New research reveals how an unusual celestial object resembling a cosmic “bullseye” could offer clues to one of the most perplexing phenomena in our universe.

The research, undertaken by physicists Pierre Sikivie and Yuxin Zhao at the University of Florida, focuses on an astronomical feature quite unlike any other—the galaxy LEDA 1313424, whose concentric rings have garnered it the apt nickname of the “Bullseye galaxy.”

Based on Sikivie and Zhao’s recent research, detailed in a new paper that appeared in The Astrophysical Journal, quantum behavior related to long-mysterious dark matter may hold the key to the galaxy’s unusual appearance.

Unraveling a Bullseye Galaxy’s Circular Secrets

In the past, astronomers have attempted to explain LEDA 1313424’s unusual concentric appearance as the likely result of an ancient collision between two galaxies.

Not so, according to Sikivie and Zhao, who now take a vastly different approach by arguing that a better explanation may involve a Bose-Einstein condensate of axions, a variety of extremely light hypothetical particles.

This concept has much deeper roots that go back close to a century ago, when the first galaxy featuring an unusual concentric ring was discovered. This example, dubbed the Cartwheel galaxy, was only the first in a succession of similar astronomical discoveries involving galaxies with “ringed” appearances that incorporate as many as three annular features.

Unraveling an Annular Enigma

Explaining why these galaxies look this way has remained elusive. One theory proposed by astronomers, as noted earlier, involves impacts between two galaxies, which could create waves of density whose ripples become evident in ring-like structures surrounding the resulting galaxies.

However, a discovery in 2025 by Imad Pasha, a Yale University astronomer, and his colleagues presented an even stranger development in this ongoing concentric saga: the discovery of a galaxy that possesses not three rings, but nine of them. With its discovery, this remarkable celestial object’s unusual appearance garnered the nickname now familiar to us: “The Bullseye galaxy.”

One theory that astronomers believed may account for the galaxy’s nine concentric structures had been a hypothetical impact—and a very energetic one—that occurred about 56 million years ago. However, while investigating this possibility, Sikivie and Zhao say a problem emerged: this would mean that the outermost ring in the galaxy would have had to travel at a speed of 758 miles per second as it blasted outward from the galaxy’s center.

This speed, they say, is impossible to reconcile with what astronomers can already discern about the nature of the supposed collusion, as well as the well-characterized behavior of galactic material observed in other regions of the cosmos.

An Alternative Approach

In an effort to get to the bottom of the mystery behind this unique galaxy, Sikivie and Zhao have put forward an alternative idea: that the rings in the Bullseye galaxy might instead be a byproduct of dark matter’s mysterious properties.

According to astronomers, dark matter is believed to be a nonluminous material that populates our universe, contributing around 85% of its total mass. Despite its ubiquity, direct detections that could confirm its existence have remained elusive.

However, there are some intriguing cosmic phenomena that astronomers are aware of that are very good candidates for being indirect evidence of this mysterious material. These observations include gravitational motion that occurs in galaxies, whereby the accumulation of dark matter is believed to manifest in the appearance of spherical halos encircling them.

In the past, Sikivie has explored this possibility, and whether current prevailing theories about dark matter, such as the idea that hypothetical particles known as axions, may offer the best interpretation of the physical reality behind dark matter.

If dark matter is indeed related to axions, it would also be required to obey what astrophysicists call conservation of angular momentum. According to Sikivie, when axions and galaxies interact, a phenomenon should emerge where portions of dark matter halos closest to the center would become oriented in structural forms known as “caustic rings.”

“We had earlier proposed that disk galaxies have caustic rings of dark matter with a pattern of ring radii, very similar to the pattern of ring radii observed in LEDA 1313424,” Sikivie recently said.

With this idea in mind, Sikivie and Lhao then turned their attention to the mysterious bullseye of LEDA 1313424 and “fitted the observed ring radii to the pattern of ring radii in the caustic ring interpretation,” from which they discerned that “LEDA’s rings are the imprint of caustic rings of dark matter on the baryonic matter in LEDA’s disk.”

Strange Quantum Behaviors

A further aspect of the researchers’ theory involves the quantum behavior that axions are expected to engage in.

Under extremely cold conditions, these hypothetical dark matter particles could collectively occupy the same quantum state, forming what is known as a Bose-Einstein condensate—a strange state of matter in which quantum effects become visible on macroscopic scales.

According to the team, such a condensate could interact with ordinary matter in ways that match astronomical observations. If the duo is correct, this could imply that ring-shaped galaxies such as LEDA may not result from galactic collisions at all—instead, these unique cosmic structures might be evidence of dark matter axions that have undergone Bose-Einstein condensation.

Presently, much like dark matter, axions have remained hypothetical and frustratingly difficult to detect. Of course, given their elusiveness, this also makes them a very good candidate in terms of helping scientists close in on the dark matter mystery, and new models are helping to provide potentially compelling new resolutions for such problems.

For now, mysterious ringed galaxies remain mysterious. However, as the work of researchers like Sikivie and Lhao progresses, one of astronomy’s more unusual recent discoveries is gradually coming into focus, while providing fresh insight into the intriguing quantum nature of dark matter.

The team’s recent paper, “The Nine Rings of the Galaxy LEDA 1313424,” appeared in The Astrophysical Journal.

Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.

Dark matter remains one of the biggest mysteries in science. Scientists cannot see it directly, yet they believe it makes up most of the matter in the universe. Now, a new study suggests that dark matter may gather around supermassive black holes, forming large invisible clouds that influence their surroundings. Researchers from Virginia Tech have […]

The post Scientists use light echoes to hunt dark matter around giant black holes appeared first on Knowridge Science Report.

A new study has taken a fresh look at a late 2025 Yonsei University study claiming the universe’s expansion is stalling out and instead has found that the universe is still expanding as previously believed.

Led by researchers from the University of Southampton, the team behind the new analysis acknowledged the scientific value of the 2025 team’s work, noting that their calculations were not wrong but lacked the additional data needed to confirm the universe’s expansion rather than refute it.

The lead author of the 2025 study, Dr. Chul Chung, told The Debrief that he and his colleagues “have already submitted our response to Wiseman, Riess et al. 2026 on May 14,” adding that the paper, “Still accelerating: age bias correction in supernova cosmology is robust to host-progenitor age mapping,” has also been posted on arXiv.

“The abstract summarizes the two main issues we identify in Wiseman et al., so I think it provides a concise overview of our position,” Chung told The Debrief.

Challenging Universal Expansion

According to a statement announcing the findings, scientists first confirmed the universe’s expansion was accelerating in 2011, a discovery that won astrophysicists Professor Adam Riess and Professor Brian Schmidt the Nobel Prize in Physics. Since that discovery, cosmological models have incorporated this accelerating expansion, with experiments on dark matter and dark energy attempting to explain it.

Then last year, a separate team of astronomers announced that a new analysis of stellar phenomena indicated that the universe was no longer expanding. Adding to the controversy was the team’s claim that dark energy may not exist, citing the lack of concrete evidence for a universe undergoing accelerating expansion.

According to the Southampton-led team behind the new study, the 2025 study suggested that previous methods used to measure the universe’s expansion, such as measuring supernovae, “were fundamentally flawed.”

“This points to a time-varying dark energy and a universe that is no longer accelerating, representing a potential paradigm shift in cosmology if confirmed,” explained Professor Chul Chung, co-lead author on the study, in a 2025 email to The Debrief.

The claims sent shockwaves through the scientific community, a result that the researchers note could have dismantled the Nobel Prize-winning team’s historic findings, and “nearly three decades of astronomical progress.”

When discussing the 2025 team’s work, the lead author of the new study, Dr Phil Wiseman, said the team’s findings and the resulting debate were not necessarily incorrect. Instead, the Southampton researcher said it was primarily a result of a scientific understanding.

“The previous and well-accepted measurements were, in fact, fine and our current understanding of the fate of the universe remains robust.”

Recalculating Galaxy Masses and White Dwarf Star Ages Reestablishes Expansion

In their reanalysis, the Southampton-led team recalibrated the original data to account for variables not in the original study. According to Wiseman, these included “different host environments and populations.”

For example, the research team looked at Type 1a supernovae, which are violet, extremely bright explosions of white dwarf stars. The authors of the 2025 study claimed that this class of supernova had “different maximum brightness,” which had tricked astronomers into thinking the cosmos was accelerating, when their analysis said it was actually slowing down.

When they took a fresh look at the data, the Southampton team found that they had incorrectly estimated the age of these stars. The new study also says the 2025 team didn’t account for the mass of the galaxies hosting these cosmological phenomena, which they described as “a standard correction used in modern cosmology to prove accuracy.”

When his team factored those data points in, Wiseman said the evidence for a stalling expansion was not supported. Instead, the researcher observed that “the evidence for cosmic acceleration remains remarkably consistent.”

In their reply on ArXiv, the Yonsei University team said the new research failed to factor in two variables of their own, adding that “when these two effects are consistently combined in computing the redshift-dependent magnitude correction, the final correction, and hence the resulting cosmological impact, remain largely unchanged,” from the original 2025 calculations.

“Extraordinary Claims Require Especially Careful Testing”

When discussing the new study’s findings, Southampton Professor Mark Sullivan highlighted the benefits of scientific review, noting that “this is how progress is made.”

“Although this idea did not turn out correct, it has opened up new ways of thinking about how supernovae explode and how we can measure dark energy more accurately,” the researcher added.

Paper co-author, Dr Brodie Popovic agreed, noting that the 2025 study provided astronomers a good opportunity to “go back over all of our assumptions,” adding that “it turns out, yes, we do understand this stuff, and we’re accounting for it in our cosmology measurement.”

In a salute to the Sagan Standard established by astronomer Carl Sagan, which says “extraordinary claims require extraordinary evidence,” Reiss seemed to celebrate the new findings that reestablish his original work by quipping that “extraordinary claims require especially careful testing.”

With the accelerating expansion of the universe seemingly reestablished, at least for now, Dr Wiseman said: “we can get back to trying to understand what dark energy actually is, rather than wondering if it exists at all.”

“Thankfully we have averted this crisis, but the mystery about why the universe is still accelerating in size remains,” he added. 

The study “Still Accelerating: Type Ia supernova cosmology is robust to host galaxy age evolution” was published in Monthly Notices of the Royal Astronomical Society.

Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.

There is a pattern found in nature. It is indeed the largest pattern there is. It is so vast that it spans the universe, filling up its observable volume. The pattern is made of individual units in the same way your body is made of cells — if each of your cells were a galaxyContinue reading "Untangling the cosmic web"

The post Untangling the cosmic web appeared first on Astronomy Magazine.

Scientists believe an unusual LIGO detection may be evidence of a primordial black hole, potentially linking these long-theorized objects to the mystery of dark matter. Scientists at the University of Miami believe they may be closing in on evidence for one of the most elusive objects in the universe: primordial black holes. While definitive proof [...]

New research suggests that the mysterious “little red dots” spotted in the early universe could be supermassive black holes birthed in the collapse of dark matter halos.

Another Theory for Little Red Dots

Many theories exist for the origins of little red dots: compact, reddish objects spotted mainly in the first billion years after the Big Bang. The leading theory states that that little red dots are growing black holes encased in dense gas. The black holes at the centers of these little red dots appear to be tens or even hundreds of millions of times the mass of the Sun, raising questions about how they grew so massive so quickly. One possibility is that they started out quite massive, arising from black holes born from the collapse of massive gas clouds in the early universe.

A recent research article explores that theory, with a twist: that the black holes at the heart of little red dots were born not out of regular, baryonic matter, but out of dark matter.

A Dark (Matter) Past

In the early days of the universe, tiny fluctuations in the density of dark matter began to grow, eventually collapsing into vast spheroidal structures called dark matter halos. In the leading theory of cosmology, ΛCDM, dark matter interacts with itself and normal matter only through gravity. This leads to the creation of stable dark matter halos that act as the invisible scaffolding within which the first stars and galaxies grow.

In today’s article, Fangzhou Jiang (Peking University) and collaborators investigated the evolution of halos containing self-interacting dark matter. Particles of self-interacting dark matter, as the name suggests, can interact with one another through collisions and exchange heat. This small adjustment, sometimes invoked in alternatives to ΛCDM to explain the properties of dwarf galaxies, can lead to the collapse of dark matter halos into black holes.

Seeding Black Holes

Jiang and coauthors examined whether this process could explain the observed population of little red dots. First, the team examined whether the timescales are feasible: is it possible for dark matter to assemble itself into massive halos and collapse into black holes, all within the first billion years of the universe?

The answer appears to be yes — in this framework, dark matter halos with masses between 10^6.5 and 10^8.5 solar masses were able to form black holes between 10^4.5 and 10^6.5 solar masses by a redshift of z = 8.5, when the universe was just 600 million years old.

Jiang’s team then performed semianalytic modeling to explore how these “seed” black holes grow and evolve through accretion and mergers. This analysis generated a distribution of black hole masses consistent with what has been observed in little red dots. The team cautioned that their results are sensitive to the interaction cross-section of the dark matter particles, as well as various other parameters like the black hole accretion duty cycle. Their modeling also assumes that the halo collapse takes place before any baryonic matter collects within it. Intriguingly, the authors found that the presence of baryons accelerates the collapse of the dark matter halo, speeding the seeding process.

To close, Jiang and collaborators noted that this process is not mutually exclusive with other black hole seeding mechanisms, and as studies of self-interacting dark matter continue, its ability to create black holes in the early universe should be examined further.

Citation

“Formation of the Little Red Dots from the Core Collapse of Self-Interacting Dark Matter Halos,” Fangzhou Jiang et al 2026 ApJL 996 L19. doi:10.3847/2041-8213/ae247a

The post A Dark Matter Origin for Little Red Dots appeared first on AAS Nova.

While classic spiral and elliptical galaxies shine brightly, another class of galaxy flies under the radar. A recent study uses a new technique to search for galaxies so faint they are nearly completely dark.

Ultra-Diffuse Galaxies

Hard to see and harder to understand, ultra-diffuse galaxies are a galaxy class with extremely low surface brightness. With low luminosities but large physical extents, these galaxies appear as barely visible smudges, making them difficult to detect. Historically, astronomers have searched for ultra-diffuse galaxies using methods that search for diffuse stellar light; however, this method only catches those bright enough to spot — if even darker galaxies exist, they will not be found this way. Is there another way to discover these elusive galaxies?

Recent studies have shown that ultra-diffuse galaxies tend to have more globular clusters on average than typical galaxies. While this raises questions surrounding their formation and evolution, the abundance of globular clusters provides another way to search for ultra-diffuse galaxies. In general, globular clusters do not clump together in space unless there is additional mass, like dark matter, pulling them together. Thus, searching for overdensities of globular clusters could reveal a hidden population of ultra-diffuse galaxies.

Candidate Confirmation

Discovered as an overdensity of three globular clusters, Candidate Dark Galaxy-2 (CDG-2) was found in data from a Hubble Space Telescope (HST) program aimed at imaging the Perseus galaxy cluster. With an updated globular cluster catalog and improved statistical method for detecting overdensities, Dayi (David) Li (University of Toronto) and collaborators perform a follow-up search and analysis to confirm CDG-2 as a galaxy. Their method searches for clustering of globular clusters that do not appear to belong to any bright galaxy and are unlikely to be randomly grouped so closely in the intergalactic medium.

From their analysis, the authors find an additional globular cluster that belongs to CDG-2, making the object even more statistically likely to be a galaxy as opposed to four randomly clumped globular clusters. Further pointing to CDG-2’s galactic nature, two stacked HST images reveal a very faint light surrounding these globular clusters. To confirm that this diffuse emission is truly associated with CDG-2, the authors use data from Euclid, a space telescope that is optimized for detecting diffuse structures. The Euclid data confirm a low-surface-brightness fuzzy emission that coincides with the emission seen with HST and corresponds to the location of the four globular clusters — strong evidence that CDG-2 is indeed a galaxy.

Globular Cluster and Dark Matter Dominated

Further analysis of CDG-2 reveals that at least ~16.6% of its total light is contained within its globular clusters. However, if CDG-2 follows the typical distribution of globular clusters in galaxies and some are still undetected, this fraction could be as high as 33%, which would make CDG-2 have the highest globular cluster-to-stellar mass ratio of any galaxy to date. With additional analysis of the mass contained within the observed globular clusters, the authors estimate that CDG-2 is highly dark-matter dominated with 99.94%–99.98% of its total mass contained within dark matter. The dark-matter fraction could be even higher if CDG-2 contains more undetected globular clusters. Future observations of CDG-2 with deeper imaging and spectroscopy will more confidently determine the galaxy’s extreme and unique structure.

Through this study, the authors have confirmed the first galaxy ever detected solely through its globular cluster population. This technique opens a promising avenue to discovering more galaxies hiding out in the dark.

Citation

“Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster,” Dayi (David) Li et al 2025 ApJL 986 L18. doi:10.3847/2041-8213/adddab

The post Finding Galaxies in the Dark: Using Globular Clusters to Identify Ultra-Diffuse Galaxies appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: A High-Caliber View of the Bullet Cluster Through JWST Strong and Weak Lensing Analyses
Authors: Sangjun Cha et al.
First Author’s Institution: Yonsei University
Status: Published in ApJL

Despite the extreme depth and resolution of JWST observations, there’s still no smoking gun to be found in the hunt for dark matter. Yet, the extremely high-velocity collision in today’s article might aid in the search for what this mysterious material is made of. The target of today’s article is the Bullet Cluster (Figure 1). Despite its name, the Bullet Cluster is not one, but actually two massive clusters of galaxies gravitationally interacting with one another. Due to the high speed with which the smaller “bullet” subcluster collided and passed through the larger one, a strong bow shock caused the gas in the system to compress and heat to millions of degrees. Typically, this alone would make this system interesting to astronomers, but the real kicker here is that this collision was powerful enough to separate the dark matter from the normal, luminous matter. This has made the Bullet Cluster an important testing ground for theories on the nature of dark matter and is exactly what the authors of today’s article attempt to use it for.

Mapping Hidden Mass

The authors used JWST imaging of the majority of the Bullet Cluster system to obtain high-resolution, multi-wavelength data. They then identified over 146 areas in the image subject to strong lensing, which is when the light from a background galaxy is bent around mass in the foreground Bullet Cluster system, since that mass distorts spacetime. They also obtained a weak lensing model for the image through statistical analysis, since the effects of weak lensing, while similar, are much more subtle than strong lensing. They then created a mass map for the system by feeding their strong lensing and weak lensing model into a reconstruction algorithm. Essentially, this algorithm creates the most likely distribution of mass, both dark matter and normal matter, that would re-create all the lensing that they observed.

What’s special about the algorithm that they used here is that it does not assume that light traces mass. In general, this assumption is correct, so other algorithms use it to get the most accurate mass estimate. But the authors here wanted to confirm that even in extreme environments like the Bullet Cluster that this is still the case, so they sent their algorithm in “blind.”

Comparing How the Light Traces the Dark

Once they had their mass map, which is dominated mostly by the dark matter in the system, they could compare it to the optical light between the system’s galaxies, referred to as the intracluster light (ICL). They used a statistical test called the modified Hausdorff distance to determine how similar the distribution of ICL and dark matter were to each other. They found that, despite the extreme gravitational interactions going on in the Bullet Cluster, the ICL still does an excellent job overall of tracing the system’s mass (see Figure 2).

Next, they compared their mass map to the distributions of hot plasma, as observed in X-ray wavelengths with Chandra, and of hydrogen gas, as observed in radio wavelengths with MeerKAT (see Figure 1). They found that the dark matter was offset from both of these distributions, which was already known about this system. However, their measurement of these mass offsets indicated that it is unlikely that the Bullet Cluster was formed from a merger between just two galaxy clusters. The authors suggest that the Bullet Cluster’s formation is much more complicated than previously thought, but they aren’t able to give any guesses about the initial conditions.

What a Massive Cluster Tells Us About a Tiny Particle

Finally, the authors were able to use their mass map to put some constraints on one of the theories of dark matter: self-interacting dark matter. If dark matter particles interact with each other, then they have a cross section that can be calculated. Here, the authors calculate it from the offset between the position of the most massive galaxy in the Bullet Cluster and the position of the highest density of dark matter, which is about 60,000 light-years (see Figure 2). They were able to determine that the upper limit of this cross section is 0.5 cm²/g. This is the tightest constraint on self-interacting dark matter’s cross section to date, but we still have that pesky “per gram” in our units since we don’t know the mass of a dark matter particle (but this is being studied!).

This cross section is near the lower limit of what is thought to be a reasonable range for self-interacting dark matter theories (about 0.5–2 cm²/g), but it certainly doesn’t rule anything out. For further context, when dividing by mass, the cross section of a hydrogen atom is about 1.7×10⁷ cm²/g, which is much bigger, but we would expect that when comparing an atom to a potential subatomic dark matter particle.

Altogether, the authors of this article were able to use both the precision of JWST and the impartiality of their mass-reconstruction algorithm to learn new things about both the structure of the Bullet Cluster and the nature of dark matter. Although there are still some open questions that will need to be answered, this study certainly is not just a shot in the dark!

Original astrobite edited by Maggie Verrico.

About the author, Veronika Dornan:

Veronika is a final-year PhD candidate at McMaster University. Her research is in observations of extragalactic globular star clusters and what they can tell us about galaxy evolution and dark matter distribution in the universe.

The post NCIS JWST: Analyzing the Aftermath of the Bullet Cluster’s Collision appeared first on AAS Nova.

JWST observations have begun uncovering some of the oldest star clusters in the universe, but their formation mechanisms are still uncertain. A recent study uses high-resolution simulations to understand early star formation and how local globular clusters may have originated. 

Supersonically Induced Gas Objects

Our leading cosmological model of the universe suggests that stars and galaxies coalesce within dark matter halos, eventually forming the larger structures we see today. However, some of the oldest astronomical objects in the universe — massive collections of stars called globular clusters — contain little to no dark matter, making their formation mechanisms somewhat mysterious. How do large clusters with thousands of stars form outside of dark matter halos?

Recent studies have suggested that as the dark matter and baryons (i.e., normal matter) in the early universe began to clump together and form slightly denser areas, the pristine gas and dark matter began to move relative to one another. Baryons, still tightly coupled to photons, did not have the same freedom to collect in dense regions that the dark matter did, creating supersonic (about five times the speed of sound) relative motions that separated gas clumps and early dark matter halos. These separate, dark-matter-free gas clumps formed baryon-enriched structures known as supersonically induced gas objects.

Over the past decade, simulations and theory have shown that supersonically induced gas objects can form star clusters, which may be the origin story of many of the local globular clusters seen today. However, these simulations have been limited by their resolution. Unable to resolve individual star formation and not including key aspects like feedback, these simulations have left open questions about these dark-matter-free objects.  

High-Resolution Simulations

Seeking to more deeply understand supersonically induced gas objects, William Lake (University of California, Los Angeles; Dartmouth College) and collaborators perform high-resolution simulations that include detailed physics and can track the formation of individual stars. The simulations varied the metallicity and presence of protostellar jets, properties that are both known to impact the star formation process in gas clouds.

From the simulations, the authors find that star formation occurs naturally in these early systems. Both feedback from jets and the metallicity of the gas affect star-formation outcomes — jets tend to have a minor impact on the star formation efficiency in low-metallicity clouds, but disruption from the jets can cause low-mass stars to form in metal-enriched systems. However, in general, the authors conclude that supersonically induced gas clouds tend to form higher-mass stars than local star clusters, which agrees with what is expected for the universe’s first stellar population. 

Given that these objects likely form very high-mass stars, these primordial star clusters will have extremely high stellar mass surface densities and brightnesses compared to what is observed locally. With such enhanced brightness, these objects may be bright enough for JWST to observe them. Further high-redshift observations of star clusters will allow for further constraints on the formation of the universe’s first stars and will yield more information about the possible origins of the globular clusters we see in the local universe.

Citation

“The Stellar Initial Mass Function of Early Dark Matter–Free Gas Objects,” William Lake et al 2025 ApJL 985 L6. doi:10.3847/2041-8213/add347

The post From Gas to Cluster: Simulating Star Formation in the Early Universe appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Examining the Nature of the Starless Dark Matter Halo Candidate Cloud-9 with Very Large Array Observations
Authors: Alejandro Benítez-Llambay et al.
First Author’s Institution: University of Milano-Bicocca
Status: Published in ApJ

Astronomy, in many ways, is the study of light — in every wavelength that exists — coming at us from light-years away. But most of the mass in our universe is made of dark matter, a material that neither emits nor reflects any light. So, how exactly are we supposed to find this dark matter? Well, the dominant theory of what our universe is made of, Lambda cold dark matter (ΛCDM), predicts that this dark matter can form small halos — regions of gravitationally bound matter — that host reservoirs of cold gas but no stars. While we cannot directly observe dark matter, this cold gas can reflect light at radio wavelengths. Finding these dark halos, referred to as reionization-limited neutral atomic hydrogen clouds (RELHICs), is therefore rather important to confirm ΛCDM. The authors of this article are attempting to do just that by taking a close look at Cloud-9, a potential RELHIC.

How to “See” in the Dark

Cloud-9 was first discovered in 2023 when a team of astronomers noticed an excess of neutral hydrogen (HI) gas with no bright stars around it, near the spiral galaxy Messier 94 (M94). This discovery was made with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), which doesn’t have the best resolution. For reference, this article found Cloud-9’s diameter to be roughly 3 arcminutes in the sky, while FAST’s resolution is 2.9 arcminutes. So, the authors of this article decided to look at Cloud-9 again, this time with the Very Large Array (VLA), an array of radio telescopes that can pick out more of the structure of this system.

With this upgraded resolution, the authors were able to get a clearer picture of the shape of Cloud-9 and compare it to the previous study (see Figure 1). They found that Cloud-9 was slightly smaller and much more lopsided than previously thought. This funky shape is likely due to gravitational interactions with the larger nearby galaxy M94, causing the gas in Cloud-9 to squish and stretch.

RELHIC or Just RELHIC-tively Dim?

To determine if Cloud-9 really is a RELHIC, the authors needed to compare its HI radial density profile to the profiles of simulated RELHICs. Density profiles are a good tool to use here since although we don’t fully understand what dark matter is, we do know how it will affect things around it gravitationally. If the only matter hosted by Cloud-9 is HI gas and dark matter, then that gas will have a specific distribution that we can observe. If other baryonic matter is hosted by Cloud-9 that we just aren’t picking up right now, it will affect that density profile.

The authors found that for a detailed, general simulation the profiles did not match well, specifically in the outer regions. But when they compared it to a sphere of gas with a constant, specific temperature, they were able to get the fits to line up (see Figure 2). However, the authors point out that these simulations are isolated, meaning there is no big galaxy next to them affecting their shape, unlike Cloud-9.

The authors also tried to test another explanation for Cloud-9’s apparent lack of stars: the stars are there but just too dim for us to see right now. The Milky Way sits near many ultra-faint dwarf galaxies, which are small and have quite dim stellar populations. We’ve only just developed telescopes in the last few years that can take in enough light to see these galaxies, and we might not be able to detect stars in an ultra-faint dwarf galaxy as far away from us as Cloud-9. The authors compare the HI density profile of Cloud-9 to that of Leo T, a nearby ultra-faint dwarf galaxy. They find that these profiles match up better than for the general RELHIC simulations, particularly in the innermost regions of Cloud-9 and Leo T.

What Are the Next Steps?

The authors conclude that, as of right now, they can’t definitively say whether Cloud-9 is or is not a RELHIC, but there are a few things astronomers can do to clear things up. First, theorists can re-run the RELHIC simulations with halos that have been affected by gravity like Cloud-9. This will allow them to better compare their observations to the simulations.

Second, observers can image Cloud-9 with an optical telescope that is powerful enough to detect any stars that could be there. They suggest the Hubble Space Telescope since it would be able to detect the brightest handful of stars in the potential dim population. JWST would be an even better choice, with its even deeper imaging capabilities, but it is very difficult to get observing time on it. (JWST recently received the most proposals in one year for any telescope ever!)

We will likely have to wait a bit for a definitive answer on the nature of Cloud-9, but either way the authors are excited by this gas cloud. Either it is the first starless dark matter halo, a confirmation of one of the predictions of ΛCDM, or it is the most distant ultra-faint dwarf galaxy ever detected! Although Cloud-9 is not much to look at right now, you should still definitely keep your eye on it.

Original astrobite edited by Maggie Verrico and Cole Meldorf.

About the author, Veronika Dornan:

Veronika is a final-year PhD candidate at McMaster University. Her research is in observations of extragalactic globular star clusters and what they can tell us about galaxy evolution and dark matter distribution in the universe.

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Particle physics is the epitome of ‘big science’. To answer our most fundamental questions out about physics requires world class experiments that push the limits of whats technologically possible. Such incredible sophisticated experiments, like those at the LHC, require big facilities to make them possible,  big collaborations to run them, big project planning to make dreams of new facilities a reality, and committees with big acronyms to decide what to build.

Enter the Particle Physics Project Prioritization Panel (aka P5) which is tasked with assessing the landscape of future projects and laying out a roadmap for the future of the field in the US. And because these large projects are inevitably an international endeavor, the report they released last week has a large impact on the global direction of the field. The report lays out a vision for the next decade of neutrino physics, cosmology, dark matter searches and future colliders. 

P5 follows the community-wide brainstorming effort known as the Snowmass Process in which researchers from all areas of particle physics laid out a vision for the future. The Snowmass process led to a particle physics ‘wish list’, consisting of all the projects and research particle physicists would be excited to work on. The P5 process is the hard part, when this incredibly exciting and diverse research program has to be made to fit within realistic budget scenarios. Advocates for different projects and research areas had to make a case of what science their project could achieve and a detailed estimate of the costs. The panel then takes in all this input and makes a set of recommendations of how the budget should be allocated, what should projects be realized and what hopes are dashed. Though the panel only produces a set of recommendations, they are used quite extensively by the Department of Energy which actually allocates funding. If your favorite project is not endorsed by the report, its very unlikely to be funded. 

Particle physics is an incredibly diverse field, covering sub-atomic to cosmic scales, so recommendations are divided up into several different areas. In this post I’ll cover the panel’s recommendations for neutrino physics and the cosmic frontier. Future colliders, perhaps the spiciest topic, will be covered in a follow up post.

The Future of Neutrino Physics

For those in the neutrino physics community all eyes were on the panels recommendations regarding the Deep Underground Neutrino Experiment (DUNE). DUNE is the US’s flagship particle physics experiment for the coming decade and aims to be the definitive worldwide neutrino experiment in the years to come. A high powered beam of neutrinos will be produced at Fermilab and sent 800 miles through the earth’s crust towards several large detectors placed in a mine in South Dakota. Its a much bigger project than previous neutrino experiments, unifying essentially the entire US community into a single collaboration.

DUNE is setup to produce world leading measurements of neutrino oscillations, the property by which neutrinos produced in one ‘flavor state’, (eg an electron-neutrino) gradually changes its state with sinusoidal probability (eg into a muon neutrino) as it propagates through space. This oscillation is made possible by a simple quantum mechanical weirdness: neutrino’s flavor state, whether it couples to electrons muons or taus, is not the same as its mass state. Neutrinos of a definite mass are therefore a mixture of the different flavors and visa versa.

Detailed measurements of this oscillation are the best way we know to determine several key neutrino properties. DUNE aims to finally pin down two crucial neutrino properties: their ‘mass ordering’, which will solidify how the different neutrino flavors and measured mass differences all fit together, and their ‘CP-violation’ which specifies whether neutrinos and their anti-matter counterparts behave the same or not. DUNE’s main competitor is the Hyper-Kamiokande experiment in Japan, another next-generation neutrino experiment with similar goals.

Construction of the DUNE experiment has been ongoing for several years and unfortunately has not been going quite as well as hoped. It has faced significant schedule delays and cost overruns. DUNE is now not expected to start taking data until 2031, significantly behind Hyper-Kamiokande’s projected 2027 start. These delays may lead to Hyper-K making these definitive neutrino measurements years before DUNE, which would be a significant blow to the experiment’s impact. This left many DUNE collaborators worried about its broad support from the community.

It came as a relief then when P5 report re-affirmed the strong science case for DUNE, calling it the “ultimate long baseline” neutrino experiment. The report strongly endorsed the completion of the first phase of DUNE. However, it recommended a pared-down version of its upgrade, advocating for an earlier beam upgrade in lieu of additional detectors. This re-imagined upgrade will still achieve the core physics goals of the original proposal with a significant cost savings. With this report, and news that the beleaguered underground cavern construction in South Dakota is now 90% complete, was certainly welcome holiday news to the neutrino community. This is also sets up a decade-long race between DUNE and Hyper-K to be the first to measure these key neutrino properties.

Cosmic Implications

While we normally think of particle physics as focused on the behavior of sub-atomic particles, its really about the study of fundamental forces and laws, no matter the method. This means that telescopes to study the oldest light in the universe, the Cosmic Microwave Background (CMB), fall into the same budget category as giant accelerators studying sub-atomic particles. Though the experiments in these two areas look very different, the questions they seek to answer are cross-cutting. Understanding how particles interact at very high energies helps us understand the earliest moments of the universe, when such particles were all interacting in a hot dense plasma. Likewise, by studying the these early moments of the universe and its large-scale evolution can tell us about what kinds of particles and forces are influencing its dynamics. When asking fundamental questions about the universe, one needs both the sharpest microscopes and the grandest panoramas possible.

The most prominent example of this blending of the smallest and largest scales in particle physics is dark matter. Some of our best evidence for dark matter comes analyzing the cosmic microwave background to determine how the primordial plasma behaved. These studies showed that some type of ‘cold’, matter that doesn’t interact with light, aka dark matter, was necessary to form the first clumps that eventually seeded the formation of galaxies. Without it, the universe would be much more soup-y and structureless than what we see to today.

To determine what dark matter is then requires an attack from two fronts: design experiments here on earth attempting directly detect it, and further study its cosmic implications to look for more clues as to its properties.

The panel recommended next generation telescopes to study the CMB as a top priority. The so called ‘Stage 4’ CMB experiment would deploy telescopes in both the south pole and Chile’s Atacama desert to better characterize sources of atmospheric noise. The CMB has been studied extensively before, but the increased precision of CMS-S4 could shed light on mysteries like dark energy, dark matter, inflation, and the recent Hubble Tension. Given the past fruitfulness of these efforts, I think few doubted the science case for such a next generation experiment.

The P5 report recommended a suite of new dark matter experiments in the next decade, including the ‘ultimate’ liquid Xenon based dark matter search. Such an experiment would follow in the footsteps of massive noble gas experiments like LZ and XENONnT which have been hunting for a favored type of dark matter called WIMP’s for the last few decades. These experiments essentially build giant vats of liquid Xenon, carefully shield from any sources of external radiation, and look for signs of dark matter particles bumping into any of the Xenon atoms. The larger the vat of Xenon, the higher chance a dark matter particle will bump into something. Current generation experiments have ~7 tons of Xenon, and the next generation experiment would be even larger. The next generation aims to reach the so called ‘neutrino floor’, the point as which the experiments would be sensitive enough to observe astrophysical neutrinos bumping into the Xenon. Such neutrino interactions would look extremely similar to those of dark matter, and thus represent an unavoidable background which would signal the ultimate sensitivity of this type of experiment. WIMP’s could still be hiding in a basement below this neutrino floor, but finding them would be exceedingly difficult.

WIMP’s are not the only dark matter candidates in town, and recent years have also seen an explosion of interest in the broad range of dark matter possibilities, with axions being a prominent example. Other kinds of dark matter could have very different properties than WIMPs and have had much fewer dedicated experiments to search for them. There is ‘low hanging fruit’ to pluck in the way of relatively cheap experiments which can achieve world-leading sensitivity. Previously, these ‘table top’ sized experiments had a notoriously difficult time obtaining funding, as they were often crowded out of the budgets by the massive flagship projects. However, small experiments can be crucial to ensuring our best chance of dark matter discovery, as they fill in the blinds pots missed by the big projects.

The panel therefore recommended creating a new pool of funding set aside for these smaller scale projects. Allowing these smaller scale projects to flourish is important for the vibrancy and scientific diversity of the field, as the centralization of ‘big science’ projects can sometimes lead to unhealthy side effects. This specific recommendation also mirrors a broader trend of the report: to attempt to rebalance the budget portfolio to be spread more evenly and less dominated by the large projects.

What Didn’t Make It

Any report like this comes with some tough choices. Budget realities mean not all projects can be funded. Besides the pairing down of some of DUNE’s upgrades, one of the biggest areas that was recommended against were ‘accessory experiments at the LHC’. In particular, MATHUSULA and the Forward Physics Facility were two experiments that proposed to build additional detectors near already existing LHC collision points to look for particles that may be missed by the current experiments. By building new detectors hundreds of meters away from the collision point, shielded by concrete and the earth, they can obtained unique sensitivity to ‘long lived’ particles capable of traversing such distances. These experiments would follow in the footsteps of the current FASER experiment, which is already producing impressive results.

While FASER found success as a relatively ‘cheap’ experiment, reusing detector components from and situating itself in a beam tunnel, these new proposals were asking for quite a bit more. The scale of these detectors would have required new caverns to be built, significantly increasing the cost. Given the cost and specialized purpose of these detectors, the panel recommended against their construction. These collaborations may now try to find ways to pare down their proposal so they can apply to the new small project portfolio.

Another major decision by the panel was to recommend against hosting a new Higgs factor collider in the US. But that will discussed more in a future post.

Conclusions

The P5 panel was faced with a difficult task, the total cost of all projects they were presented with was three times the budget. But they were able to craft a plan that continues the work of the previous decade, addresses current shortcomings and lays out an inspiring vision for the future. So far the community seems to be strongly rallying behind it. At time of writing, over 2700 community members from undergraduates to senior researchers have signed a petition endorsing the panels recommendations. This strong show of support will be key for turning these recommendations into actual funding, and hopefully lobbying congress to even increase funding so that more of this vision can be realized.

For those interested the full report as well as executive summaries of different areas can be found on the P5 website. Members of the US particle physics community are also encouraged to sign the petition endorsing the recommendations here.

And stayed tuned for part 2 of our coverage which will discuss the implications of the report on future colliders!

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