large amounts of neutral hydrogen fall into it,

• but the hydrogen remains diffuse and has difficulty cooling,

• largely due to the presence of ultraviolet radiation from all the stars formed throughout the cosmos (i.e., the cosmic ultraviolet background),

• which gradually ejects some (or most) of the hydrogen gas inside,

• leaving only a small reservoir of hydrogen inside a large, massive dark matter halo,

• where only a small number, or perhaps even zero, stars have formed inside.

Two years ago, the Green Bank Telescope “accidentally” (because of an operator error) observed a region of sky that wasn’t supposed to have a target of interest within it, but it led to an incredible discovery. What has since become known as a dark, primordial galaxy appears to have somewhere around a billion solar masses worth of hydrogen gas in it, but no visible stars at all. However, the gas is indeed rotating and in a disk configuration, and is high enough in mass to strongly suggest that there are stars within it, just below the detection limits of the tools we’ve used to observe it so far.

But Cloud 9 is an entirely different story: representing a discovery that could be our first example of a long-predicted, but never-before-observed population of astronomical objects. In theory, you fully expect that there will be systems that form with both dark matter and neutral gas, and while most of the systems with gas will collapse-and-cool to form stars, some won’t. Some, instead, will be heated up by the cosmic ultraviolet background light so that the normal matter won’t collapse, won’t form a disk, and won’t lead to the creation of stars. Instead, they might actually be reservoirs of truly pristine material: material that has not been part of the life cycle of even a single star since the Big Bang.

If that’s true for this newly discovered and probed Cloud 9 system, it would be an incredibly exciting find. If there are background quasars shining through the cloud, we could perform absorption spectroscopy to detect the presence of hydrogen, helium, and nothing else. If the cosmic ultraviolet background is gradually ionizing the hydrogen within the cloud, we should be able to detect faint, diffuse emission line signatures that could reveal which elements are and aren’t present, and in what abundance. And because we already see that there’s no evidence for rotation within Cloud 9, we can conclude that this isn’t a system that’s formed a disk out of its normal matter, disfavoring any recent or ongoing star-formation episodes.

Interestingly, the authors of the most recent paper about this system dutifully considered a number of plausible-sounding alternatives to this interpretation of Cloud 9.

• Could it be a foreground high-velocity gas cloud in the Milky Way? That’s disfavored by the precise match between the recessional velocity of Cloud 9 to the suspected host galaxy Messier 94.

• Could Cloud 9 instead be part of the Magellanic streams located a few hundred thousand light-years away? It’s not impossible, but the streams are located in the opposite direction of the sky to where Messier 94 is located.

• Could it be a neutral gas cloud in thermal equilibrium with the hot, ionized circumgalactic medium surrounding Messier 94? It fits many of the observed criteria, but not this one: the Universe is 13.8 billion years old, but the ongoing ram pressure interactions would destabilize the gas cloud on timescales of ~10 million years, or a factor of 1000 faster than the observed lifetime of Cloud 9.

All of this points to the REHLIC (or Reionization-Limited HI Cloud) interpretation as the leading explanation for Cloud 9. Remarkably, future observations — either longer-period observations with Hubble or superior, longer-wavelength observations with JWST — could directly probe this object to see whether the upper limit on the number of stars inside can be lowered even further, or could even be confirmed to be zero. As is so often the case in astronomy, by using different observatories sensitive to different wavelengths of light together, we can glean more comprehensive information about a system, and its nature, than even by using the newest and best observatory on its own. In astronomy, more is better, but different is also better.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Despite all we’ve learned about ourselves and the physical reality that we all inhabit, the giant question of whether we’re alone in the Universe remains unanswered. We’ve explored the surfaces and atmospheres of many worlds in our own Solar System, but only Earth shows definitive signs of life: past or present. We’ve discovered more than 5,000 exoplanets over the past 30 years, identifying many Earth-sized, potentially inhabited worlds among them. Still, none of them have revealed themselves as actually inhabited, although the prospects for finding extraterrestrial life in the near future are tantalizing.

And finally, we’ve begun searching directly for any signals from space that might indicate the presence of an intelligent, technologically advanced civilization, through endeavors such as SETI (the Search for Extra-Terrestrial Intelligence) and Breakthrough Listen. All of these searches have returned only null results so far, despite memorably loud claims to the contrary. However, the fact that we haven’t met with success just yet should in no way discourage us from continuing to search for life on all three fronts, to the limits of our scientific capabilities. After all, when it comes to the biggest existential question of all, we have no right to expect that the lowest-hanging branches on the cosmic tree of life should bear fruit so easily.

Each of the three main ways we have of searching for life beyond the life that arose and continues to thrive on planet Earth has its own sets of advantages and disadvantages.

1. We can access the surfaces and atmospheres of other worlds in our Solar System, enabling us to look for even tiny, microscopic signs of biological activity, including imprints left by ancient, now-extinct forms of life. But we may have to dig through tens of kilometers of ice to find it, or recognize life forms wholly unrelated to life-as-we-know-it on Earth.

2. With thousands of exoplanets now known, the imminent technological advances that will enable transit spectroscopy and/or direct imaging of Earth-sized worlds could lead us to discover living planets with unmistakable biosignatures in their atmospheres. If life thriving on an Earth-sized world is common, positive detections are only a matter of time and resources.

3. And searches for extraterrestrial intelligence offer the most profound rewards: a chance to make contact with another, perhaps even a technologically superior, intelligent species. The odds are unknown, but the payoff could be unfathomable.

For these (and other) reasons, the only sensible strategy is to continue to pursue all three methods to the limits of our capabilities, as without superior information, we have no way of knowing what sorts of probabilities any of these methods will have of yielding our first positive detection. After all, “Absence of evidence is not evidence of absence,” and that saying certainly applies to life in the Universe.

From a cosmic perspective, the laws that govern the Universe as well as the nature of the components that make it up indicate that the potential for life as a common occurrence might be absolutely inevitable. Initially, at the start of the hot Big Bang, our Universe was hot, dense, and filled with particles, antiparticles, and radiation moving at or indistinguishably close to the speed of light. In these beginning stages, neither the ingredients nor the conditions necessary for chemical-based life were in place; the Universe was born life-free. And yet, as time went on, the potential for biological activity would rise and rise.

As the Universe expanded and cooled, the following steps sequentially occurred:

• the particles and antiparticles annihilated away, leaving a tiny excess of matter behind,

• quarks and gluons formed bound states, giving rise to protons and neutrons,

• fusion reactions occurred, creating the light elements,

• atoms formed from these atomic nuclei and the surrounding bath of electrons,

• gravitational contraction and collapse takes place, giving rise to stars,

• star clusters and other clumps of matter attract, forming galaxies,

• and within those galaxies, successive generations of stars are formed, creating heavy elements.

Once a galaxy becomes enriched enough with these heavy elements, the new generations of stars that follow can form with rocky worlds within those stellar systems, many of which will have the potential for life.

Within our observable Universe, since the dawn of the hot Big Bang, sextillions of stars have formed. Of those, the majority of them are found in large, massive, rich galaxies: galaxies comparable in size and mass to the Milky Way or greater. By the time billions of years have gone by, most of the new stars will have sufficiently large fractions of heavy elements to lead to the formation of rocky planets and molecules that are known as precursors to life. These precursor molecules have been found everywhere, from comets and asteroids to the interstellar medium to stellar outflows to planet-forming disks.

And, at this critical step, we find ourselves face-to-face with the end of our scientific certainty.

• Where, and under what conditions, does life come into existence?

• On those worlds where life arises, how frequently does it survive and thrive, persisting for billions of years?

• How often does that life saturate its habitable regions, transforming and feeding back on its biosphere?

• Where this occurs, how often does life diversify, becoming complex and differentiated?

• And where that occurs, how frequently does life become intelligent enough to become technologically advanced, capable of communicating across or even traversing the vast interstellar distances?

These questions aren’t merely there for us to philosophically ponder; they’re there for us to gather information about, and eventually, to draw scientifically valid conclusions about such probabilities.

Of course, there are plenty of valid explanations for why we haven’t succeeded in our searches for life just yet. The most sobering — and the most pessimistic — is that it’s possible that one or more of the steps required to give rise to the type of life we’d be sensitive to are particularly difficult, and only rarely can the Universe achieve them. In other words, it’s possible that any one of life, sustained life, complex and differentiated life, or intelligent and technologically advanced life are rare, and none of the worlds we’ve surveyed possess them. That’s a possibility we have to keep in mind so long as we’re committed to remaining intellectually honest.

But there’s no reason, at least so far, to suspect that’s the primary reason we haven’t discovered life beyond Earth just yet. The old saying, “If at first you don’t succeed, try, try again,” applies wherever the odds of success are unknown, but we have every indication that success is possible under the right circumstances. Here on Earth, the evidence strongly indicates that our home planet is an example of such circumstances, and hence it’s likely that there are places all throughout space and time where life sustains itself, evolves to become complex and differentiated, and achieves a level of technological advancement sufficient for interstellar communication.

The big unknowns are in the probabilities of the various types of alien life that are actually out there, not in the question of whether such achievements are possible within our Universe.

That doesn’t mean that we should take seriously every claim that’s been made — even by scientists — that alien life has been found. The “Wow!” signal, for example, was a high-powered radio signal received over the span of 72 seconds back in 1977; although its nature is unknown, it has never been replicated, either back at the original source or anywhere else. Without confirmation or repeatability, we can draw no affirmative, definitive conclusions.

Fast radio bursts, like many signatures observed astronomically, appear in many locations both in and out of our galaxy, but have no indication that they were intelligently created; they are likely simply a natural astronomical phenomena whose origins have yet to be determined.

NASA’s Mars Viking lander conducted numerous tests for life on the Martian surface, with one experiment (the Labeled Release experiment) giving a positive signature. However, the possibility of contamination, the lack of reproducibility, and the lack of a verified follow-up experiment has cast tremendous doubt on the “biological positive” interpretation of the experiment.

And despite the possibility of encountering interstellar space probes, direct alien contact, or even the ubiquity of alien abduction stories, no robust verification of any of these claims has ever come forth. We have to keep our minds open while at the same time remaining skeptical of any grand claims. The conclusions we draw can only be as strong as the supporting evidence for them.

It’s primarily for these reasons — we have every indication that the Universe has all the necessary ingredients for life, but no indication that we’ve found it just yet — that it’s so vital to keep looking in a scientifically scrupulous way. When we do announce that we’ve found extraterrestrial life, we don’t want it to be another instance of crying “wolf” with insufficient evidence that we’ve found a wolf; we want the claim to be supported by overwhelming, unassailable evidence.

1. That means building fleets of orbiters, landers, sample-return missions, and laboratory-equipped rovers to explore a wide variety of worlds in our Solar System: Venus’s atmosphere, Mars’s surface, Titan’s lakes, and the oceans of Europa, Enceladus, Triton, and Pluto, among others.

2. That means building superior coronagraphs on world-class space-based and ground-based telescopes, considering the construction of a starshade, and investing in transit spectroscopy. By imaging the atmospheres and surfaces of exoplanets, including breaking down their molecular and atomic constituents and abundances over time, we should be able to identify any world with a life-saturated biosphere.

3. And that means continuing to search, with greater precisions and sensitivities across the electromagnetic spectrum, for any signals that might come from an intelligent species seeking to communicate or announce their presence.

If you don’t find fruit on the lowest-hanging branches, that doesn’t necessarily mean you give up on the tree; it means you find a way to climb higher, where the fruit may be present but out of your present reach.

This might also include expanding our searches for extraterrestrial intelligence. While most searches focus on far-reaching radio transmissions, it’s possible that alien civilizations who seek to communicate across the stars and galaxies will rely on a different technology. Perhaps we should be monitoring the tails of water maser lines or the 21 cm spin-flip transition of hydrogen. Perhaps we should be looking for patterns in pulsar signals, including correlating signals between pulsars. Perhaps we should even be looking for extraterrestrial intelligences in gravitational wave signals that we have yet to discover. Wherever a signal can be encoded by a sufficiently advanced species, humanity should be looking and listening.

There are also avenues to explore that won’t reveal alien life, but could help us understand how it arose and arises throughout the Universe. We can recreate the atmospheric conditions found on other worlds or even as they were on Earth long ago in the lab, with an eye toward recreating the origin of life-from-non-life. We can continue exploring the possibility of having nucleic acids (RNAs, DNAs, even PNAs: peptide-based nucleic acids) coevolve with peptides in an early prebiotic environment: perhaps the most compelling candidate for how life first arose on Earth.

The rewards of finding out that we’re not alone in the Universe would be immeasurable. Perhaps we could learn how to survive the great environmental threats that face us: hazardous asteroids, a changing climate, or violent space weather events. Perhaps there are even more important lessons to be learned about how to overcome our own insufficiencies as human beings: the great challenge of moving beyond our primal nature. Perhaps other civilizations have success stories to offer us, recounting how, in the early days of their technological infancy, they overcame such issues as:

• overconsumption, where they devoured their planet’s resources beyond the point of sustainability,

• short-term thinking, where they addressed the immediate, urgent problems at the expense of long-term ones that threatened their existence,

• or the emergence of disease, famine, pestilence, and ecological collapse, resulting from the global changes wrought by a post-industrial society.

Our impulses toward greed, plunder, and self-gratification may not be unique, and there may be more experienced, wiser species out there that have figured out solutions that elude us today. Perhaps, if we’re lucky, they may have lessons waiting-in-the-wings for us that could guide us toward a more successful future.

Many of us can imagine two different futures unfolding for the enterprise of human civilization. There’s the one we should strive to avoid: where we resort to infighting, squabbling over the limited resources of our world, descending into ideologically-driven wars and ensuring our own eventual destruction. If we never find life beyond Earth — never find anyone else to communicate with, exchange information and culture with, and to give us hope that there’s a future for humanity out there among the stars — perhaps extinction will indeed be our most likely outcome.

But there’s another possible outcome for humanity: a future where we come together collectively to face the gargantuan challenges facing humans, the environment, planet Earth, and our long-term future. Perhaps the discovery of life beyond Earth — and potentially, of one or more intelligent, spacefaring, extraterrestrial civilizations — might give us not only the guidance and knowledge we need to survive our growing pains, but something far grander than any terrestrial achievements to hope for. Until that day arrives, we must make do with the knowledge that, at present, we have only one another to extend our kindnesses and compassion to.

This article was first published in December of 2022. It was updated in December of 2025.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Perhaps the most remarkable fact about the Universe, if you think about it on a truly fundamental level, is that it exists at all. And yet, not only does it exist, but there’s matter within it, which obeys the same rules everywhere and at all times, and assembles according to the physical laws governing reality to create, among other things:

• atomic nuclei,

• neutral atoms,

• molecules,

• stars and planets,

• galaxies,

• and a large-scale cosmic web.

Not only that, but in at least one relatively unremarkable corner of this Universe, a planet arose some 4.5 billion years ago where life survived and thrived, eventually giving rise to an intelligent, self-aware species that can ask deep questions about the Universe they inhabit.

In doing so, we’re also asking fundamental, deep questions about our own selves, as we’re just as much a part of this Universe as any inanimate objects that exist. That’s what led Steve Cordon to write in and ask one of the deepest questions of all:

“I’ve always wondered why is there something instead of nothing? I’ve read some books authored by a variety of cosmologists. I was curious as to your explanation on that as well. I look forward to your plausible explanation! As Carl Sagan was known to say, ‘Extraordinary claims require extraordinary evidence!’”

This is one of those questions, I’m sorry to say, that science not only doesn’t have a satisfactory answer for right now, but will probably never have one. Although there’s a lot we can say about this question, we face fundamental limitations by the very nature of the scientific enterprise. Here’s why.

How did we get to be here?

There’s an enormous difference between a “why” question, which science isn’t really well-equipped to answer, with a “how” question, which is the bread-and-butter of what science is good for. If we were to ask the question of why we’re all here, there isn’t a scientific way to approach this question: we can’t formulate a testable hypothesis and derive what sorts of things we can go out and measure to answer that. Even if we determine the underlying rules that reality follows, there’s a limit to what we can derive from them: we can derive physical consequences that stem from those rules and some set of initial conditions, but we cannot derive any sort of purpose behind those consequences using the tools of science.

The question of “how” we got to be here, on the other hand, is one that science can tell us literally everything about. Especially if we’ve already done the hard work to understand the laws of science, from the biological and geological and atmospheric evolution that’s occurred here on Earth to the chemical evolution that’s occurred through the differing environments of space and on Earth to the particle-based evolution that occurred inside stars and in the crucible of the Big Bang, we can put those pieces of the story together to understand precisely how this Universe gave rise to our Solar System, our planet, the origin of life, and after some 4-ish billion years, even a full-fledged human being.

Although that story — of how we came to be the way we are — still has a few gaps in it as far as definitive knowledge is concerned, these are gaps that are forever shrinking as we accumulate more and more knowledge about:

• the laws and rules that govern the Universe,

• the past and present phenomena that have occurred here on Earth,

• and the great variety of possible outcomes, given similar starting conditions, that occur or could have occurred elsewhere in the cosmos.

We still don’t know how, specifically, life originated from non-life, how many planets once existed and were either destroyed or ejected at some point in our Solar System’s past, or what predecessor stars existed that enabled the Sun to form with the properties it possessed at its birth.

And if we step back and take a look at the bigger cosmic picture, it turns out that although there are some really fascinating details that we do profoundly understand, there remain gaps there, too: things that we know must have happened in our cosmic past at some point and in some way, but whose details — the specifics of “how” they occurred — remain obscure to us. Some of these cosmic unknowns are among the biggest questions in theoretical physics and astrophysics today.

The knowns

This isn’t to undersell what we actually do know, which is perhaps the greatest achievement of the entire enterprise of human civilization. The fact that we have asked the Universe so many questions about itself and how it works have revealed an incredible amount of information.

Biologically, we understand the inheritance of traits and the role that mutation and selection plays, leading to what we commonly know as Darwinian evolution. We understand that Darwinian evolution is not a fundamental law, but rather that inheritance is driven by genetics, which was further developed by Gregor Mendel. And Mendelian genetics, while profoundly important, itself is a consequence of the genetic code written down in every plant, animal, and fungus’s DNA, discovered by James Watson, Francis Crick, and Rosalind Franklin.

But DNA is simply a molecule: assembled and synthesized together out of atoms through a series of chemical (and biochemical) reactions. And chemistry is just a consequence of atoms and molecules (and ions) reacting together in an environment with a temperature or energy gradient: a complex process, sure, but one whose interactions and reactions can be understood at a fundamental level.

We can go back further still: to the origin of the atoms that make up every molecule. Within the heart of every star, light elements are fused into heavier ones through incredibly energetic nuclear reactions. In a star like the Sun, hydrogen is fused in a chain reaction to build up helium, while carbon, nitrogen, and oxygen also play an important (but sub-dominant) role. In more evolved stars, helium is fused into carbon, and in very massive stars, elements like neon, oxygen, magnesium, silicon, sulfur, argon, and calcium are also produced in chain reactions, eventually leading up to the formation of the most stable elements of all: iron, cobalt, and nickel.

When the most massive stars die, they do so in catastrophic core-collapse supernova explosions, typically creating elements in abundance all the way up the periodic table to zirconium (element #40), but few beyond that. (Sun-like stars, in their final, giant phase, process pre-existing nuclei to create a smattering of elements from strontium, element 38, up through bismuth, element 83.)

When stars generally die, the remnants they leave behind include white dwarfs, neutron stars, and black holes. When white dwarfs and white dwarfs merge, they create a different kind of supernova (type Ia) that also produces a wide variety of elements. When neutron stars merge with neutron stars, they can produce a cataclysm known as a kilonova, where the majority of still-heavier elements are produced. All told, we can trace back the astrophysical origin of every element found in the periodic table using modern science.

Going back to the beginning

In fact, this is one of the most important properties of science overall: the notion that, if you understand a physical system well enough, then simply by providing

• a set of initial conditions for that system

• plus the physical laws that govern it,

then we’ll be able to calculate that system’s properties, or at worst, a probabilistic set of outcomes for that system’s properties, at any arbitrary moment as far into the future as we like. This, in many ways, is the heart of what science is: our best and most accurate models of reality, and an attempt to account for all observable and measurable phenomena in the process.

We can use all we’ve learned to extrapolate backward in time, and learn what the Universe was like at earlier and earlier times. We can go back to a time before any stars had formed, and by applying the laws of physics, learn how stars, galaxies, and the cosmic web assembled. Moreover, we can learn what “seeds” of structure — or initial conditions — needed to be in place to give rise to what we observe at later times. We can go back still farther and learn about the formation of neutral atoms (and the prediction of the cosmic microwave background), the formation of atomic nuclei (and the prediction for the abundance of the light elements), the creation of a primordial neutrino background (and predict the observable signatures it leaves today), and back all the way to the Big Bang and even before, to the phase of cosmic inflation that preceded it and set it up.

The unknowns

But that still doesn’t tell us everything. For example, we know that, after the primordial soup from the hot Big Bang cooled off, we were left with a slight overabundance of matter compared to antimatter. It’s as though, during the earliest phases, for every 500 million antiquarks that were present, there were 500 million and one quarks that were present. For every 1.5 billion positrons that were present, there were 1.5 billion and one electrons that were present. And while we know some general rules that allow us to create a matter-antimatter asymmetry from an initially symmetric state, we have yet to unpack precisely how our Universe generated the matter-antimatter asymmetry that gave rise to our existence.

We don’t know what many of the specifics of cosmic inflation were, or how, precisely, it came to an end to create the hot Big Bang. We don’t know how “hot” it actually got during the hot Big Bang; we have an upper limit and a lower limit on the initial temperature, but there are some ~14 orders of magnitude in between those limits. We don’t know how the fundamental constants came to possess the values that they have, and how we came to have the four fundamental forces (and, seemingly, no others) within our Universe. And from a phenomenological point of view, we have an overwhelming amount of evidence for the existence of both dark matter and dark energy, but precious little evidence indicating what their true nature actually is.

The bigger questions

It leads to a question that almost everyone — physicists and non-physicists alike — has asked themselves at some point or other: did things have to turn out this way, or could they have been different? If you were to run the clock all the way back to the earliest state we can say something physically sensible about, the inflationary state with the same initial conditions ours possessed, would you get the same Universe with the same:

• fundamental constants,

• laws of physics,

• hot Big Bang,

• sets of species of particles and antiparticles,

• matter/antimatter asymmetry,

• and dark matter and dark energy,

that our Universe possesses? Would you get the same structures, from atoms to stars to the cosmic web? The same types of planets? The same chemical and biological outcomes?

On some of these accounts, we’re pretty certain the answer is no. There are too many possible outcomes and too many interacting particles for us to expect that, for instance, human beings would exist anywhere else in the Universe, or would come to exist in a Universe that started off with identical initial conditions to our own. A great many properties that our Universe possesses, from the cosmic microwave background to the cosmic web, would certainly be similar in a statistical sense to the ones our actual Universe possesses, but we’re certain they wouldn’t be identical. The laws of physics, after all, are known to not be fully deterministic in an inherently quantum Universe.

But what about the fundamental constants? What of the laws of physics themselves? What about the number of forces, the types of particles and antiparticles that are allowed to exist, and the properties and existence of poorly-understood entities like dark matter and dark energy?

The truth is: we don’t know. We can postulate physical and mathematical extensions to what’s known to try to answer them, and under many of those extensions, there are indeed variations that are allowed. Those possibilities include:

• extra dimensions,

• grand unified theories,

• string theory (and other ideas concerning quantum gravity),

• theories of everything (of which string theory is one),

all of which allow for the existence of new particles, new interactions, and new phenomena, such as leptoquarks, superheavy bosons, and proton decay.

But more conservative scenarios, still fully consistent with everything we observe, don’t include these new phenomena, and don’t necessarily allow different sets of fundamental constants, new laws of physics, or novel forces and interactions. These are questions — big, profound questions — that science has yet to figure out.

So why is there something instead of nothing?

We are certain that “something” exists. We are certain that if you take away the particles and antiparticles and photons and quanta in a region of space, that empty space will still exist. If you move far away from any sources of mass or energy and clear the space of all external electric, magnetic, and gravitational fields, and prevent any photons or gravitational waves from entering that space, that “physical nothingness” will still exist in that region. And in that region, certain things cannot be removed:

• there will still be quantum fields in the vacuum of that empty space,

• the fundamental constants and underlying laws of physics will still exist in that empty space,

• and there will still be a “zero-point energy” inherent to that space, and it will still possess a finite, positive, and non-zero value.

As far as we can tell, that’s as close to nothing as we can get within our Universe.

You might be able to imagine, in your mind, a state of pure nothingness that’s even more “nothing-like” than this, but that doesn’t represent anything physically real. There’s no experiment you can design that can create such a condition. The best we can say — assuming that we’re sticking to science and not moving into the realm of theology, philosophy, or pure imagination — is that the reason there’s something rather than nothing is that “nothing” cannot exist compatibly within our Universe. Of course, that leads back to the original question: why? And for that, dissatisfying though it is, science has no answer. The Universe is the way it is, and though we strive to understand it as best we can, we are compelled to be humble before the great cosmic unknown. The only advice I can give you is this: beware of anyone who claims to “know” the unknown. They may or may not be fooling themselves, but you certainly shouldn’t allow them to fool you.

This article was first published in November of 2023. It was updated in December of 2025.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

For millennia, supernovae were rare, once-per-century sights.

The last naked-eye Milky Way supernova occurred way back in 1604.

But with modern astronomy, they’ve appeared all across the Universe.

Similarly, gravitational lenses abound, with mass bending and distorting space.

The light from background objects often appears multiple times.

When galactic brightness varies — from quasars or supernovae — those multiple images vary, too.

Different images possess different path-lengths, causing delays in those features’ appearances.

Our first multiply lensed supernova exhibited significant delays across five separate images.

These lensing path delays, wherever they occur, yield distance and redshift information.

Multiply lensed, time-varying objects enable measurements of the cosmic expansion rate.

In 2025, supernova SN 2025wny became humanity’s first multiply imaged superluminous supernova.

First identified with the Zwicky Transient Facility, follow-up ground based imaging revealed its nature.

These observations demonstrate that existing ground-based facilities are sufficient for time-delay cosmography.

Absorption lines, magnification information, and light-curve features are all easily revealed.

With the advent of the Vera C. Rubin Observatory, ~20 billion galaxies are continuously imaged.

Many are lensed, yielding expectations of thousands of multiply lensed supernovae by 2035.

This unprecedented sensitivity advances our cosmic understanding: one lensed system and one transient/supernova at a time.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.