Brian Koberlein

The Quadratic Equation

Tue, 31 Mar 2026 00:00:00 +0000

A Venn Diagram of how the main theories of physics are interconnected.

Modern cosmology is built upon three theoretical pillars: special relativity, Newtonian gravity, and quantum mechanics. Each is supported by a wealth of experimental evidence, but each describes the physical world in a way that contradicts the other two.

Quantum theory describes the tiny. Objects driven by the electromagnetic, the strong, and weak forces. The fuzzy world of atoms and molecules. Newton’s model describes the huge. Galaxies, black holes, and the orbits of planets. Special relativity describes space and time. The background through which atoms, planets, and humans move and interact.

Any two of these theories can be unified into a consistent model. Connect special relativity with gravity, and you get general relativity, which describes how gravity is a warping of spacetime. Connect special relativity with quantum mechanics, and you get quantum field theory. Connect quantum mechanics and Newtonian gravity, and you get weak quantum gravity, which can describe how atoms and molecules behave in a weak gravitational field such as Earth’s.

What we don’t have is a theory that unifies all three. One of the major difficulties is the renormalization problem. For example, in special relativity matter can be converted to energy and energy to matter. In quantum theory, particles can spontaneously appear or disappear as virtual particles within the bounds of quantum uncertainty. When you connect the two, the virtual particles have energy, which triggers more virtual particles. If you try to calculate the total energy of all virtual particles, it blows up to infinity.

Fortunately, it is only the relative energy that matters. Through a mathematical process known as renormalization, you can cancel out the virtual energy of quantum particles to get the answer you need. But when you add gravity into the mix, this all breaks down. The energy of the virtual particles should warp spacetime, and without a fixed spacetime background, you can’t renormalize.

Many approaches to quantum gravity have this problem and can’t be renormalized. But one approach, known as quadratic quantum gravity, can be renormalized. Essentially the model adds quadratic terms to the Einstein field equations so that it can be renormalized like quantum field theory. The problem is that it also introduces a quantum field of “ghost particles.” These particles don’t appear in particle physics experiments, so quadratic quantum gravity isn’t very popular. It’s possible that the ghost particles are just too massive to appear in current particle physics experiments, but this makes the theory untestable.

Or so we thought.

Liu, et al.

A comparison of the quadratic gravity model with observations.

A new paper in The Physical Review Letters argues that quadratic quantum gravity is the reason the Universe expanded rapidly in its youth. The authors show that within quadratic quantum gravity, the quadratic terms drive cosmic expansion naturally. Once the cosmos undergoes its early expansion, the spacetime structure is dominated by the usual effects of general relativity.

The authors go on to demonstrate that the model also predicts a minimum level of background gravitational waves created during the inflationary period. These waves are too small to detect with current observatories, but they are in the range of future observatories such as LISA. So the model will be testable in time.

Testing the Waves

Mon, 23 Mar 2026 00:00:00 +0000

LIGO/Caltech/MIT/R. Hurt (IPAC)

Discoveries made by the LIGO-Virgo-KAGRA (LVK) network since LIGO’s first detection of gravitational waves emanating from pairs of colliding black holes.

General relativity stands as one of the bedrock theories in modern physics. Its strange view of relative time and space has been confirmed by countless experimental and observational tests, from rotational frame dragging to the radiation of gravitational waves. But there is reason to believe that it is not the final say on the nature of space and time.

One of the big reasons for this is that general relativity breaks down on the scale of the very tiny. The world of atoms and molecules is a quantum one, but general relativity is a classical theory. What we need is a quantum theory of gravity. There are plenty of proposed models for quantum gravity, but they often assume alternative models of gravity. Theories that give the same results as GR for weak gravitational interactions but that deviate from GR in strong gravitational fields. The predictions of these alternative models have been untestable with current observations. But that’s starting to change, as a recent¹ set² of papers³ shows.

The three papers look at data from the 4th run of the LIGO–Virgo–KAGRA detections of black hole mergers, which is the latest and most advanced set of observations. The first paper looks at the overall comparison of the data with general relativity to see if GR is consistent with the data. The second looks at what are known as post-Newtonian parameters, which is a way to look for deviations from GR. The third paper looks specifically at the “ringdown” data as the newly merged black hole settles down into its new stable state.

As you might expect, all the results support general relativity. The first work found that within the limits of observation, GR is a solid fit. There is no need for an alternative model. There are alternative gravitational models that also fit the data, but we have no reason to assume they are correct.

The second paper further constrained alternative models. In the post-Newtonian approach, you look at how observations deviate from Newtonian gravity by tweaking a set of parameters. The more parameters you can fit to the data, the more precise your model is. The merger data is precise enough to look at the dipole and quadrupole parameters and found no deviation from GR. This means that any alternative model that predicts, for example, a quadrupole deviation is ruled out.

Interestingly, since post-Newtonian approximations of gravity can be quantized, this second paper also gives a new experimental bound on the mass of gravitons. Based on GR and basic quantum theory, gravitons should be massless, just like photons. This new work proves the mass of the gravition must be less than 2 x 10^-23 eV/c². In comparison, in particle physics, the upper bound of photon mass is 10^-18 eV/c².

The third paper focused on the prediction of some alternative theories that merging black holes could create gravitational echoes. That is, after gravitational waves of the merger settle down, there should be a second burst of gravitational waves. These echoes are impossible under general relativity, so detecting them would prove GR is incomplete. The authors found no evidence for gravitational echoes and thus no evidence for alternative gravitational models.

These results are not surprising given how strongly GR has been supported by previous experiments. But the big news here isn’t that we’ve proven Einstein right once again. What is key with these papers is that we now have gravitational wave data good enough to test GR. We can now test how space and time behave in the regions of black holes. All with only a decade of observations. The next few decades of gravitational wave astronomy will finally give us the kind of data we need to truly explore the limits of gravity.

Abac, A. G., et al. “GWTC-4.0: Tests of General Relativity. I. Overview and General Tests.” arXiv preprint arXiv:2603.19019 (2026). ↩︎
Abac, A. G., et al. “GWTC-4.0: Tests of General Relativity. II. Parameterized Tests.” arXiv preprint arXiv:2603.19020 (2026). ↩︎
Abac, A. G., et al. “GWTC-4.0: Tests of General Relativity. III. Tests of the Remnants.” arXiv preprint arXiv:2603.19021 (2026). ↩︎

Soupy Sails

Thu, 12 Mar 2026 00:00:00 +0000

Jose-Luis Olivares, MIT

In this illustration a quark zooms through quark-gluon plasma, creating a wake in the plasma.

In its earliest moments, the Universe was hot and dense. A plasma sea of quarks and gluons out of which hydrogen, helium, and humans eventually formed. This early cosmic state is sometimes called the primordial soup, and thanks to new research, we now know just how fitting the term is.¹

Although we’ve long known of the early Big Bang, it is difficult to understand its full nature. Some of it we can glean through theoretical calculations and things such as the ratio of hydrogen to helium, but theory can only take you so far. Imagine trying to calculate whether a region of water is solid, liquid, or gas simply from computer simulations of water molecules. There is no bulk material on Earth with the density and temperature of the early Universe.

But the interiors of atomic nuclei come close. So one way to study the early Universe is through particle physics experiments. Recently a team at CERN has been colliding heavy ions. The particles collide with each other at nearly the speed of light, creating a brief mix of quarks and gluons similar to the primordial soup. This state only exists for a tiny fraction of a second, so scientists can’t observe the state directly. What they can do is look at the cascade of particles created by the plasma state. This is similar to studying water waves by looking at how the spray wets the shore.

In this recent study, the team focused on the interaction of Z-bosons, which are a type of weak interaction carrier particle, similar to photons for electromagnetism. They compared the observed results with different models of the quark-gluon plasma (QGP). The best model is one in which the plasma is soupy.

What they actually found was evidence of wakes within the plasma field. If you move your hand through a pool of water, it creates a wake of ripples. This is because water is a liquid. If you move your hand through sand, the sand grains move, but they don’t create ripples through the sand. By finding evidence of wakes, the team now knows that particles moving through a quark-gluon plasma behave like a stone through water. The plasma has a soupy, fluid-like behavior.

Knowing that matter in the early Universe was a thick soup will help us better understand its earliest moment. Things such as shock waves behave differently through fluids than through gas or solids, and this can affect not only how the first atoms formed but also how the seeds for galaxies and black holes first appeared.

In future studies, the team hopes to pin down more details of these wakes, such as how quickly they move through the plasma and how large they are. This will let them determine bulk properties such as the fluid density and viscosity. In other words, how soupy the primordial soup really was.

Cms Collaboration. “Evidence of medium response to hard probes using correlations of Z bosons with hadrons in heavy ion collisions.” Physics Letters B (2025): 140120. ↩︎

Galaxy Flash

Wed, 11 Mar 2026 00:00:00 +0000

Max Planck Institute for Gravitational Physics

Simulation of the gravitational lensing of starlight by a binary black hole.

Most galaxies have a supermassive black hole at their center, but some galaxies have two. These supermassive binaries form when two galaxies collide and merge. We can detect some of these binaries, such as by observing the periodic changes of a quasar or by observing the binary directly, such as in the case of NGC 7727. But most supermassive binaries remain hidden. They are too far away to be observed directly or too inactive to be observed by jets. And while gravitational wave observatories can detect the mergers of stellar-mass black holes, we can’t yet detect the mergers of supermassive black holes. But a new study shows how we might detect some of them.¹

The idea is based on gravitational lensing, where the mass of an object deflects a path of light similar to the way the glass lens of a telescope focuses starlight. Gravitational lensing is most commonly seen when a distant quasar is lensed by a foreground galaxy. A similar effect known as microlensing occurs when a mass passes in front of a more distant star.

In this case, the study looks at what would happen as the supermassive black holes orbit each other against the background of stars within their galaxy. The black holes always lens nearby starlight, but when the two black holes are positioned just so, they can act as a particularly strong lens. This could cause a background star to appear much brighter than usual. Since this alignment occurs every orbit or half orbit, the brightness would occur periodically.

Wang, et al.

How binary black holes can gravitationally lens a background star.

It’s not quite this simple, since there could be more than one star with just the right alignment, and the black holes and stars are all moving, so alignments would change over time. But the authors show that quasiperiodic flashes could still occur. Based on simulations, the authors predict that, given our current technology, we should be able to detect quasiperiodic starlight flashes in about 50 nearby galaxies. The flashes should occur within periods of less than a decade or so.

Given that timescale, it’s not surprising that we haven’t detected this yet. You’d need to watch lots of galaxies over a long time frame. Fortunately, we will be doing exactly that in the near future. For example, the Vera Rubin Observatory will capture images of thousands of galaxies on the order of a few days. Not only will this capture the periodic flashes, it will allow astronomers to construct the shape of the black hole orbits in time. This will allow us to look for the gravitational waves of specific binaries as our gravitational telescopes improve.

Wang, Hanxi, Miguel Zumalacárregui, and Bence Kocsis. “Black Holes as Telescopes: Discovering Supermassive Binaries through Quasiperiodic Lensed Starlight.” Physical Review Letters 136.6 (2026): 061403. ↩︎

Moon and Shadows

Wed, 21 Jan 2026 00:00:00 +0000

Vladimir Vustyansky / NASA

This illustration depicts a conceptual Lunar Crater Radio Telescope on the Moon’s far side.

We now have direct images of two supermassive black holes: M87* and Sag A*. The fact that we can capture such images is remarkable, but they might be the only black holes we can observe. That is, unless we take radio astronomy to a whole new level.¹

It’s incredibly difficult to get high-resolution images in radio astronomy. Radio wavelengths are on the order of millimeters or larger, compared to nanometers for visible light. Since the resolution of a telescope depends on the wavelength size, radio telescopes have to be huge. It would take a radio dish nearly 10 kilometers wide to get the resolution of a large optical telescope. This is why we now build radio telescopes as arrays of smaller dishes and use interferometry to create a virtual dish the size of the array.

Both M87* and Sag A* have an apparent size of about 40 microarcseconds, which is roughly that of a baseball on the surface of the Moon. To observe such a small apparent object, astronomers had to create a virtual telescope the size of Earth. It took an array of telescopes all over the world, and even then the resolution of the Event Horizon Telescope was only about 20 microarcseconds. That’s part of the reason the images are so blurry. Updates to the EHT could sharpen the resolution to 10 microarcseconds, but that’s about it.

Unfortunately, M87* and Sag A* have the largest apparent size of nearby supermassive black holes. And M87* is particularly bright, making it easy to observe. While there are dozens of other black holes we’d love to observe, they are beyond the limits of the EHT. So why not make an even larger virtual telescope?

This is the idea behind a new work on the arXiv. A lunar radio telescope has been proposed many times before. Usually the idea is to locate it on the lunar far side so it will be hidden from all the radio noise from Earth. In this new work, the authors consider five possible locations: two on the far side, two on the near side, and one at the lunar south pole. Multiple locations would allow astronomers to continue observing objects as the Moon orbits the Earth and Sun.

Zhao, et al.

Comparative apparent sizes and brightness of black holes that could be observed with an Earth-Moon array.

The sensitivity of the proposed telescopes would depend on their overall size, but if we assume the sensitivity is similar to current Earth-based observatories, then observing other black holes comes down to the resolution of the Earth-Moon array. That depends on where the Earth and Moon are relative to their target. If the Earth and Moon are along the same line of sight, then having lunar dishes wouldn’t help much. If they see the object with a baseline of the full lunar radius, then we could get resolutions less than 1 microarcsecond.

The authors look at the orientation of the Earth-Moon system relative to black holes in our greater cosmic neighborhood and find nearly 30 black holes that could be observed. These range from the supermassive black hole in the Andromeda Galaxy to Cyg A*, which is at the heart of a radio galaxy 760 light-years away.

We are decades away from operating radio telescopes on the Moon, and there are plenty of engineering challenges we have yet to solve. But studies such as this show why we should rise to that challenge. Lunar observatories would not only capture some of the faintest radio objects; they would also reveal the light around black holes in unprecedented detail.

Zhao, Shan-Shan, et al. “Beyond Sgr A* and M87*: Sub-Microarcsecond Black Hole Shadow Detection via Lunar-based Extremely Long Baseline Interferometry.” arXiv preprint arXiv:2601.02812 (2026). ↩︎

Interrobang

Mon, 12 Jan 2026 00:00:00 +0000

NSF/AUI/NSF NRAO/B. Saxton

New research suggests that the highlighted Wolf-Rayet star may explode as a supernova within a million years.

Supernovae play a central role in the birth of new stars. They provide a rich source of gas and dust to form stellar nurseries, and their explosions can trigger shockwaves that trigger the birth of new stars. But it all depends on where supernovae occur. A supernova that happens within a dense cloud of gas will have a very different effect than one that occurs in a large void. Understanding these effects is a challenge. It is much easier to observe a supernova while it happens rather than long before or after. But a new radio study of the Triangulum Galaxy explores these interactions.¹

The study is based on data from the Very Large Array (VLA), which looks at distributions of cold neutral hydrogen, and data from the Atacama Large Millimeter/submillimeter Array (ALMA), which looks at cold and warm molecular gas. Taken together, the data allowed the team to create a radio map of a section within Triangulum showing dense and void cloud regions.

The team then compared this map to the locations of three types of objects: red supergiants, Wolf-Rayet stars, and supernova remnants. The remnants, of course, indicate where supernovae have occurred. Red supergiants are dying stars that will become supernovae in the cosmic near future. Wolf-Rayet stars are old stars that cast off their outer layers gradually, though some may trigger shock waves, supernovae, or gamma-ray bursts.

They found that only about a third of red supergiants and supernova remnants are located within higher density regions of molecular hydrogen. The rest are generally found in low-density regions and voids. This suggests that supernova shock waves play a smaller role in triggering star formation than expected. Wolf-Rayet stars are more typically found near dense regions. This makes sense given they are short-lived and have little time to drift away from their origin location. Even within these general patterns, the authors found subtle variations. For example, a Wolf-Rayet star that first appeared to be within a dense region was found to be surrounded by a 10 light-year cavity upon closer examination.

The authors plan to use this study to develop computer simulations of the birth and death of stars within the Andromeda galaxy. They also plan to expand their survey to star-forming regions within other nearby galaxies.

Sarbadhicary, Sumit K., et al. “Where do stars explode in the ISM?–The distribution of dense gas around massive stars and supernova remnants in M33.” arXiv preprint arXiv:2310.17694 (2023). ↩︎

Do the Dark Dance

Sat, 10 Jan 2026 00:00:00 +0000

NASA/GSFC

Illustration the evolution of the Universe from the Big Bang to today.

Time again for a tale of things dark and mysterious. A tale of dark matter. It’s a well-told tale, but this time it involves an interactive dance between dark matter and neutrinos.¹

Dark matter is, of course, the majority of matter in the cosmos according to the standard model and, by definition, cannot interact strongly with light. There has been some debate as to whether dark matter interacts with itself, but so far there isn’t strong evidence to suggest that it does. Neutrinos also don’t interact strongly with light. Technically, neutrinos meet the conditions of dark matter, but neutrino particles zip around so quickly that they are a hot form of dark matter. But observational evidence we have for dark matter suggests that it is cold. So neutrinos aren’t the dark matter we’re looking for.

Given that neither neutrinos nor cold dark matter interact strongly with matter, there hasn’t been much to suggest that they interact with each other. But a new study argues that they do. The authors even suggest that these interactions would help solve the Hubble tension problem.

Zu, et al.

The interaction model straddles the range between conflicting cosmological data sets.

The study looked at an effect known as cosmic shear. It’s a subtle distortion in the way galaxies gravitationally lens distant light. If a galaxy were perfectly spherical, then the lensing of distant objects would be circular. But since galaxies aren’t perfectly circular, the lensed light is distorted. For an individual galaxy this doesn’t mean much, but galaxies within a large structure of galaxies have an intrinsic alignment, and this gives a slight alignment or shear to lensed objects. By making large surveys of gravitationally lensing galaxies, we can measure the cosmic shear and understand the large-scale structure of the Universe.

The key is that if neutrinos and dark matter interact, this would affect the large scale structures of galactic clusters and voids, which would impact our measure of cosmic shear. Using cosmic shear data gathered by the 3-year Dark Energy Survey gathered by the Blanco Telescope in northern Chile, the authors found an interaction level of about 1 part in 10,000. While this suggests there is an interaction between the two, the statistical significance of their result is only 3σ, which isn’t strong enough to be considered proof.

Future cosmic shear surveys, such as ones that can be done with Rubin Observatory data, will soon allow the team to narrow things down. If new observations confirm their results, we will have to re-examine our standard comsological model. But it’s also possible that the data won’t hold up, and this idea will join the multitude of others that give us possibilities but no answers. For now the dark mystery continues.

Zu, Lei, et al. “A solution to the S₈ tension through neutrino–dark matter interactions.” Nature Astronomy (2026): 1-9. ↩︎

Anvil of Worlds

Thu, 08 Jan 2026 00:00:00 +0000

Astrobiology Center, NINS

Illustration of four baby planets in the V1298 Tau system in the process of becoming super-Earths and sub-Neptunes.

The closest planet to the Sun is Mercury. It’s a tiny world, even smaller than Saturn’s moon Titan and Jupiter’s moon Ganymede. That’s unusual for a planetary system. Most star systems have a large world between the size of Earth and Neptune orbiting much closer than Mercury. A new study has figured out how these close-orbiting super-Earths form and clues about why they are so common.¹

When we started discovering exoplanets, it was generally thought that we mostly found large planets close to their star because they are the easiest to find. Most planets are discovered by the transit method, which favors larger planets with many transits in a short time. But as we found thousands of exoplanets, it became clear that it wasn’t observation bias. Hot super-Earths are common in the galaxy.

This led to two main ideas as to their origin. The first is that the planets form farther away from their star but then migrate inward due to gravitational interactions from other protoplanets in the system. Computer simulations of our solar system suggest that gravitational interactions between Jupiter and Saturn caused the planets to shift orbits dramatically early on. In our case it drove the large planets farther away from the Sun. In other systems it could push worlds closer to their star. The other idea is that hot super-Earths form close to their star. This is what the new study suggests, but with an extra twist.

The team looked at a very young star known as V1298 Tau. It’s only about 20 million years old but already has four planets orbiting it closer than Mercury orbits the Sun. Based on their transits, the planets have diameters 5 to 10 times that of Earth, which is the size range of Neptune to Jupiter. We’ve known about these planets for several years, but we’ve only known their sizes, not their masses. We had no idea whether they were dense worlds like Earth and Jupiter or less dense like Saturn.

In this new study, the team was able to determine the masses by looking at small timing shifts in the planetary transits. A single planet orbiting its star would follow a simple Keplerian orbit with transits happening with clockwork precision. But when there are multiple planets, the gravitational tugs between them mean that the orbits shift slightly and the transit times have small fluctuations. By modeling these fluctuations, the team was able to determine the masses of all four worlds. They found the masses ranged from 5 to 15 Earth masses, ranging from super-Earth to sub-Neptune.

With the masses known, the team calculated the density of each planet and found they were all extremely low density. All four of the planets are super-puff worlds with average densities less than packing foam. This means they are planets with large, diffuse atmospheres of hydrogen and helium. Since young stars are notorious for flares and strong stellar winds, those atmospheres will be stripped from the planets over time. The team estimates that by the time the system matures, these planets will be whittled down to masses and densities similar to the hot super-Earths that are so common.

Gas giants form early in a star system. For close-orbiting planets, their star hammers away at them until they become smaller, denser worlds.

Livingston, John H., et al. “A young progenitor for the most common planetary systems in the Galaxy.” Nature 649.8096 (2026): 310-314. ↩︎

Black Holes and Galaxies

Mon, 05 Jan 2026 00:00:00 +0000

NOIRLab/NSF/AURA/J. da Silva

Artist’s impression of an active supermassive black hole in the early universe.

Nearly every galaxy has a supermassive black hole in its core. Whether the black hole forms first and then the galaxy around it—or the other way around—is still a matter of some debate, but we know the evolution of both are deeply connected. We can use that relationship to study the black holes.

When a supermassive black hole is active, we can look at things such as core luminosity and the black hole jets to determine its mass. But when it’s quiet, we can’t do that and have to use indirect means. One of these is the M-sigma relation. By measuring the spectra of stars at the core of a galaxy, we can determine their motion thanks to the Doppler effect. The spectra on one side of the core are blue-shifted because those stars are orbiting toward us and redshifted on the other side because they are moving away from us. This means the core spectrum has a statistical spread, or sigma. The bigger the black hole, the faster the core stars orbit the galactic center, and the bigger the sigma. Hence the M-sigma relation.

While the M-sigma relation is a simple and powerful tool to measure the mass of galactic black holes, it turns out it isn’t always true. A new study has found that it doesn’t work well for the largest supermassive black holes.¹ The team focused on what are known as ultra-massive black holes (UMBHs). Those with masses of more than 10 billion Suns. In comparison, the only two black holes we’ve observed directly, M87* and our own SagA*, have respective masses of 6 billion and 4 million Suns.

de Nicola, et al

The M-sigma relation doesn’t hold well for UMBHs.

The team looked at 16 of the brightest cluster galaxies and compared the data to what is known as the triaxial Schwarzschild model. This is a model simulation where the various orbits of stars around the core are simulated to create a core brightness curve. The core is assumed to be an elliptical spheroid with three different axes, hence the name. It turns out that when you can get the necessary observations, the model gives an excellent measure of black hole mass.

The team was able to do this for 8 of the cluster galaxies. They then plotted them on an M-sigma graph, comparing them to other galaxies with known black holes. They found that these ultra-massive black holes trend higher than the M-sigma relation, meaning that the relation would underestimate the mass of UMBHs. The relation just isn’t a good fit on the high end.

The authors then go on to show how you can use a different relation, known as the central light-deficient region. Since the largest black holes tend to consume more nearby stars, the brightness curves will have a dip in brightness right at the center. The bigger that region, the bigger the black hole mass. So even if you can’t get the triaxial Schwarzschild data, you can still determine the mass of UMBHs.

de Nicola, Stefano, et al. “Eight New Ultramassive Black Hole Masses confirm Best Correlation with Galaxy Core Sizes.” arXiv preprint arXiv:2512.04178 (2025). ↩︎

Immersion

Sat, 03 Jan 2026 00:00:00 +0000

ESO/L. Calçada/M. Kornmesser

Artist’s impression of strontium emerging from a neutron star merger.

It’s quite a challenge to make an Earth-like world. You need enough mass to hold an atmosphere and generate a good magnetic field, but not so much mass that you hang on to light elements such as hydrogen and helium. You also need to be close enough to your star that you remain comfortably warm, but not so warm that all your water gets baked away. And then you need an abundance of short-lived radioisotopes (SLRs).

SLRs are isotopes with half-lives of less than 5 million years. That’s a blink of an eye at cosmic timescales, which means their decay helps warm up the early solar system. The idea is that a warmer early solar system had enough heat to prevent terrestrial planets such as Earth from holding too much water. Without SLRs most Earth-sized planets would become Hycean worlds. We know the solar system was rich in SLRs because of the isotopes we find in meteorites. For example, the SLR aluminum-26 decays into magnesium-26. So if you find an excess of magnesium in a meteor fragment, you know radioactive aluminum was around long ago. The same is true for other radioisotopes such as titanium-44.

Sawada, et al

How an immersion of cosmic rays could enrich a young star system.

The only problem with this idea is that short-lived radioisotopes are formed in supernovae, and nearby supernovae would tend to rip apart the protoplanetary disk of a young star. Somehow the Sun’s early disk survived intact, and if that is a rare thing, it would mean that Earth-like planets are rare. But a new study suggests they could actually be common.¹

The authors suggest that rather than being blasted by a nearby supernova shockwave, our early solar system was bathed in cosmic rays from a more distant supernova. According to their model, if at least one supernova occurred within a parsec of us, it would bathe the solar system with enough cosmic rays to create the level of radioactive isotopes necessary to match those of meteorites. Since sun-like stars form within star clusters, the odds of experiencing such a supernova are pretty good. This means terrestrial planets such as Earth should be fairly common.

We know that supernovae can enrich the galaxy with radioactive aluminum. In fact, the level of aluminum-26 in our galaxy gives us a good estimation of the average rate of supernovae in the Milky Way. So this model is certainly plausible.

Sawada, Ryo, et al. “Cosmic-ray bath in a past supernova gives birth to Earth-like planets.” Science Advances 11.50 (2025): eadx7892. ↩︎

Spot the Flares

Fri, 02 Jan 2026 00:00:00 +0000

NASA/SDO/Goddard Space Flight Center

A giant sunspot seen at the edge of the Sun.

The Sun is not only our closest stellar neighbor, it’s also the star we understand the most. As we’ve observed it over the centuries, we’ve learned that the Sun is not an immortal constant. It goes through active and quiet cycles, it has become warmer over geologic time scales, and it occasionally batters the Earth with solar flares. We’ve generally thought that other main sequence stars behave in much the same way, but when it comes to solar flares, that isn’t always true.

There are many similarities. For example, just as the Sun has sunspots, other stars have starspots. The number of sunspots varies along an 11-year cycle, and this correlates with the amount of magnetic activity and solar flares. Observing starspots is difficult, but we have been able to do it for about 400 stars. Through this, we know that other stars also have a cycle of activity, though the period varies from 3 to 20 years depending on the star. By looking at the spectral lines of these stars, we also know that magnetic activity generally follows the same cycle.

Since a star’s magnetic field is what drives solar flares, you would think that stellar flares would follow the same cycle. So we should see more flares where there are lots of starspots and fewer in a region without starspots. This is precisely what we see with the Sun. Sunspots and solar flares appear in similar locations around the same time. But a new study shows that isn’t the case.¹

We can’t observe starspots on thousands of stars, so the team used an indirect measure. Using data from the Transiting Exoplanet Survey Satellite (TESS), they looked at how the brightness of stars varied. The idea is that when there are lots of starspots, a star is slightly dimmer when the spots face us than when they are on the far side of the star. As the star rotates, the fluctuation of brightness follows that cycle. They also looked for short spikes of brightness that would indicate a stellar flare. Since we can only see flares on the side of the star facing us, the team could correlate the two.

Based on observations of more than 14,000 stars, the authors observed more than 200,000 stellar flares. They then looked at the correlation. For the Sun, this would strongly correlate. So when we see a solar flare, it’s almost certain that there are also sunspots facing us. But for the stars, the team found this was only true half the time. In other words, when we see a stellar flare, the chance that there are sunspots in the area is essentially random chance. The two aren’t correlated at all.

It turns out the Sun is unusual. For most stars, the exact mechanism driving flares and spots is different. Why sunspots and solar flares correlate so strongly is something we don’t yet understand.

Zhang, Andy B., et al. “Starspots and Flares are Generally Not Correlated.” arXiv preprint arXiv:2512.01051 (2025). ↩︎

Do You Have the Time?

Fri, 19 Dec 2025 00:00:00 +0000

NASA GISS

The Mars24 software displays a Mars sunclock, a graphical representation of the planet Mars showing its current standard Mars time.

Do you know what time it is? It’s an easy question, right? Just look at your phone or watch. But is that really the exact time? Oh, well, for that you can look to Coordinated Universal Time, or UTC. It’s what your phone clock is synced to, give or take, but you can get a more accurate measure of UTC with a device that can pick up the UTC radio time signal. Of course, UTC is only an agreed-upon standard that tries to stay in sync with Earth’s rotation. It, in turn, is based upon International Atomic Time (TAI), which is a weighted average of 450 atomic clocks located all over the world.

Whew!

Knowing the time accurately is hard. But at least it’s based on Earth, where we live. But in the future some of you could be living on Mars. So what time would it be then?

In practical terms, the answer is pretty simple. Just as we sync our clocks to solar noon, so can people on Mars. A Martian “sol” is about 37 minutes and 23 seconds longer than an Earth day, so we just can make Martian hours a bit longer so that a sol is 24 hours. We could even use the Darian calendar system, which defines a Martian year (1.88 Earth years) in terms of 24 months, each with 27 or 28 sols.¹

That system would be fine if you no longer wanted anything to do with Earth but isn’t great for interplanetary communication. For Martian lander missions, we usually use what is known as Airy Mean Time (AMT), which is similar to Greenwich Mean Time (GMT). It defines a prime meridian on Mars by the location of a small crater known as Airy-0. But the exact location of the meridian isn’t known, so missions keep track of things based on Earth time and count days since mission touchdown. This is why you’ll often hear about a rover doing a task on Sol 25, or the 25th day of its mission.

Ideally we would define a Coordinated Mars Time (MTC) and set up atomic clocks all over Mars to create a Mars Atomic Time. But as a recent study shows, that isn’t as simple as it seems.²

Ashby and Patla

The calculated rate clock offset between Mars and Earth.

To begin with, the rate of time is affected by gravity. The stronger your gravitational acceleration, the slower your clocks. Since the surface gravity of Earth is about 3 times that of Mars, an atomic clock on Mars would tick faster by about 477 microseconds a day. And clocks are also affected by relative motion. Since the speed of Mars relative to Earth varies depending on where Earth and Mars are in their orbits, atomic clocks on Mars would vary by 226 microseconds. And then there is the travel time of light between the two worlds, which varies based on the distance between Earth and Mars.

In this latest study, the authors looked at the challenges of keeping Earth and Mars clocks in sync and what measurements would need to be included to make them accurate. They found that, given our current technology, a time system could be synchronized with reasonable accuracy, but the challenges of an Earth-Mars system are even greater than those of an Earth-Moon system. They found that given current uncertainties with the Earth-Moon system, Mars clocks would still have a daily residual of 100 nanoseconds, give or take.

So we can’t make an exact clock system that is perfectly synchronized between Earth, the Moon, and Mars. But given how complex the relativistic nature of “now” really is, it’s safe to say that we can sync them close enough.

Gangale, T. E. “Martian standard time.” Journal of the British Interplanetary Society 39.6 (1986): 282-288. ↩︎
Ashby, Neil, and Bijunath R. Patla. “A comparative study of time on Mars with lunar and terrestrial clocks.” The Astronomical Journal 171.1 (2025): 2. ↩︎

You've Got the Look

Tue, 09 Dec 2025 00:00:00 +0000

NASA/JPL-Caltech

The typical artistic view of a Jupiter-like world (left) compared to a look based on new research (right).

Jupiter is the largest planet in the solar system. It’s also one of the largest planets in the Universe. There are planets out there with much more mass, but thanks to gravity, they are generally more dense, not “bigger.” This raises an interesting question about massive exoplanets. Do they look similar to Jupiter? A new study finds they probably don’t.¹

Before getting into the details, let’s talk about the difference between a planet, a brown dwarf, and a star. Very broadly, a planet is massive enough to compress into a sphere under hydrostatic equilibrium but not massive enough to trigger any kind of nuclear fusion in its core. Stars are massive enough to trigger the fusion of hydrogen. Brown dwarfs lie in the middle ground. They are too small to trigger hydrogen fusion like a true star but large enough to have a bit of deuterium fusion. In terms of mass, anything up to about 10 Jupiters is a planet, anything over 90 Jupiters is a star, and brown dwarfs are the middle child.

ESO VHS

A direct image of the exoplanet VHS 1256b and its star.

We’ve long known that the most massive brown dwarfs look very star-like. They can have surface temperatures of nearly 3000 K. If we were to see one up close, it would look like a deep red dwarf star. But what about the smallest brown dwarfs? With a mass of around 10 Jupiters, it would have a diameter slightly smaller than our largest planetary sibling and a surface temperature of a few hundred Kelvin. That’s a bit warmer than Jupiter’s 170 K, but not warm enough to glow. So generally we’ve imagined these “Super-Jupiters” to look very Jupiter-like. Most of the artistic representations show them as a gas planet with banded clouds similar to Jupiter or Saturn.

To test this idea, the new study looks at an exoplanet known as VHS 1256b. It has a mass of around 20 Jupiters and is one of the few exoplanets we can directly image. Images of the world from JWST show it is a reddish planet with a surface temperature of around 1300 K. It glows faintly in the deep red, so even with banded clouds, it would look like an alien world. But thanks to observations of its spectra, the team found evidence of large and dusty storms in its atmosphere. This causes the exoplanet to vary in brightness, similar to the fluctuations we see in tiny stars.

Based on this data, the team modeled the atmosphere of VHS 1256b and planets more similar to Jupiter. The banded cloud pattern we observe on Jupiter is caused by large winds moving parallel to its equator. Some winds stream eastward while others stream westward, creating the different cloud regions. This structure is reinforced by heat exchange between layers. But super-Jupiters are warmer, driving more energy into their atmospheres. The team found that the atmospheres of super-Jupiters react to the heat more strongly, creating turbulent regions that would break apart banded structures. In other words, many super-Jupiters wouldn’t look like their smaller cousin but would have a more chaotic surface.

Super-Jupiters have a look all their own.

Tan, Xianyu, et al. “Large-amplitude variability driven by giant dust storms on a planetary-mass companion.” Science Advances 11.48 (2025): eadv3324. ↩︎

On the Move

Wed, 03 Dec 2025 00:00:00 +0000

Mike Peel; Jodrell Bank Centre for Astrophysics, University of Manchester

The Lovell Radio Telescope at Jodrell Bank Observatory, near Goostrey, Cheshire, England.

If you ever feel like you are constantly on the move, that’s because you are. And not only in your daily life. You spin around the world once a day, the Earth dances with the Moon around the Sun, and the Sun and everything else in the solar system bob around the Milky Way. Even our galaxy moves through the cosmos, and it might be moving faster than we thought.¹

The thing about motion is that it’s all relative. We can’t feel a constant motion in a straight line. So when we talk about motion, we always have to specify what the motion is relative to. For the Milky Way, we can measure our motion relative to nearby galaxies such as Andromeda, but on a cosmic scale we can measure our motion relative to the cosmic microwave background.

Wilkinson Microwave Anisotropy Probe

The dipole in the raw cosmic background.

You’ve probably seen images of the cosmic microwave background from Planck or WMAP with their little fluctuations of color showing local variations in temperature. But that is a processed image that has corrected for our motion relative to the CMB. The raw data shows a clear dipole effect where the CMB in one direction appears hotter in one direction and cooler in the opposite direction. This is due to our relative motion to the CMB. Based on the data, we are moving through the cosmos at about 360 km/s, which is surprisingly fast.

Böhme, et al

The galaxy results vs. the CMB (green lines).

Here’s where it gets interesting. We should get about the same result if we take a statistical average of our speeds relative to the most distant galaxies. Cosmic expansion due to dark energy would average out to zero, which would leave an overall net motion. According to the standard model of cosmology, there might be a small difference between the two, but our relative motion should be about the same.

To verify this, the authors of this new work looked at sky survey data of radio galaxies. These are galaxies that are bright at radio frequencies, so we can measure their relative motion pretty easily. Many radio galaxies are so bright that we can see across billions of light-years, so there shouldn’t be a local bias. But their result skewed the dipole even further. Based on the radio galaxy observations, the Milky Way is moving through the Universe even faster than the CMB result.

The two results don’t entirely disagree. Both the radio galaxy and CMB measure have us moving in about the same direction, but the radio galaxy result is statistically faster by 5.4⁢𝜎, which contradicts our expectation. This is similar to the Hubble tension issue with dark energy, where different results contradict. Further observations may resolve the issue, but this might another way in which our current standard model isn’t quite right. Clearly, our understanding of the cosmos is also on the move.

Böhme, Lukas, et al. “Overdispersed radio source counts and excess radio dipole detection.” Physical Review Letters 135.20 (2025): 201001. ↩︎

Gordian Knot

Tue, 02 Dec 2025 00:00:00 +0000

Volker Springel/Max Planck Institute For Astrophysics/SPL

Simulation showing the web of galaxies and voids in the cosmos.

Suppose you slammed together two neutrons at near-luminous speed. The resulting collision would create a cascade of particles from protons, electrons, and neutrinos to more exotic fare. We can’t predict the exact number or type of particles produced, but we do know one thing: the total charge of all the particles would be zero. This is because charge is a conserved quantity, and since the neutrons have zero total charge, their resulting particles must have the same.

Flip Tanedo

Examples of symmetry in physics.

Electric charge (C) is just one of the inherent symmetries that governs particle physics. The others are parity (P), or handedness, and time (T). These symmetries can be combined to create CP symmetry, PT symmetry, or CPT symmetry. One consequence of this is that, when taken together, these symmetries require a symmetry between matter and antimatter. With enough energy you can create a negatively charged electron, but only if you also create a positively charged positron. Symmetry is a powerful tool in physics, but it creates a deep mystery.

The standard cosmological model says that matter was created out of the dense energy of the Big Bang. According to symmetry, matter and antimatter appeared in equal amounts, but when we look at the Universe it is made of matter. There aren’t antimatter galaxies or clouds of anti-hydrogen streaming through the cosmos. Just the regular matter that makes up you and me. This mystery is known as the matter asymmetry problem.

One possible solution to this problem is to introduce an asymmetry to particle decay. It turns out that the only really crucial symmetry for cosmology is CPT symmetry. If CPT is violated, then so is Lorentz invariance, which means special and general relativity are violated, and chaos reigns.¹ But other symmetries might be violated on rare occasions. We know, for example, that the weak force can violate CP symmetry. If PT can also be violated, then charge wouldn’t be perfectly conserved. But nothing in the standard model of particle physics seems to allow for PT violation.

Eto, et al

3D plots of the numerical solution for a knot soliton.

So a new work looks at an extended model of particle physics and how extended symmetries might create matter asymmetry.² The authors look at two specific symmetries. The first is the Peccei-Quinn (PQ) symmetry, which is based on hypothetical particles known as axions. Axions are also often proposed as the particles of dark matter. The second is Baryon Number Minus Lepton Number (B-L) symmetry. Protons and neutrons are baryons; electrons and neutrinos are leptons. In standard physics, baryon and lepton numbers are each conserved, but some models propose the existence of heavy “sterile” neutrinos that would decay into lighter particles.

Rather than looking at specific extended models, the team focuses on symmetries and their consequences. They found that PQ and B-L symmetries allow for the formation of soliton knots within energy fields. These knots could act as a kind of pseudo-particle that would trigger an asymmetrical decay into more matter particles than antimatter ones. In other words, these knots may have formed during the earliest moments of the Big Bang, thus allowing matter to form much more readily than antimatter. The team also found that the presence of these knots in the early Universe would have left a gravitational fingerprint. In principle, a gravitational wave signal could verify the existence of these knots, though this is far beyond the current ability of gravitational wave astronomy.

While the study isn’t conclusive, it’s an interesting approach to the problem of matter asymmetry. Right now it’s the Gordian Knot of cosmology, so perhaps it will take a new kind of knot to resolve it.

Greenberg, Oscar W. “CPT violation implies violation of Lorentz invariance.” Physical Review Letters 89.23 (2002): 231602. ↩︎
Eto, Minoru, Yu Hamada, and Muneto Nitta. “Tying knots in particle physics.” Physical review letters 135.9 (2025): 091603. ↩︎

Getting Warmer

Mon, 01 Dec 2025 00:00:00 +0000

ESA/Hubble & NASA, ESO/ Lutz Wisotzki et al

The early Universe as seen by the MUSE spectrograph on ESO’s Very Large Telescope.

So first the Big Bang happens. Everything is incredibly hot and dense; there are photons flying everywhere, but they keep colliding with electrons and ionized nuclei. Then, finally after about 380,000 years the cosmos is becomes cool enough for atoms to stabilize. The Universe becomes optically transparent, and all those photons are able to roam free for billions of years, allowing us to see them as the cosmic microwave background.

And then what? Darkness…

For roughly a billion years the Universe didn’t really produce new light. Cosmic hydrogen and helium continued to cool, and their vast clouds hadn’t yet collapsed to form the first stars. The Universe remained dark until the rise of the first stars, which were bright enough to ionize hydrogen again.

The period between recombination and reionization is known as the cosmic dark ages. While we know the period exists, we don’t have a great deal of observational evidence for it. There were no bright stars, no clouds of cooling plasma, nothing to emit the kind of light we see in the Universe today. But there was one kind of light around back then, and it’s known as the 21 cm line.

Wikipedia

Hydrogen emits light through the interaction of proton and electron spin.

Most of the light we see every day is caused when atoms in an excited state emit light to drop to a lower energy state. If the electrons in an atom are all in their lowest energy states, then they can’t emit light. During the cosmic dark age, the neutral hydrogen and helium had cooled to their ground states, so their electrons couldn’t emit any light. But it turns out that neutral hydrogen can emit a very faint radio signal thanks to a spin interaction between its proton and electron. When the electron and proton have the same spin orientation, there is a teeny amount of energy that can be released. The electron can flip its spin and release that energy. The wavelength of the light released is about 21 centimeters, hence the name.

Hydrogen is by far the most abundant element in the Universe, so the 21 cm line is a great way to map the distribution of matter. And since the emitted light has a very specific wavelength, we can use things such as Doppler shift to look at how hydrogen is moving. This is how we first discovered that galactic rotation pointed to the existence of dark matter.

To study the cosmic dark ages, astronomers focus on the 21cm line during the Epoch of Reionization (EoR). It’s the period just as the first stars and galaxies were starting to form. The challenge for observing this period is that the 21cm line is not only faint but also highly redshifted. It’s only recently that we’ve had the technology to observe this period well.¹ Now a couple of new studies have found the late period of the cosmic dark age was dark, but not cold.²

The team used data from the Murchison Widefield Array telescope in Western Australia. To pull the cosmological signal out of the background radio noise, they combined a decade’s worth of data to determine the hydrogen line power spectrum during the epoch of reionization. From this they found that the hydrogen of the Universe started warming up about 800 million years after the Big Bang. It was warm before the first stars ignited.

This result is interesting because it raises the question of what could have been heating it. One idea is that the warming was caused by x-rays produced by early black holes. Regardless of the source, the results rule out the “cold start” model for reionization. Even during the dark period, the cosmos was actively laying the groundwork for the stars and galaxies we see today.

Nunhokee, C. D., et al. “Limits on the 21 cm power spectrum at z= 6.5-7.0 from MWA observations.” Astrophysical Journal, vol. 989.1 (2025): 57. ↩︎
Trott, Cathryn M., et al. “Improved Limits on the 21 cm Signal at z= 6.5–7.0 with the Murchison Widefield Array Using Gaussian Information.” The Astrophysical Journal 991.2 (2025): 211. ↩︎

In the Area Again

Wed, 19 Nov 2025 00:00:00 +0000

Pixabay - Public Domain

An illustration of Hawking radiation near a black hole.

Hawking radiation has never been proved, but it’s generally thought to be real. Essentially, the argument is that when you combine black hole event horizons with quantum fuzziness, thermal energy can escape a black hole. We don’t have a fully quantum theory of gravity, but we do have several semi-classical models that support the existence of Hawking radiation. And if Hawking radiation is true, then the interaction of black holes is governed by the laws of thermodynamics.

The argument is pretty simple. If black holes quantum radiate, then they emit heat energy and have a thermal temperature. So the laws of thermodynamics apply to black holes just as they do to a really hot cup of tea. This idea is incredibly powerful because it means the complex interactions of merging black holes can be described as black hole thermodynamics.

In simple terms, there are four basic laws of thermodynamics:

Zeroth: If objects A-C and B-C are in thermal equilibrium, then so is A-B.

First: Energy is conserved, so thermal energy lost by an object is gained by others.

Second: Heat energy naturally flows from hotter objects to colder objects, not the other way around. This is often stated as “the entropy of a system can never decrease.”

Third: You can never cool an object to absolute zero.

For black holes, these laws become:

Zeroth: A non-rotating black hole has uniform gravity at its event horizon.

First: The temperature—and therefore entropy—of a black hole is determined by the surface area of its event horizon.

Second: Since entropy can never decrease, the surface area of a merged black hole must be no less than the surface area of the two original black holes.

Third: You can’t remove the event horizon of a black hole by spinning it up or giving it an electric charge.

Tang, et al

The Area Theorem is supported by black hole mergers.

The second law is also known as the Area Theorem for black holes, and in many ways it’s the most powerful. Black holes can emit several stars’ worth of gravitational wave energy when they merge. It’s the reason we can detect them. But the Area Theorem says there is a hard limit to the amount of gravitational energy mergers can produce. The final black hole must have at least the same surface area as the originals. And since the surface area of a black hole’s event horizon depends upon its mass, that means the mass of a merger is likewise constrained.

The good news is that the Area Theorem is something we can test by looking at black hole mergers. Back in 2021, a study of the black hole merger GW150914 supported the Area Theorem with a 97% confidence level.¹ Now a new study of the merger GW230814 confirms the theorem to a 99.5% confidence level.² Both of these results are less than the usual 5σ level used for scientific certainty, which would be a 99.99994% certainty, but the results are still quite strong.

Black hole thermodynamics seems to be true, which is an astonishing idea when you think about it. It means when you have a hot cuppa on a cold morning, the same physics that warms your hands steers the dynamics of black hole event horizons.

Isi, Maximiliano, et al. “Testing the black-hole area law with GW150914.” Physical Review Letters 127.1 (2021): 011103. ↩︎
Tang, Shao-Peng, et al. “Verification of the Black Hole Area Law with GW230814.” arXiv preprint arXiv:2509.03480 (2025). ↩︎

Message for You, Sir!

Sun, 16 Nov 2025 00:00:00 +0000

ESO/L. Calçada

An animation showing two white dwarf stars merging and creating a Type Ia supernova.

So I got an email from Adam Reiss. You know, the guy who was awarded the 2011 Nobel Prize in Physics along with Saul Perlmutter and Brian Schmidt for discovering the rate of cosmic expansion is accelerating. He pointed out a few issues with the decelerating Universe paper, and with his permission I’d like to share them with you.

So let’s start with the central claims of the original paper.¹ Based on observations of about 300 supernovae, the authors found a correlation between the peak brightness of Type-Ia supernovae and the age of its host galaxy. Basically, the younger the galaxy, the dimmer the supernova. As a result, the authors argue, our measure of galactic distances is wrong. Based on their results, the Universe is decelerating, which would also mean the standard ΛCDM is wrong. Although the paper is peer reviewed, Reiss finds a couple of major flaws.

The first is on the issue of galactic ages. The authors emphasize that SN-Ia light curves don’t take the age of their host galaxies into account. That’s somewhat true, but they do take galactic mass into account. Determining the age of a galaxy is difficult to do. It’s also model dependent, so the results can be a bit tweaked. Galactic mass, on the other hand, is much simpler to measure.

Studies have shown that the mass of a supernova’s host galaxy should be considered.² This is why modern catalogs such as Pantheon+ adjust for mass. The reason they don’t worry about galactic age is because the age of a galaxy and its mass correlate pretty strongly.³ Once you adjust for mass, adjusting for age buys you nothing.

Since around 2010, Type-Ia supernova catalogs all include the mass adjustment, which also serves as an age proxy. Since the authors wanted to focus on age directly, they used older databases without the mass adjustment. That’s a bit of a red flag. If you want to disprove the current theory, don’t use old data. But this leads to the second issue, which is the connection between galaxy age and progenitor age.

The authors focus on the measured age of the host galaxies, since that’s something that can be measured. They don’t focus on the age of a supernova’s progenitor star because we don’t have a good way to measure that. In the paper, the team uses galaxy age as a proxy for progenitor age, assuming that the progenitor formed when the galaxy formed. Thus, distant supernovae progenitors are young, while the progenitors of nearby supernovae are old. But local supernovae are typically found in young star-forming regions. In fact, studies suggest that Type-Ia supernovae occur less than a billion years after the formation of their progenitor star.⁴ So that very basis of their argument is shaky at best.

Of course, don’t take my word for it. Just wait for the peer-reviewed papers that will look at all these issues and more. I don’t think we’ll have to wait long.

With thanks to Professor Reiss for his kind feedback.

Son, Junhyuk, et al. “Strong progenitor age bias in supernova cosmology–II. Alignment with DESI BAO and signs of a non-accelerating universe.” Monthly Notices of the Royal Astronomical Society 544.1 (2025): 975-987. ↩︎
Brout, Dillon, and Daniel Scolnic. “It’s dust: solving the mysteries of the intrinsic scatter and host-galaxy dependence of standardized type Ia supernova brightnesses.” The Astrophysical Journal 909.1 (2021): 26. ↩︎
Rose, B. M., et al. “Host Galaxy Mass Combined with Local Stellar Age Improve Type Ia Supernovae Distances.” The Astrophysical Journal 909.1 (2021): 28. ↩︎
Mannucci, F., et al. “The supernova rate in local galaxy clusters.” Monthly Notices of the Royal Astronomical Society 383.3 (2008): 1121-1130. ↩︎

The Fulness of Time

Fri, 14 Nov 2025 00:00:00 +0000

Robin Dienel / Carnegie Institution for Science

Illustration showing different objects that can be used to measure cosmic expansion.

So if the standard model of cosmology is wrong, what alternative is there?

For the past couple of posts I’ve been talking about a new study that suggests we’ve been measuring supernova distances a bit wrong. While these new results could solve the long-standing Hubble tension problem, it does so by overturning the ΛCDM model.¹ Specifically, it would mean that dark energy cannot be due to a cosmological constant that is inherent to the structure of spacetime. What would this mean?

To begin with, much of what we know about the Universe would still hold up. The Big Bang, cosmic expansion, and general relativity are all still valid. The only thing that changes is that the Λ in ΛCDM isn’t constant. But while the ΛCDM model has stood the test of time, it’s also the simplest cosmological model. Plenty of other models have been proposed, and it’s worth looking at a few of them.

One approach is to treat dark energy as a scalar field.² Known as quintessence, or the “fifth force,” the model proposes that expansion is due to a kind of potential energy. Just as the Big Bang created protons, electrons, and neutrinos, maybe it also created a potential field. The simplest version of quintessence allows for a uniform energy density that is greater or less than the cosmological constant, which is fixed at a value of w_q = -1. What this means is that you could tweak the model to fit what we observe.

For example, the amount of matter and dark matter we observe in the cosmos isn’t nearly enough to slow down the rate of cosmic expansion. As the Universe expands, the mass density of the Universe decreases, meaning that eventually dark energy dominates cosmic evolution. If you tweak dark energy to be weaker, then there could be enough matter and dark matter to slow down cosmic expansion. The authors of the supernova paper looked at this kind of idea, referring to it as the Flat wCDM model. They found that while it fits the data better than the standard model in some ways, overall it isn’t a particularly good match.

Another approach is what’s known as a variable dark energy model. The approach here is to describe dark energy as an equation of state rather than a specific physical phenomenon. This allows the density of dark energy to vary both in space and time. The most popular approach is known as the Chevallier–Polarski–Linder model (CPL),³ and it gained popularity when we could use the scale of galactic clustering at different cosmological distances to test whether dark energy changed over time.

Son, et al

A comparison of the standard model with a variable dark energy model.

The supernova authors also look at this approach, specifically the Flat w₀w_aCDM model, which is the simplest CPL model. In it, w₀ represents an initial dark energy density, similar to Λ, which w_a is a scale factor that can vary in time. In this way you can have dark energy start out strong in the early Universe, but weaken over time. The team found that this model fits their data quite well. The correlation is even stronger when their data is combined with Baryon Accousing Oscillation (BAO) and Cosmic Microwave Background (CMB) data. Based on their study alone, this seems to be the best model. Of course, as noted before, their data set is small, with only about 300 supernovae.

When Rubin Observatory starts gathering supernova data in the near future, we’ll be able to determine whether this new study holds up. Perhaps the Flat w₀w_aCDM model will become the new standard model. Or maybe we’ll find even stranger results and we’ll have to look to things such as modified gravity or some interacting process that combines both dark energy and dark matter. Whatever we find, we will have a better understanding of the cosmos in the fulness of time.

Son, Junhyuk, et al. “Strong progenitor age bias in supernova cosmology–II. Alignment with DESI BAO and signs of a non-accelerating universe.” Monthly Notices of the Royal Astronomical Society 544.1 (2025): 975-987. ↩︎
Ratra, Bharat, and Philip JE Peebles. “Cosmological consequences of a rolling homogeneous scalar field.” Physical Review D 37.12 (1988): 3406. ↩︎
Linder, Eric V. “Exploring the expansion history of the universe.” Physical review letters 90.9 (2003): 091301. ↩︎

Far from the Madding Crowd

Wed, 12 Nov 2025 00:00:00 +0000

Gabriela Secara, Perimeter Institute CC-BY-4.0

An illustration showing how Baryon Acoustic Oscillations (BAOs) expand with the Universe.

Last time I wrote about new data that overturns the standard cosmological model. Before anyone starts dusting off their fringe cosmological models, we should note what this new study doesn’t overturn. It doesn’t say the Big Bang model is wrong, nor does it say that the Universe isn’t expanding or that Hubble’s redshift-distance relation needs to be thrown out. It really only says that our Hubble constant model is wrong.¹ But we already knew that thanks to a little thing known as the Hubble tension. These new results could solve that mystery as well.

Before we dive into the Hubble tension, let’s talk about the Hubble constant and the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. In 1929, thanks to the work of Henrietta Leavitt and others, Edwin Hubble was able to show that—beyond the local group—the more distant a galaxy is the greater its redshift. He found the relation between distance and redshift was linear, leading him to propose a cosmological constant, now known as the Hubble constant.²

In 1917 Einstein had added a cosmological constant to general relativity to balance out the gravity of galaxies. Like most astronomers at the time, Einstein assumed the Universe was in a steady state. Without the constant, a steady state wasn’t possible. With Hubble’s discovery, Einstein tossed the idea, but Alexander Friedmann and Georges Lemaître discovered independently that solutions to the Einstein’s equations with a cosmological constant could describe an expanding universe that begins with a Big Bang. In 1935, Howard Robertson and Arthur Walker proved that the FLRW metric is the only solution to GR that describes a uniform expanding universe.³ This is the metric used in the standard model. Since the FLRW metric uses Λ as the symbol for the cosmological constant, it’s the ΛCDM model.

NASA and A. Feild (STScI)

Graph showing how the fate of the Universe depends on dark energy, dark matter, and matter.

The Hubble constant H₀ and the cosmological constant Λ are related, but they aren’t exactly the same. The rate of cosmic expansion depends on several things: the cosmological constant (dark energy), the amount of dark matter and regular matter in the cosmos, and the distribution of that matter. In simple terms, matter tries to pull everything together, while dark energy tries to push everything apart, and the balance between the two gives the rate of cosmic expansion, or Hubble constant. Naturally, since the early Universe was denser than the current Universe, you’d expect the rate of cosmic expansion to increase a bit over time. This is why the discovery of an accelerating cosmic expansion was such a big deal. It proved the existence of dark energy and the cosmological constant.⁴ This is also why the Hubble constant is often called the Hubble parameter these days.

Wendy Freedman

Measured Hubble values don’t agree.

For decades observational evidence supported the ΛCDM model. But in the past decade or so our measurements of the Hubble parameter became problematic. There are several ways to find the Hubble parameter, but the big three are distant supernovae, the cosmic microwave background (CMB), and a pattern in clustering galaxies known as Baryon Accoustic Oscillation (BAO). The supernovae observations give us an expansion rate of about H₀ = 71 – 75 (km/s)/Mpc, while the scale of fluctuations in the CMB give a value of H₀ = 67 – 68 (km/s)/Mpc. The BAO measure gives a result of H₀ = 66 – 69 (km/s)/Mpc. This is what we call the Hubble tension. These results should agree, but they absolutely don’t.

Now you might think this means the supernova measurements are wrong, but things aren’t so clear. All three of these methods rely upon assumptions about models and evidence hierarchies. Early on, astronomers figured better data would bring the values together, but they only got worse. Even other methods using things such as gravitational lensing or astronomical masers contradict each other. Which is why this new study is so interesting.

Son, et al

Old methods disagreed, but this new result brings things together.

The work doesn’t make a full survey of how their results would change various Hubble measurements, but it does look at the big three. When the age of host galaxies is taken into account, the supernova measure shifts much closer to the other two. The team even did an initial test of their results using host galaxies of about the same age regardless of their redshift, and the results are slightly better. Accounting for galactic age in supernova data appears to solve much of the Hubble tension.

The authors point out that their results are still somewhat tentative. There are only about 300 distant galaxies that have both an observed supernova and a spectrum from which you can determine the age of the host galaxy. That’s a small sample size, so while the results are compelling, they aren’t conclusive. The good news is that when Rubin Observatory comes online later this year we’ll be able to determine the ages of thousands of distant galaxies. Within a few years we’ll know whether this new model holds up. If it does, then we’ll have to toss the cosmological constant as the single source of dark energy.

So what then? If ΛCDM is wrong, what alternative is there? I’ll talk about that next time.

Son, Junhyuk, et al. “Strong progenitor age bias in supernova cosmology–II. Alignment with DESI BAO and signs of a non-accelerating universe.” Monthly Notices of the Royal Astronomical Society 544.1 (2025): 975-987. ↩︎
Hubble, Edwin. “A relation between distance and radial velocity among extra-galactic nebulae.” Proceedings of the national academy of sciences 15.3 (1929): 168-173. ↩︎
Robertson, Howard Percy. “Kinematics and world-structure.” Astrophysical Journal, vol. 82, p. 284 82 (1935): 284. ↩︎
Peebles, P. James E., and Bharat Ratra. “The cosmological constant and dark energy.” Reviews of modern physics 75.2 (2003): 559. ↩︎