A wobble from Mars could be sign of dark matter

In a new study, MIT physicists propose that if most of the dark matter in the universe is made up of microscopic primordial black holes -- an idea first proposed in the 1970s -- then these gravitational dwarfs should zoom through our solar system at least once per decade. A flyby like this, the researchers predict, would introduce a wobble into Mars' orbit, to a degree that today's technology could actually detect.

Such a detection could lend support to the idea that primordial black holes are a primary source of dark matter throughout the universe.

"Given decades of precision telemetry, scientists know the distance between Earth and Mars to an accuracy of about 10 centimeters," says study author David Kaiser, professor of physics and the Germeshausen Professor of the History of Science at MIT. "We're taking advantage of this highly instrumented region of space to try and look for a small effect. If we see it, that would count as a real reason to keep pursuing this delightful idea that all of dark matter consists of black holes that were spawned in less than a second after the Big Bang and have been streaming around the universe for 14 billion years."

Kaiser and his colleagues report their findings today in the journal Physical Review D. The study's co-authors are lead author Tung Tran '24, who is now a graduate student at Stanford University; Sarah Geller '12, SM '17, PhD '23, who is now a postdoc at the University of California at Santa Cruz; and MIT Pappalardo Fellow Benjamin Lehmann.

Beyond particles

Less than 20 percent of all physical matter is made from visible stuff, from stars and planets, to the kitchen sink. The rest is composed of dark matter, a hypothetical form of matter that is invisible across the entire electromagnetic spectrum yet is thought to pervade the universe and exert a gravitational force large enough to affect the motion of stars and galaxies.

Physicists have erected detectors on Earth to try and spot dark matter and pin down its properties. For the most part, these experiments assume that dark matter exists as a form of exotic particle that might scatter and decay into observable particles as it passes through a given experiment. But so far, such particle-based searches have come up empty.

In recent years, another possibility, first introduced in the 1970s, has regained traction: Rather than taking on a particle form, dark matter could exist as microscopic, primordial black holes that formed in the first moments following the Big Bang. Unlike the astrophysical black holes that form from the collapse of old stars, primordial black holes would have formed from the collapse of dense pockets of gas in the very early universe and would have scattered across the cosmos as the universe expanded and cooled.

These primordial black holes would have collapsed an enormous amount of mass into a tiny space. The majority of these primordial black holes could be as small as a single atom and as heavy as the largest asteroids. It would be conceivable, then, that such tiny giants could exert a gravitational force that could explain at least a portion of dark matter. For the MIT team, this possibility raised an initially frivolous question.

"I think someone asked me what would happen if a primordial black hole passed through a human body," recalls Tung, who did a quick pencil-and-paper calculation to find that if such a black hole zinged within 1 meter of a person, the force of the black hole would push the person 6 meters, or about 20 feet away in a single second. Tung also found that the odds were astronomically unlikely that a primordial black hole would pass anywhere near a person on Earth.

Their interest piqued, the researchers took Tung's calculations a step further, to estimate how a black hole flyby might affect much larger bodies such as the Earth and the moon.

"We extrapolated to see what would happen if a black hole flew by Earth and caused the moon to wobble by a little bit," Tung says. "The numbers we got were not very clear. There are many other dynamics in the solar system that could act as some sort of friction to cause the wobble to dampen out."

Close encounters

To get a clearer picture, the team generated a relatively simple simulation of the solar system that incorporates the orbits and gravitational interactions between all the planets, and some of the largest moons.

"State-of-the-art simulations of the solar system include more than a million objects, each of which has a tiny residual effect," Lehmann notes. "But even modeling two dozen objects in a careful simulation, we could see there was a real effect that we could dig into."

The team worked out the rate at which a primordial black hole should pass through the solar system, based on the amount of dark matter that is estimated to reside in a given region of space and the mass of a passing black hole, which in this case, they assumed to be as massive as the largest asteroids in the solar system, consistent with other astrophysical constraints.

"Primordial black holes do not live in the solar system. Rather, they're streaming through the universe, doing their own thing," says co-author Sarah Geller. "And the probability is, they're going through the inner solar system at some angle once every 10 years or so."

Given this rate, the researchers simulated various asteroid-mass black holes flying through the solar system, from various angles, and at velocities of about 150 miles per second. (The directions and speeds come from other studies of the distribution of dark matter throughout our galaxy.) They zeroed in on those flybys that appeared to be "close encounters," or instances that caused some sort of effect in surrounding objects. They quickly found that any effect in the Earth or the moon was too uncertain to pin to a particular black hole. But Mars seemed to offer a clearer picture.

The researchers found that if a primordial black hole were to pass within a few hundred million miles of Mars, the encounter would set off a "wobble," or a slight deviation in Mars' orbit. Within a few years of such an encounter, Mars' orbit should shift by about a meter -- an incredibly small wobble, given the planet is more than 140 million miles from Earth. And yet, this wobble could be detected by the various high-precision instruments that are monitoring Mars today.

If such a wobble were detected in the next couple of decades, the researchers acknowledge there would still be much work needed to confirm that the push came from a passing black hole rather than a run-of-the-mill asteroid.

"We need as much clarity as we can of the expected backgrounds, such as the typical speeds and distributions of boring space rocks, versus these primordial black holes," Kaiser notes. "Luckily for us, astronomers have been tracking ordinary space rocks for decades as they have flown through our solar system, so we could calculate typical properties of their trajectories and begin to compare them with the very different types of paths and speeds that primordial black holes should follow."

To help with this, the researchers are exploring the possibility of a new collaboration with a group that has extensive expertise simulating many more objects in the solar system.

"We are now working to simulate a huge number of objects, from planets to moons and rocks, and how they're all moving over long time scales," Geller says. "We want to inject close encounter scenarios, and look at their effects with higher precision."

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Dark matter could have helped make supermassive black holes in the early universe

Supermassive black holes typically take billions of years to form. But the James Webb Space Telescope is finding them not that long after the Big Bang -- before they should have had time to form.

It takes a long time for supermassive black holes, like the one at the center of our Milky Way galaxy, to form. Typically, the birth of a black hole requires a giant star with the mass of at least 50 of our suns to burn out -- a process that can take a billion years -- and its core to collapse in on itself.

Even so, at only about 10 solar masses, the resulting black hole is a far cry from the 4 million-solar-masses black hole, Sagittarius A*, found in our Milky Way galaxy, or the billion-solar-mass supermassive black holes found in other galaxies. Such gigantic black holes can form from smaller black holes by accretion of gas and stars, and by mergers with other black holes, which take billions of years.

Why, then, is the James Webb Space Telescope discovering supermassive black holes near the beginning of time itself, eons before they should have been able to form? UCLA astrophysicists have an answer as mysterious as the black holes themselves: Dark matter kept hydrogen from cooling long enough for gravity to condense it into clouds big and dense enough to turn into black holes instead of stars. The finding is published in the journal Physical Review Letters.

"How surprising it has been to find a supermassive black hole with a billion solar mass when the universe itself is only half a billion years old," said senior author Alexander Kusenko, a professor of physics and astronomy at UCLA. "It's like finding a modern car among dinosaur bones and wondering who built that car in the prehistoric times."

Some astrophysicists have posited that a large cloud of gas could collapse to make a supermassive black hole directly, bypassing the long history of stellar burning, accretion and mergers. But there's a catch: Gravity will, indeed, pull a large cloud of gas together, but not into one large cloud. Instead, it gathers sections of the gas into little halos that float near each other but don't form a black hole.

The reason is because the gas cloud cools too quickly. As long as the gas is hot, its pressure can counter gravity. However, if the gas cools, pressure decreases, and gravity can prevail in many small regions, which collapse into dense objects before gravity has a chance to pull the entire cloud into a single black hole.

"How quickly the gas cools has a lot to do with the amount of molecular hydrogen," said first author and doctoral student Yifan Lu. "Hydrogen atoms bonded together in a molecule dissipate energy when they encounter a loose hydrogen atom. The hydrogen molecules become cooling agents as they absorb thermal energy and radiate it away. Hydrogen clouds in the early universe had too much molecular hydrogen, and the gas cooled quickly and formed small halos instead of large clouds."

Lu and postdoctoral researcher Zachary Picker wrote code to calculate all possible processes of this scenario and discovered that additional radiation can heat the gas and dissociate the hydrogen molecules, altering how the gas cools.

"If you add radiation in a certain energy range, it destroys molecular hydrogen and creates conditions that prevent fragmentation of large clouds," Lu said.

But where does the radiation come from?

Only a very tiny portion of matter in the universe is the kind that makes up our bodies, our planet, the stars and everything else we can observe. The vast majority of matter, detected by its gravitational effects on stellar objects and by the bending of light rays from distant sources, is made of some new particles, which scientists have not yet identified.

The forms and properties of dark matter are therefore a mystery that remains to be solved. While we don't know what dark matter is, particle theorists have long speculated that it could contain unstable particles which can decay into photons, the particles of light. Including such dark matter in the simulations provided the radiation needed for the gas to remain in a large cloud while it is collapsing into a black hole.

Dark matter could be made of particles that slowly decay, or it could be made of more than one particle species: some stable and some that decay at early times. In either case, the product of decay could be radiation in the form of photons, which break up molecular hydrogen and prevent hydrogen clouds from cooling too quickly. Even very mild decay of dark matter yielded enough radiation to prevent cooling, forming large clouds and, eventually, supermassive black holes.

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Dark matter: A camera trap for the invisible

It sounds fantastical, but it's a reality for the scientists who work at the world's largest particle collider:

In an underground tunnel some 350 feet beneath the France-Switzerland border, a huge device called the Large Hadron Collider sends beams of protons smashing into each other at nearly the speed of light, creating tiny eruptions that mimic the conditions that existed immediately after the Big Bang.

Scientists like Duke physicist Ashutosh Kotwal think the subatomic debris of these collisions could contain hints of the universe's "missing matter." And with some help from artificial intelligence, Kotwal hopes to catch these fleeting clues on camera, using a design described May 3 in the journal Scientific Reports.

Ordinary matter -- the stuff of people and planets -- is only part of what's out there. Kotwal and others are hunting for dark matter, an invisible matter that's five times more abundant than the stuff we can see but whose nature remains a mystery.

Scientists know it exists from its gravitational influence on stars and galaxies, but other than that we don't know much about it.

The Large Hadron Collider could change that. There, researchers are looking for dark matter and other mysteries using detectors that act like giant 3D digital cameras, taking continuous snapshots of the spray of particles produced by each proton-proton collision.

Only ordinary particles trigger a detector's sensors. If researchers can make dark matter at the LHC, scientists think one way it could be noticeable is as a sort of disappearing act: heavy charged particles that travel a certain distance -- 10 inches or so -- from the point of collision and then decay invisibly into dark matter particles without leaving a trace.

If you retraced the paths of these particles, they would leave a telltale "disappearing track" that vanishes partway through the detector's inner layers.

But to spot these elusive tracks they'll need to act fast, Kotwal says.

That's because the LHC's detectors take some 40 million snapshots of flying particles every second.

That's too much raw data to hang on to everything and most of it isn't very interesting. Kotwal is looking for a needle in a haystack.

"Most of these images don't have the special signatures we're looking for," Kotwal said. "Maybe one in a million is one that we want to save."

Researchers have just a few millionths of a second to determine if a particular collision is of interest and store it for later analysis.

"To do that in real time, and for months on end, would require an image recognition technique that can run at least 100 times faster than anything particle physicists have ever been able to do," Kotwal said.

Kotwal thinks he may have a solution. He has been developing something called a "track trigger," a fast algorithm that is able to spot and flag these fleeting tracks before the next collision occurs, and from among a cloud of tens of thousands of other data points measured at the same time.

His design works by divvying up the task of analyzing each image among a large number of AI engines running simultaneously, built directly onto a silicon chip. The method processes an image in less than 250 nanoseconds, automatically weeding out the uninteresting ones.

Kotwal first described the approach in a sequence of two papers published in 2020 and 2021. In the more recent paper published this May in Scientific Reports, he and a team of undergraduate student co-authors show that his algorithm can run on a silicon chip.

Kotwal and his students plan to build a prototype of their device by next summer, though it will be another three or four years before the full device -- which will consist of about 2000 chips -- can be installed at detectors at the LHC.

As the performance of the accelerator continues to crank up, it will produce even more particles. And Kotwal's device could help make sure that, if dark matter is hiding among them, scientists won't miss it.

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Dark matter flies ahead of normal matter in mega galaxy cluster collision

Astronomers have untangled a messy collision between two massive clusters of galaxies in which the clusters' vast clouds of dark matter have decoupled from the so-called normal matter. The two clusters each contain thousands of galaxies and are located billions of light-years away from Earth. As they plowed through each other, the dark matter -- an invisible substance that feels the force of gravity but emits no light -- sped ahead of the normal matter. The new observations are the first to directly probe the decoupling of the dark and normal matter velocities.

Galaxy clusters are among the largest structures in the universe, glued together by the force of gravity. Only 15 percent of the mass in such clusters is normal matter, the same matter that makes up planets, people, and everything you see around you. Of this normal matter, the vast majority is hot gas, while the rest is stars and planets. The remaining 85 percent of the cluster mass is dark matter.

During the tussle that took place between the clusters, known collectivity as MACS J0018.5+1626, the individual galaxies themselves largely went unscathed because so much space exists between them. But when the enormous stores of gas between the galaxies (the normal matter) collided, the gas became turbulent and superheated. While all matter, including both normal matter and dark matter, interacts via gravity, the normal matter also interacts via electromagnetism, which slows it down during a collision. So, while the normal matter became bogged down, the pools of dark matter within each cluster sailed on through.

Think of a massive collision between multiple dump trucks carrying sand, suggests Emily Silich, lead author of a new study describing the findings in The Astrophysical Journal. "The dark matter is like the sand and flies ahead." Silich is a graduate student working with Jack Sayers, research professor of physics at Caltech and principal investigator of the study.

The discovery was made using data from the Caltech Submillimeter Observatory (which was recently removed from its site on Maunakea in Hawai'i and will be relocated to Chile), the W.M. Keck Observatory on Maunakea, NASA's Chandra X-ray Observatory, NASA's Hubble Space Telescope, the European Space Agency's now-retired Herschel Space Observatory and Planck observatory (whose affiliated NASA science centers were based at Caltech's IPAC), and the Atacama Submillimeter Telescope Experiment in Chile. Some of the observations were made decades ago, while the full analysis using all the datasets took place over the past couple of years.

Such decoupling of dark and normal matter has been seen before, most famously in the Bullet Cluster. In that collision, the hot gas can be seen clearly lagging behind the dark matter after the two galaxy clusters shot through each other. The situation that took place in MACS J0018.5+1626 (referred to subsequently as MACS J0018.5) is similar, but the orientation of the merger is rotated, roughly 90 degrees relative to that of the Bullet Cluster. In other words, one of the massive clusters in MACS J0018.5 is flying nearly straight toward Earth while the other one is rushing away. That orientation gave researchers a unique vantagepoint from which to, for the first time, map out the velocity of both the dark matter and normal matter and elucidate how they decouple from each other during a galaxy cluster collision.

"With the Bullet Cluster, it's like we are sitting in a grandstand watching a car race and are able to capture beautiful snapshots of the cars moving from left to right on the straightaway," says Sayers. "In our case, it's more like we are on the straightaway with a radar gun, standing in front of a car as it comes at us and are able to obtain its speed."

To measure the speed of the normal matter, or gas, in the cluster, researchers used an observational method known as the kinetic Sunyaev-Zel'dovich (SZ) effect. Sayers and his colleagues made the first observational detection of the kinetic SZ effect on an individual cosmic object, a galaxy cluster named MACS J0717, back in 2013, using data from CSO (the first SZ effect observations taken of MACS J0018.5 date back to 2006).

The kinetic SZ effect occurs when photons from the early universe, the cosmic microwave background (CMB), scatter off electrons in hot gas on their way toward us on Earth. The photons undergo a shift, called a Doppler shift, due to the motions of the electrons in the gas clouds along our line of sight. By measuring the change in brightness of the CMB due to this shift, researchers can determine the speed of gas clouds within galaxy clusters.

"The Sunyaev-Zeldovich effects were still a very new observational tool when Jack and I first turned a new camera at the CSO on galaxy clusters in 2006, and we had no idea there would be discoveries like this," says Sunil Golwala, professor of physics and Silich's faculty PhD advisor. "We look forward to a slew of new surprises when we put next-generation instruments on the telescope at its new home in Chile."

By 2019, the researchers had made these kinetic SZ measurements in several galaxy clusters, which told them the speed of the gas, or normal matter. They had also used Keck to learn the speed of the galaxies in the cluster, which told them by proxy the speed of the dark matter (because the dark matter and galaxies behave similarly during the collision). But at this stage in the research, the team had a limited understanding of the orientations of the clusters. They only knew that one of them, MACS J0018.5, showed signs of something strange going on -- the hot gas, or normal matter, was traveling in the opposite direction to the dark matter.

"We had this complete oddball with velocities in opposite directions, and at first we thought it could be a problem with our data. Even our colleagues who simulate galaxy clusters didn't know what was going on," Sayers says. "And then Emily got involved and untangled everything."

For part of her PhD thesis, Silich tackled the conundrum of MACS J0018.5. She turned to data from the Chandra X-ray Observatory to reveal the temperature and location of the gas in the clusters as well as the degree to which the gas was being shocked. "These cluster collisions are the most energetic phenomena since the Big Bang," Silich says. "Chandra measures the extreme temperatures of the gas and tells us about the age of the merger and how recently the clusters collided." The team also worked with Adi Zitrin of the Ben-Gurion University of the Negev in Israel to use Hubble data to map the dark matter using a method known as gravitational lensing.

Additionally, John ZuHone of the Center for Astrophysics at Harvard & Smithsonian helped the team simulate the cluster smashup. These simulations were used in combination with data from the various telescopes to ultimately determine the geometry and evolutionary stage of the cluster encounter. The scientists found that, prior to colliding, the clusters were moving toward each other at approximately 3000 kilometers/second, equal to roughly one percent of the speed of light. With a more complete picture of what was going on, the researchers were able to figure out why the dark matter and normal matter appeared to be traveling in opposite directions. Though the scientists say it's hard to visualize, the orientation of the collision, coupled with the fact that dark matter and normal matter had separated from each other, explains the oddball velocity measurements.

In the future, the researchers hope that more studies like this one will lead to new clues about the mysterious nature of dark matter. "This study is a starting point to more detailed studies into the nature of dark matter," Silich says. "We have a new type of direct probe that shows how dark matter behaves differently from normal matter."

Sayers, who recalls first collecting the CSO data on this object almost 20 years ago, says, "It took us a long time to put all the puzzle pieces together, but now we finally know what's going on. We hope this leads to a whole new way to study dark matter in clusters."

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Astrophysicists uncover supermassive blackhole/dark matter connection in solving the 'final parsec problem'

Researchers have found a link between some of the largest and smallest objects in the cosmos: supermassive black holes and dark matter particles.

Their new calculations reveal that pairs of supermassive black holes (SMBHs) can merge into a single larger black hole because of previously overlooked behaviour of dark matter particles, proposing a solution to the longstanding "final parsec problem" in astronomy.

The research is described in Self-interacting dark matter solves the final parsec problem of supermassive black hole mergers published this month in the journal Physical Review Letters.

In 2023, astrophysicists announced the detection of a "hum" of gravitational waves permeating the universe. They hypothesized that this background signal emanated from millions of merging pairs of SMBHs each billions of times more massive than our Sun.

However, theoretical simulations showed that as pairs of these mammoth celestial objects spiral closer together, their approach stalls when they are roughly a parsec apart -- a distance of about three light years -- thereby preventing a merger.

Not only did this "final parsec problem" conflict with the theory that merging SMBHs were the source of the gravitational wave background, it was also at odds with the theory that SMBHs grow from the merger of less massive black holes.

"We show that including the previously overlooked effect of dark matter can help supermassive black holes overcome this final parsec of separation and coalesce," says paper co-author Gonzalo Alonso-Álvarez, a postdoctoral fellow in the Department of Physics at the University of Toronto and the Department of Physics and Trottier Space Institute at McGill University. "Our calculations explain how that can occur, in contrast to what was previously thought."

The paper's co-authors include Professor James Cline from McGill University and the CERN Theoretical Physics Department in Switzerland and Caitlyn Dewar, a master of science student in physics at McGill.

SMBHs are thought to lie in the centres of most galaxies and when two galaxies collide, the SMBHs fall into orbit around each other. As they revolve around each other, the gravitational pull of nearby stars tugs at them and slows them down. As a result, the SMBHs spiral inward toward a merger.

Previous merger models showed that when the SMBHs approached to within roughly a parsec, they begin to interact with the dark matter cloud or halo in which they are embedded. They indicated that the gravity of the spiraling SMBHs throws dark matter particles clear of the system and the resulting sparsity of dark matter means that energy is not drawn from the pair and their mutual orbits no longer shrink.

While those models dismissed the impact of dark matter on the SMBH's orbits, the new model from Alonso-Álvarez and his colleagues reveals that dark matter particles interact with each other in such a way that they are not dispersed. The density of the dark matter halo remains high enough that interactions between the particles and the SMBHs continue to degrade the SMBH's orbits, clearing a path to a merger.

"The possibility that dark matter particles interact with each other is an assumption that we made, an extra ingredient that not all dark matter models contain," says Alonso-Álvarez. "Our argument is that only models with that ingredient can solve the final parsec problem."

The background hum generated by these colossal cosmic collisions is made up of gravitational waves of much longer wavelength than those first detected in 2015 by astrophysicists operating the Laser Interferometer Gravitational-Wave Observatory (LIGO). Those gravitational waves were generated by the merger of two black holes, both some 30 times the mass of the Sun.

The background hum has been detected in recent years by scientists operating the Pulsar Timing Array. The array reveals gravitational waves by measuring minute variations in signals from pulsars, rapidly rotating neutron stars that emit strong radio pulses.

"A prediction of our proposal is that the spectrum of gravitational waves observed by pulsar timing arrays should be softened at low frequencies," says Cline. "The current data already hint at this behavior, and new data may be able to confirm it in the next few years."

In addition to providing insight into SBMH mergers and the gravitational wave background signal, the new result also provides a window into the nature of dark matter.

"Our work is a new way to help us understand the particle nature of dark matter," says Alonso-Álvarez. "We found that the evolution of black hole orbits is very sensitive to the microphysics of dark matter and that means we can use observations of supermassive black hole mergers to better understand these particles."

For example, the researchers found that the interactions between dark matter particles they modeled also explains the shapes of galactic dark matter halos.

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Astronomers' simulations support dark matter theory

Computer simulations by astronomers support the idea that dark matter -- matter that no one has yet directly detected but which many physicists think must be there to explain several aspects of the observable universe -- exists, according to the researchers, who include those at the University of California, Irvine.

The work addresses a fundamental debate in astrophysics -- does invisible dark matter need to exist to explain how the universe works the way it does, or can physicists explain how things work based solely on the matter we can directly observe? Currently, many physicists think something like dark matter must exist to explain the motions of stars and galaxies.

"Our paper shows how we can use real, observed relationships as a basis to test two different models to describe the universe," said Francisco Mercado, lead author and recent Ph.D. graduate from the UC Irvine Department of Physics & Astronomy who is now a postdoctoral scholar at Pomona College. "We put forth a powerful test to discriminate between the two models."

The test involved running computer simulations with both types of matter -- normal and dark -- to explain the presence of intriguing features measured in real galaxies. The team reported their results in Monthly Notices of the Royal Astronomy Society.

The features in galaxies the team found "are expected to appear in a universe with dark matter but would be difficult to explain in a universe without it," said Mercado. "We show that such features appear in observations of many real galaxies. If we take these data at face value, this reaffirms the position of the dark matter model as the one that best describes the universe we live in."

These features Mercado noted describe patterns in the motions of stars and gas in galaxies that seem to only be possible in a universe with dark matter.

"Observed galaxies seem to obey a tight relationship between the matter we see and the inferred dark matter we detect, so much so that some have suggested that what we call dark matter is really evidence that our theory of gravity is wrong," said co-author James Bullock, professor of physics at UCI and dean of the UCI School of Physical Sciences. "What we showed is that not only does dark matter predict the relationship, but for many galaxies it can explain what we see more naturally than modified gravity. I come away even more convinced that dark matter is the right model."

The features also appear in observations made by proponents of a dark matter-free universe. "The observations we examined -- the very observations where we found these features -- were conducted by adherents of dark matter-free theories," said co-author Jorge Moreno, associate professor of physics and astronomy at Pomona College. "Despite their obvious presence, little-to-no analysis was performed on these features by that community. It took folks like us, scientists working with both regular and dark matter, to start the conversation."

Moreno added that he expects debate within his research community to follow in the wake of the study, but that there may be room for common ground, as the team also found that such features only appear in their simulations when there is both dark matter and normal matter in the universe.

"As stars are born and die, they explode into supernovae, which can shape the centers of galaxies, naturally explaining the existence of these features," said Moreno. "Simply put, the features we examined in observations require both the existence of dark matter and the incorporation of normal-matter physics."

Now that the dark matter model of the universe appears to be the leading one, the next step, Mercado explained, is to see if it remains consistent across a dark matter universe.

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Physicists solve puzzle about ancient galaxy found by Webb telescope

Last September, the James Webb Space Telescope, or JWST, discovered JWST-ER1g, a massive ancient galaxy that formed when the universe was just a quarter of its current age. Surprisingly, an Einstein ring is associated with this galaxy. That's because JWST-ER1g acts as a lens and bends light from a distant source, which then appears as a ring -- a phenomenon called strong gravitational lensing, predicted in Einstein's theory of general relativity.

The total mass enclosed within the Einstein radius -- the radius of the Einstein ring -- has two components: stellar and dark matter components.

"If we subtract the stellar mass from the total mass, we get the dark matter mass within the Einstein radius," said Hai-Bo Yu, a professor of physics and astronomy at the University of California, Riverside, whose team has published new work about JWST-ER1g in the journal The Astrophysical Journal Letters. "But the value for the dark matter mass seems higher than expected. This is puzzling. In our paper, we offer an explanation."

A dark matter halo is the halo of invisible matter that permeates and surrounds a galaxy like JWST-ER1g. Although dark matter has never been detected in laboratories, physicists are confident dark matter, which makes up 85% of the universe's matter, exists.

"When ordinary matter -- pristine gas and stars -- collapses and condenses into the dark matter halo of JWST-ER1g, it may be compressing the halo, leading to a high density," said Demao Kong, a second-year graduate student at UCR, who led the analysis. "Our numerical studies show that this mechanism can explain the high dark matter density of JWST-ER1g -- more dark matter mass in the same volume, resulting in higher density."

According to Daneng Yang, a postdoctoral researcher at UCR and co-author on the paper, JWST-ER1g, formed 3.4 billion years after the Big Bang, provides "a great chance to learn about dark matter."

"This strong lensing object is unique because it has a perfect Einstein ring, from which we can obtain valuable information about the total mass within the Einstein radius, a critical step for testing dark matter properties," he said.

Launched on Christmas Day in 2021, NASA's JWST is an orbiting infrared observatory. Also called Webb, it is designed to answer questions about the universe. It is the largest, most complex, and powerful space telescope ever built.

"JWST provides an unprecedented opportunity for us to observe ancient galaxies formed when the universe was young," Yu said. "We expect to see more surprises from JWST and learn more about dark matter soon."

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Astrophysicist's research could provide a hint in the search for dark matter

Dark matter is one of science's greatest mysteries. Although it is believed to make up about 85 percent of the cosmos, scientists know very little about its fundamental nature. Research by Clemson University postdoctoral fellow Alex McDaniel provides some of the most stringent constraints on the nature of dark matter yet. It also revealed a small hint of a signal that, if real, could be confirmed in the next decade or so.

Dark matter is one of science's greatest mysteries.

It doesn't absorb, reflect or emit light, so we can't see it. But its presence is implied by the gravitational effects it appears to have on galaxies.

Although dark matter makes up about 85 percent of the cosmos, scientists know very little about its fundamental nature.

Theories abound, and research by Clemson University postdoctoral fellow Alex McDaniel provides some of the most stringent constraints on the nature of dark matter yet.

His research also reveals a small hint of a signal that, if real, could be confirmed sometime in the next decade or so.

"With data collection and new discoveries in the future, this small hint could potentially turn into a very concrete detection of a dark matter model," McDaniel said.

Detecting dark matter would be groundbreaking.

"Dark matter is one of the most important things in astrophysics, and we know next to nothing about it. Discovering it will be a tremendous breakthrough," said Marco Ajello, an associate professor in the Clemson Department of Physics and Astronomy and McDaniel's adviser.

"Whoever discovers may win a Nobel Prize. It's that big."

In this work, McDaniel and collaborators were searching dwarf galaxies for dark matter that self-annihilates into ordinary matter and gamma rays, a form of light at the highest energy levels.

Dwarf galaxies are ideal for study because they are small, rich in dark matter and mostly lack other astrophysics phenomena such as gas, dust and supernova that could contaminate the findings.

"We look for these because, ideally, they give us a clean signal or allow us to rule out certain particle theories," McDaniel said.

Some models predict that dark matter has a certain mass or cross section, which is the probability of a specific event occurring due to the interaction of particles.

That would determine what researchers would expect to see in gamma rays.

If they don't see that, they can rule out those masses and cross sections, he said.

"In this paper, we do more ruling out, saying that dark matter can't have those masses or cross section," said Chris Karwin, a former postdoc at Clemson and a co-author of the study.

Karwin is now a postdoctoral fellow at the NASA Goddard Space Flight Center.

"But compared to previous studies, we do start to see a hint of something that might be a signal from these systems."

McDaniel's study uses the larger samples that include additional discovered dwarf galaxies and larger amounts of data than previous studies.

He included about 50 dwarf galaxies in his study but said that with new, more powerful telescopes coming online in the near future, he expects that number to increase to 150-200.

"The new telescopes are basically dwarf galaxy detectors," he said.

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New research suggests that our universe has no dark matter

The current theoretical model for the composition of the universe is that it's made of 'normal matter,' 'dark energy' and 'dark matter.' A new uOttawa study challenges this.

A University of Ottawa study published today challenges the current model of the universe by showing that, in fact, it has no room for dark matter.

In cosmology, the term "dark matter" describes all that appears not to interact with light or the electromagnetic field, or that can only be explained through gravitational force.

We can't see it, nor do we know what it's made of, but it helps us understand how galaxies, planets and stars behave.

Rajendra Gupta, a physics professor at the Faculty of Science, used a combination of the covarying coupling constants (CCC) and "tired light" (TL) theories (the CCC+TL model) to reach this conclusion.

This model combines two ideas -- about how the forces of nature decrease over cosmic time and about light losing energy when it travels a long distance.

It's been tested and has been shown to match up with several observations, such as about how galaxies are spread out and how light from the early universe has evolved.

This discovery challenges the prevailing understanding of the universe, which suggests that roughly 27% of it is composed of dark matter and less than 5% of ordinary matter, remaining being the dark energy.

Challenging the need for dark matter in the universe

"The study's findings confirm that our previous work ("JWST early Universe observations and ΛCDM cosmology") about the age of the universe being 26.7billionyears has allowed us to discover that the universe does not require dark matter to exist," explains Gupta.

"In standard cosmology, the accelerated expansion of the universe is said to be caused by dark energy but is in fact due to the weakening forces of nature as it expands, not due to dark energy."

"Redshifts" refer to when light is shifted toward the red part of the spectrum.

The researcher analyzed data from recent papers on the distribution of galaxies at low redshifts and the angular size of the sound horizon in the literature at high redshift.

"There are several papers that question the existence of dark matter, but mine is the first one, to my knowledge, that eliminates its cosmological existence while being consistent with key cosmological observations that we have had time to confirm," says Gupta.

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Bright galaxies put dark matter to the test

For the past year and a half, the James Webb Space Telescope has delivered astonishing images of distant galaxies formed not long after the Big Bang, giving scientists their first glimpses of the infant universe. Now, a group of astrophysicists has upped the ante: Find the tiniest, brightest galaxies near the beginning of time itself, or scientists will have to totally rethink their theories about dark matter.

The team, led by UCLA astrophysicists, ran simulations that track the formation of small galaxies after the Big Bang and included, for the first time, previously neglected interactions between gas and dark matter. They found that the galaxies created are very tiny, much brighter, and form more quickly than they do in typical simulations that don''t take these interactions into account, instead revealing much fainter galaxies.

Small galaxies, also called dwarf galaxies, are present throughout the universe, and are often thought to represent the earliest type of galaxy. Small galaxies are thus especially interesting to scientists studying the origins of the universe. But the small galaxies they find don't always match what they think they should find. Those closest to the Milky Way spin quicker or are not as dense as in simulations, indicating that the models might have omitted something, such as these gas-dark matter interactions.

The new research, published in The Astrophysical Journal Letters, improves the simulations by adding dark matter interactions with gas and finds that these faint galaxies may have been much brighter than expected early in the universe's history, when they were just beginning to form. The authors suggest scientists should try to find small galaxies that are much brighter than expected using telescopes like the Webb telescope. If they only find faint ones, then some of their ideas about dark matter might be wrong.

Dark matter is a type of hypothetical matter that does not interact with electromagnetism or light. Thus, it is impossible to observe using optics, electricity or magnetism. But dark matter does interact with gravity, and its presence has been inferred from the gravitational effects it has on ordinary matter -- the stuff that makes up all the observable universe. Even though 84% of the matter in the universe is thought to be made of dark matter, it has never been detected directly.

All galaxies are surrounded by a vast halo of dark matter, and scientists think that dark matter was essential to their formation. The "standard cosmological model" astrophysicists use to understand galaxy formation describes how clumps of dark matter in the very early universe drew in ordinary matter through gravity, causing the formation of stars and creating the galaxies we see today. Because most dark matter particles -- called cold dark matter -- are thought to move much slower than the speed of light, this process of accumulation would have occurred gradually.

But over 13 billion years ago, prior to the formation of the first galaxies, ordinary matter, consisting of hydrogen and helium gas from the Big Bang, and dark matter were moving relative to one another. The gas streamed at supersonic velocities past dense thickets of more slowly moving dark matter that should have pulled it in to form galaxies.

"Indeed, in models that do not take streaming into account, this is exactly what happens," said Claire Williams, a UCLA doctoral student and the paper's first author. "Gas is attracted to the gravitational pull of dark matter, forms clumps and knots so dense that hydrogen fusion can occur, and thus forms stars like our sun."

But Williams and co-authors on the Supersonic Project team, a group of astrophysicists from the United States, Italy and Japan led by UCLA physics and astronomy professor Smadar Naoz, found if they added the streaming effect of different velocities between dark and ordinary matter to the simulations, the gas landed far away from the dark matter and was prevented from forming stars right away. When the accumulated gas fell back into the galaxy millions of years later, a massive burst of star formation occurred all at once. Because these galaxies had many more young, hot, luminous stars than ordinary small galaxies for a time, they shone much brighter.

"While the streaming suppressed star formation in the smallest galaxies, it also boosted star formation in dwarf galaxies, causing them to outshine the non-streaming patches of the universe," Williams said. "We predict that the Webb telescope will be able to find regions of the universe where galaxies will be brighter, heightened by this velocity. The fact that they should be so bright might make it easier for the telescope to discover these small galaxies, which are typically extremely hard to detect only 375 million years after the Big Bang."

Because dark matter is impossible to study directly, searching for bright patches of galaxies in the early universe could offer an effective test for theories about dark matter, which has been fruitless so far.

"The discovery of patches of small, bright galaxies in the early universe would confirm that we are on the right track with the cold dark matter model because only the velocity between two kinds of matter can produce the type of galaxy we're looking for," said Naoz, the Howard and Astrid Preston Professor of Astrophysics. "If dark matter does not behave like standard cold dark matter and the streaming effect isn't present, then these bright dwarf galaxies won't be found and we need to go back to the drawing board."

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Magnetic fields in the cosmos: Dark matter could help us discover their origin

The mini-halos of dark matter scattered throughout the Cosmos could function as highly sensitive probes of primordial magnetic fields. This is what emerges from a theoretical study conducted by SISSA and published in Physical Review Letters. Present on immense scales, magnetic fields are found everywhere in the Universe. However, their origin are still subjects of debate among scholars. An intriguing possibility is that magnetic fields originated near the birth of the universe itself, that is they are primordial magnetic fields.

In the study, researchers showed that if magnetic fields are indeed primordial then it could cause an increase in dark matter density perturbations on small scales.

The ultimate effect of this process would be the formation of mini-halos of dark matter, which, if detected would hint towards a primordial nature of magnetic fields.

Thus, in an apparent paradox, the invisible part of our Universe could be useful in resolving the nature of a component of the visible one.

Shedding light on the formation of Magnetic Fields

"Magnetic fields are ubiquitous in the Cosmos," explains Pranjal Ralegankar of SISSA, the author of the research.

"A possible theory regarding their formation suggests that those observed so far could be produced in the early stages of our Universe. However, this proposition lacks explanation in the standard model of physics. To shed light on this aspect and find a way to detect "primordial" magnetic fields, with this work we propose a method that we could define as 'indirect.' Our approach is based on a question: What is the influence of magnetic fields on dark matter?" It is known that there is no direct interaction.

Still, as Ralegankar explains, "there is an indirect one that occurs through gravity."

Right from the primordial Universe

Primordial magnetic fields can enhance density perturbations of electrons and protons in the primordial Universe.

When these become too large, they influence the magnetic fields themselves.

The consequence is the suppression of fluctuations on a small scale.

Ralegankar explains: "In the study, we show something unexpected. The growth in baryon density gravitationally induces the growth of dark matter perturbations without the possibility of subsequent cancellation. This would result in their collapse on small scales, producing mini-halos of dark matter." The consequence, continues the author, is that although fluctuations in the density of baryonic matter are cancelled, they would leave traces through the mini-halos, all solely through gravitational interactions.

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Finding explanation for Milky Way's warp

The Milky Way is often depicted as a flat, spinning disk of dust, gas, and stars. But if you could zoom out and take an edge-on photo, it actually has a distinctive warp -- as if you tried to twist and bend a vinyl LP.

Though scientists have long known through observational data that the Milky Way is warped and its edges are flared like a skirt, no one could explain why.

Now, Harvard astronomers at the Center for Astrophysics | Harvard and Smithsonian (CfA) have performed the first calculations that fully explain this phenomenon, with compelling evidence pointing to the Milky Way's envelopment in an off-kilter halo of dark matter. The work also bolsters current thinking about how the galaxy evolved and may offer clues into some of the mysteries of dark matter.

The new calculations were led by Jiwon Jesse Han, a Griffin Graduate School of Arts and Sciences student affiliated with the CfA. Published in Nature Astronomy, the work includes co-authors Charlie Conroy and Lars Hernquist, both faculty members at the CfA and in the Department of Astronomy.

Our galaxy is located inside a diffuse cloud called the stellar halo, which extends much farther out into the universe. In groundbreaking work published last year, the Harvard team deduced that the stellar halo is tilted and elliptical in shape, like a zeppelin or football.

Building on that, the team assumed the same shape for the dark matter halo, the larger entity that encompasses everything in and around the Milky Way. Dark matter makes up 80 percent of the galaxy's mass but is invisible because it doesn't interact with light, so the shape of that halo must be inferred. Using models to calculate the orbits of stars within a tilted, oblong dark matter halo, the team found a near-perfect match to existing observations of a warped, flared galaxy.

"A tilted dark halo is actually fairly common in simulations, but no one had explored its effect on the Milky Way," Conroy said. "It turns out that the tilt is an elegant way to explain both the magnitude and direction of our galaxy's wobbly disk."

Scientists had long surmised that the Milky Way formed due to a galactic collision; the astronomers' work further underscores that hypothesis.

"If the galaxy was just evolving on its own, it would have had this nice, spherical halo, this nice, flat disk," Han said. "So the fact that the halo is tilted and has a football-like shape suggests that our galaxy experienced a merger event, where two galaxies collide."

Their calculation of the dark matter halo's probable shape may also provide clues as to the properties and particle nature of dark matter itself, which remain unsolved mysteries in physics. "The fact that the galaxy is not spherical in our data implies that there is some limit to which dark matter can interact with itself," Han explained.

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Pulsars may make dark matter glow

The central question in the ongoing hunt for dark matter is: what is it made of? One possible answer is that dark matter consists of particles known as axions. A team of astrophysicists, led by researchers from the universities of Amsterdam and Princeton, has now shown that if dark matter consists of axions, it may reveal itself in the form of a subtle additional glow coming from pulsating stars.

Dark matter may be the most sought-for constituent of our universe. Surprisingly, this mysterious form of matter, that physicist and astronomers so far have not been able to detect, is assumed to make up an enormous part of what is out there. No less than 85% of matter in the universe is suspected to be 'dark', presently only noticeable through the gravitational pull it exerts on other astronomical objects. Understandably, scientists want more. They want to really see dark matter -- or at the very least, detect its presence directly, not just infer it from gravitational effects. And, of course: they want to know what it is.

Cleaning up two problems

One thing is clear: dark matter cannot be the same type of matter that you and I are made of. If that were to be the case, dark matter would simply behave like ordinary matter -- it would form objects like stars, light up, and no longer be 'dark'. Scientists are therefore looking for something new -- a type of particle that nobody has detected yet, and that probably only interacts very weakly with the types of particles that we know, explaining why this constituent of our world so far has remained elusive.

There are plenty of clues for where to look. One popular assumption is that dark matter could be made of axions. This hypothetical type of particle was first introduced in the 1970s to resolve a problem that had nothing to do with dark matter. The separation of positive and negative charges inside the neutron, one of the building blocks of ordinary atoms, turned out to be unexpectedly small. Scientists of course wanted to know why. It turned out that the presence of a hitherto undetected type of particle, interacting very weakly with the neutron's constituents, could cause exactly such an effect. The later Nobel Prize winner Frank Wilczek came up with a name for the new particle: axion -- not just similar to other particle names like proton, neutron, electron and photon, but also inspired by a laundry detergent of the same name. The axion was there to clean up a problem.

In fact, despite never being detected, it might clean up two. Several theories for elementary particles, including string theory, one of the leading candidate theories to unify all forces in nature, appeared to predict that axion-like particles could exist. If axions were indeed out there, could they also constitute part or even all of the missing dark matter? Perhaps, but an additional question that haunted all dark matter research was just as valid for axions: if so, then how can we see them? How does one make something 'dark' visible?

Shining a light on dark matter

Fortunately, it seems that for axions there may be a way out of this conundrum. If the theories that predict axions are correct, they are not only expected to be mass-produced in the universe, but some axions could also be converted into light in the presence of strong electromagnetic fields. Once there is light, we can see. Could this be the key to detect axions -- and therefore to detect dark matter?

To answer that question, scientists first had to ask themselves where in the universe the strongest known electric and magnetic fields occur. The answer is: in regions surrounding rotating neutron stars also known as pulsars. These pulsars -- short for 'pulsating stars' -- are dense objects, with a mass roughly the same as that of our Sun, but a radius that is around 100,000 times smaller, only about 10 km. Being so small, pulsars spin with enormous frequencies, emitting bright narrow beams of radio emission along their axis of rotation. Similar to a lighthouse, the pulsar's beams can sweep across the Earth, making the pulsating star easily observable.

However, the pulsar's enormous spin does more. It turns the neutron star into an extremely strong electromagnet. That, in turn, could mean that pulsars are very efficient axion factories. Every single second an average pulsar would be capable of producing a 50-digit number of axions. Because of the strong electromagnetic field around the pulsar, a fraction of these axions could convert into observable light. That is: if axions exist at all -- but the mechanism can now be used to answer just that question. Just look at pulsars, see if they emit extra light, and if they do, determine whether this extra light could be coming from axions.

Simulating a subtle glow

As always in science, actually performing such an observation is of course not that simple. The light emitted by axions -- detectable in the form of radio waves -- would only be a small fraction of the total light that these bright cosmic lighthouses send our way. One needs to know very precisely what a pulsar without axions would look like, and what a pulsar with axions would look like, to be able to see the difference -- let alone to quantify that difference and turn it into a measurement of an amount of dark matter.

This is exactly what a team of physicists and astronomers have now done. In a collaborative effort between the Netherlands, Portugal and the USA, the team has constructed a comprehensive theoretical framework which allows for the detailed understanding of how axions are produced, how axions escape the gravitational pull of the neutron star, and how, during their escape, they convert into low energy radio radiation.

The theoretical results were then put on a computer to model the production of axions around pulsars, using state-of-the-art numerical plasma simulations that were originally developed to understand the physics behind how pulsars emit radio waves. Once virtually produced, the propagation of the axions through the electromagnetic fields of the neutron star was simulated. This allowed the researchers to quantitatively understand the subsequent production of radio waves and model how this process would provide an additional radio signal on top of the intrinsic emission generated from the pulsar itself.

Putting axion models to a test

The results from theory and simulation were then put to a first observational test. Using observations from 27 nearby pulsars, the researchers compared the observed radio waves to the models, to see if any measured excess could provide evidence for the existence of axions. Unfortunately, the answer was 'no' -- or perhaps more optimistically: 'not yet'. Axions do not immediately jump out to us, but perhaps that was not to be expected. If dark matter were to give up its secrets that easily, it would already have been observed a long time ago.

The hope for a smoking-gun detection of axions, therefore, is now on future observations. Meanwhile, the current non-observation of radio signals from axions is an interesting result in itself. The first comparison between simulations and actual pulsars has placed the strongest limits to date on the interaction that axions can have with light.

Read more at Science Daily

New clues to the nature of elusive dark matter

A team of international researchers, led by experts at the University of Adelaide, has uncovered further clues in the quest for insights into the nature of dark matter.

"Dark matter makes up 84 per cent of the matter in the universe but we know very little about it," said Professor Anthony Thomas, Elder Professor of Physics, University of Adelaide.

"The existence of dark matter has been firmly established from its gravitational interactions, yet its precise nature continues to elude us despite the best efforts of physicists around the world."

"The key to understanding this mystery could lie with the dark photon, a theoretical massive particle that may serve as a portal between the dark sector of particles and regular matter."

Regular matter, of which we and our physical world are made up of, is far less abundant than dark matter: five times more dark matter exists than regular matter. Finding out more about dark matter is one of the greatest challenges for physicists around the world.

The dark photon is a hypothetical hidden sector particle, proposed as a force carrier similar to the photon of electromagnetism but potentially connected to dark matter. Testing existing theories about dark matter is one of the approaches that scientists such as Professor Thomas, along with colleagues Professor Martin White, Dr Xuangong Wang and Nicholas Hunt-Smith, who are members of the Australian Research Council (ARC) Centre of Excellence for Dark Matter Particle Physics, are pursuing in order to gain more clues into this elusive but highly important substance.

"In our latest study, we examine the potential effects that a dark photon could have on the complete set of experimental results from the deep inelastic scattering process," said Professor Thomas.

Analysis of the by-products of the collisions of particles accelerated to extremely high energies gives scientists good evidence of the structure of the subatomic world and the laws of nature governing it.

In particle physics, deep inelastic scattering is the name given to a process used to probe the insides of hadrons (particularly the baryons, such as protons and neutrons), using electrons, muons and neutrinos.

"We have made use of the state-of-the-art Jefferson Lab Angular Momentum (JAM) parton distribution function global analysis framework, modifying the underlying theory to allow for the possibility of a dark photon," said Professor Thomas.

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Dark matter halos measured around ancient quasars

At the center of every galaxy is a supermassive black hole. Beyond a certain size, these become active, emitting huge amounts of radiation, and are then called quasars. It is thought these are activated by the presence of massive dark matter halos (DMH) surrounding the galaxy, directing matter towards the center, feeding the black hole. A team including researchers from the University of Tokyo have, for the first time, surveyed hundreds of ancient quasars and found this behavior is very consistent throughout history. This is surprising, as many large-scale processes show variation throughout the life of the universe, so the mechanism of quasar activation could have implications for the evolution of the entire universe.

Measuring the mass of DMHs is not easy; it's famously a very elusive substance, if substance is even the right word to use, given the actual nature of dark matter is unknown. We only know it exists at all due to its gravitational impact on large structures such as galaxies. Thus, dark matter can only be measured by making observations about its gravitational effects on things. This includes the way it might pull on something or affect its movement, or through the lensing (bending of light) of objects behind a suspected area of dark matter.

The challenge becomes greater at large distances, given how weak the light from more distant, and therefore ancient, phenomena can be. But this did not stop Professor Nobunari Kashikawa from the Department of Astronomy, and his team, from trying to answer a long-standing question in astronomy: How are black holes born, and how do they grow? The researchers are especially keen to explore this in relation to supermassive black holes, the largest kind, which exist in the heart of every galaxy. These would be very difficult to study were it not for the fact that some grow so massive they begin to output incredibly powerful jets of matter or spheres of radiation that in either case become what we call quasars. These are so powerful that even at large distances, we can now observe them using modern techniques.

"We measured for the first time the typical mass for dark matter halos surrounding an active black hole in the universe about 13 billion years ago," said Kashikawa. "We find the DMH mass of quasars is pretty constant at about 10 trillion times the mass of our sun. Such measurements have been made for more recent DMH around quasars, and those measurements are strikingly similar to what we see for more ancient quasars. This is interesting because it suggests there is a characteristic DMH mass which seems to activate a quasar, regardless of whether it happened billions of years ago or right now."

Quasars at great distances appear faint, as the light which left them long ago has spread out, was absorbed by intervening matter, and has been stretched into nearly invisible infrared wavelengths due to the universe expanding over time. So Kashikawa and his team, whose project began in 2016, used multiple surveys of the sky which incorporated a range of different instruments, the main one being Japan's Subaru Telescope, located in U.S. state of Hawaii.

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New cosmological constraints on the nature of dark matter

New research has revealed the distribution of dark matter in never before seen detail, down to a scale of 30,000 light-years. The observed distribution fluctuations provide better constraints on the nature of dark matter.

Mysterious dark matter accounts for most of the matter in the Universe. Dark matter is invisible and makes itself know only through its gravitational effects. Dark matter has never been isolated in a laboratory, so researchers must rely on "natural experiments" to study it.

One type of natural experiment is a gravitational lens. Sometimes by random chance, two objects at different distances in the Universe will lie along the same line-of-sight when seen from Earth. When this happens, the spatial curvature caused by the matter around the foreground object acts like a lens, bending the path of light from the background object and making a lensed image. However, it is difficult to achieve the high resolution to detect clumps of dark matter which are less massive than galaxies in natural experiments, so the exact nature of dark matter has been poorly constrained.

A team of Japanese researchers led by Professor Kaiki Taro Inoue at Kindai University used ALMA (Atacama Large Millimeter/submillimeter Array) to study the gravitational lens system known as MG J0414+0534 in the direction of the constellation Taurus. In this system, the foreground object forms not one, but four images of the background object due to the gravitational force of a massive galaxy acting on the light. With the help of the bending effect and their new data analysis method, the team was able to detect fluctuations in the dark matter distribution along the line-of-sight in higher resolution than ever before, down to a scale of 30,000 light-years.

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The puzzle of the galaxy with no dark matter

A team of scientists, led by the researcher at the IAC and the University of La Laguna (ULL) Sebastién Comerón, has found that the galaxy NGC 1277 does not contain dark matter.This is the first time that a massive galaxy (it has a mass several times that of the Milky Way) does not show evidence for this invisible component of the universe. "This result does not fit in with the currently accepted cosmological models, which include dark matter" explains Comerón.

In the current standard model cosmology massive galaxies contain substantial quantities of dark matter, a type of matter which does not interact in the same way as normal matter; the only evidence for its existence is the strong gravitational pull which it exerts on the stars and the gas nearby, and this interacton is observable.

NGC 1277 is considered a prototype "relic galaxy" which means a galaxy which has had no interactions with its neighbours. Galaxies of this type are very rare, and they are considered the remnants of giant galaxies which formed in the early days of the universe.

"The importance of relic galaxies in helping us to understand how the first galaxies formed was the reason we decided to observe NGC 1277 with an integral field spectrograph" explains Comerón. "From the spectra we made kinematic maps which enabled us to work out the distribution of mass within the galaxy out to a radius of some 20,000 light years" he adds.

The team discovered that the mass distribution in NGC 1277 was just the distribution of the stars, and from this they inferred that within the radius observed there cannot be more than 5% of dark matter, although the observations are consistent with the complete absence of this component.

However, present cosmological models predict that a galaxy with the mass of NGC 1277 should have at least 10 % of their mass in the form of dark matter, with a maximum of 70 % in this form. "This discrepancy between the observations and what we would expect is a puzzle, and maybe even a challenge for the standard model" notes Ignacio Trujillo, a researcher at the IAC and the ULL, who participated in the study.

The article suggests two possible explanations for the lack of dark matter in NGC 1277. "One is that the gravitational interaction with the surrounding medium within the galaxy cluster in which this galaxy is situated has stripped out the dark matter" comments Anna Ferré-Mateu, a researcher at the IAC and the ULL who also participated in the study. "The other is that the dark matter was driven out of the system when the galaxy formed by the merging of protogalactic fragments, which gave rise to the relic galaxy."

For the authors of the study neither of these explanations is fully satisfactory "so the puzzle of how a massive galaxy can form without dark matter remains a puzzle" insists Comerón. In order to continue researching the mystery the team plans to make new observations with the WEAVE instrument on the William Herschel Telescope (WHT) at the Roque de los Muchachos Observatory, in the Canary Island of La Palma.

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James Webb Telescope catches glimpse of possible first-ever 'dark stars'

Stars beam brightly out of the darkness of space thanks to fusion, atoms melding together and releasing energy. But what if there's another way to power a star?

A team of three astrophysicists -- Katherine Freese at The University of Texas at Austin, in collaboration with Cosmin Ilie and Jillian Paulin '23 at Colgate University -- analyzed images from the James Webb Space Telescope (JWST) and found three bright objects that might be "dark stars," theoretical objects much bigger and brighter than our sun, powered by particles of dark matter annihilating. If confirmed, dark stars could reveal the nature of dark matter, one of the deepest unsolved problems in all of physics.

"Discovering a new type of star is pretty interesting all by itself, but discovering it's dark matter that's powering this -- that would be huge," said Freese, director of the Weinberg Institute for Theoretical Physics and the Jeff and Gail Kodosky Endowed Chair in Physics at UT Austin.

Although dark matter makes up about 25% of the universe, its nature has eluded scientists. Scientists believe it consists of a new type of elementary particle, and the hunt to detect such particles is on. Among the leading candidates are Weakly Interacting Massive Particles. When they collide, these particles annihilate themselves, depositing heat into collapsing clouds of hydrogen and converting them into brightly shining dark stars. The identification of supermassive dark stars would open up the possibility of learning about the dark matter based on their observed properties.

The research is published in the Proceedings of the National Academy of Sciences.

Follow-up observations from JWST of the objects' spectroscopic properties -- including dips or excess of light intensity in certain frequency bands -- could help confirm whether these candidate objects are indeed dark stars.

Confirming the existence of dark stars might also help solve a problem created by JWST: There seem to be too many large galaxies too early in the universe to fit the predictions of the standard model of cosmology.

"It's more likely that something within the standard model needs tuning, because proposing something entirely new, as we did, is always less probable," Freese said. "But if some of these objects that look like early galaxies are actually dark stars, the simulations of galaxy formation agree better with observations."

The three candidate dark stars (JADES-GS-z13-0, JADES-GS-z12-0, and JADES-GS-z11-0) were originally identified as galaxies in December 2022 by the JWST Advanced Deep Extragalactic Survey (JADES). Using spectroscopic analysis, the JADES team confirmed the objects were observed at times ranging from about 320 million to 400 million years after the Big Bang, making them some of the earliest objects ever seen.

"When we look at the James Webb data, there are two competing possibilities for these objects," Freese said. "One is that they are galaxies containing millions of ordinary, population-III stars. The other is that they are dark stars. And believe it or not, one dark star has enough light to compete with an entire galaxy of stars."

Dark stars could theoretically grow to be several million times the mass of our sun and up to 10 billion times as bright as the sun.

"We predicted back in 2012 that supermassive dark stars could be observed with JWST," said Ilie, assistant professor of physics and astronomy at Colgate University. "As shown in our recently published PNAS article, we already found three supermassive dark star candidates when analyzing the JWST data for the four high redshift JADES objects spectroscopically confirmed by Curtis-Lake et al, and I am confident we will soon identify many more."

The idea for dark stars originated in a series of conversations between Freese and Doug Spolyar, at the time a graduate student at the University of California, Santa Cruz. They wondered: What does dark matter do to the first stars to form in the universe? Then they reached out to Paolo Gondolo, an astrophysicist at the University of Utah, who joined the team. After several years of development, they published their first paper on this theory in the journal Physical Review Letters in 2008.

Together, Freese, Spolyar and Gondolo developed a model that goes something like this: At the centers of early protogalaxies, there would be very dense clumps of dark matter, along with clouds of hydrogen and helium gas. As the gas cooled, it would collapse and pull in dark matter along with it. As the density increased, the dark matter particles would increasingly annihilate, adding more and more heat, which would prevent the gas from collapsing all the way down to a dense enough core to support fusion as in an ordinary star. Instead, it would continue to gather more gas and dark matter, becoming big, puffy and much brighter than ordinary stars. Unlike ordinary stars, the power source would be evenly spread out, rather than concentrated in the core. With enough dark matter, dark stars could grow to be several million times the mass of our sun and up to 10 billion times as bright as the sun.

Read more at Science Daily

Astronomers discover new link between dark matter and clumpiness of the universe

In a study published today in the Journal of Cosmology and Astroparticle Physics, researchers at the University of Toronto reveal a theoretical breakthrough that may explain both the nature of invisible dark matter and the large-scale structure of the universe known as the cosmic web. The result establishes a new link between these two longstanding problems in astronomy, opening new possibilities for understanding the cosmos.

The research suggests that the "clumpiness problem," which centres on the unexpectedly even distribution of matter on large scales throughout the cosmos, may be a sign that dark matter is composed of hypothetical, ultra-light particles called axions. The implications of proving the existence of hard-to-detect axions extend beyond understanding dark matter and could address fundamental questions about the nature of the universe itself.

"If confirmed with future telescope observations and lab experiments, finding axion dark matter would be one of the most significant discoveries of this century," says lead author Keir Rogers, Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics in the Faculty of Arts & Science at the University of Toronto. "At the same time, our results suggest an explanation for why the universe is less clumpy than we thought, an observation that has become increasingly clear over the last decade or so, and currently leaves our theory of the universe uncertain."

Dark matter, comprising 85 percent of the universe's mass, is invisible because it does not interact with light. Scientists study its gravitational effects on visible matter to understand how it is distributed in the universe.

A leading theory proposes that dark matter is made of axions, described in quantum mechanics as "fuzzy" due to their wave-like behaviour. Unlike discrete point-like particles, axions can have wavelengths larger than entire galaxies. This fuzziness influences the formation and distribution of dark matter, potentially explaining why the universe is less clumpy than predicted in a universe without axions.

This lack of clumpiness has been observed in large galaxy surveys, challenging the other prevailing theory that dark matter consists only of heavy, weakly interacting sub-atomic particles called WIMPs. Despite experiments like the Large Hadron Collider, no evidence supporting the existence of WIMPs has been found.

"In science, it's when ideas break down that new discoveries are made and age-old problems are solved," says Rogers.

For the study, the research team -- led by Rogers and including members of associate professor Renée Hložek's research group at the Dunlap Institute, as well as from the University of Pennsylvania, Institute for Advanced Study, Columbia University and King's College London -- analyzed observations of relic light from the Big Bang, known as the Cosmic Microwave Background (CMB), obtained from the Planck 2018, Atacama Cosmology Telescope and South Pole Telescope surveys. The researchers compared these CMB data with galaxy clustering data from the Baryon Oscillation Spectroscopic Survey (BOSS), which maps the positions of approximately a million galaxies in the nearby universe. By studying the distribution of galaxies, which mirrors the behavior of dark matter under gravitational forces, they measured fluctuations in the amount of matter throughout the universe and confirmed its reduced clumpiness compared to predictions.

The researchers then conducted computer simulations to predict the appearance of relic light and the distribution of galaxies in a universe with long dark matter waves. These calculations aligned with CMB data from the Big Bang and galaxy clustering data, supporting the notion that fuzzy axions could account for the clumpiness problem.

Future research will involve large-scale surveys to map millions of galaxies and provide precise measurements of clumpiness, including observations over the next decade with the Rubin Observatory. The researchers hope to compare their theory to direct observations of dark matter through gravitational lensing, an effect where dark matter clumpiness is measured by how much it bends the light from distant galaxies, akin to a giant magnifying glass. They also plan to investigate how galaxies expel gas into space and how this affects the dark matter distribution to further confirm their results.

Understanding the nature of dark matter is one of the most pressing fundamental questions and key to understanding the origin and future of the universe.

Presently, scientists do not have a single theory that simultaneously explains gravity and quantum mechanics -- a theory of everything. The most popular theory of everything over the last few decades is string theory, which posits another level below the quantum level, where everything is made of string-like excitations of energy. According to Rogers, detecting a fuzzy axion particle could be a hint that the string theory of everything is correct.

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Astrophysicists reveal the nature of dark matter through the study of crinkles in spacetime

Most of the matter in the universe, amounting to a staggering 85% by mass, cannot be observed and consists of particles not accounted for by the Standard Model of Particle Physics (see remark 1). These particles are known as Dark Matter, and their existence can be inferred from their gravitational effects on light from distant galaxies. Finding the particle that makes up Dark Matter is an urgent problem in modern physics, as it dominates the mass and, therefore, the gravity of galaxies -- solving this mystery can lead to new physics beyond the Standard Model.

While some theoretical models propose the existence of ultramassive particles as a possible candidate for Dark Matter, others suggest ultralight particles. A team of astrophysicists led by Alfred AMRUTH, a PhD student in the team of Dr Jeremy LIM of the Department of Physics at The University of Hong Kong (HKU), collaborating with Professor George SMOOT, a Nobel Laureate in Physics from the Hong Kong University of Science and Technology (HKUST) and Dr Razieh EMAMI, a Research Associate at the Center for Astrophysics | Harvard & Smithsonian (CFA), has provided the most direct evidence yet that Dark Matter does not constitute ultramassive particles as is commonly thought but instead comprises particles so light that they travel through space like waves. Their work resolves an outstanding problem in astrophysics first raised two decades ago: why do models that adopt ultramassive Dark Matter particles fail to correctly predict the observed positions and the brightness of multiple images of the same galaxy created by gravitational lensing? The research findings were recently published in Nature Astronomy.

Dark Matter does not emit, absorb or reflect light, which makes it difficult to observe using traditional astronomical techniques. Today, the most powerful tool scientists have for studying Dark Matter is through gravitational lensing, a phenomenon predicted by Albert Einstein in his theory of General Relativity. In this theory, mass causes spacetime to curve, creating the appearance that light bends around massive objects such as stars, galaxies, or groups of galaxies. By observing this bending of light, scientists can infer the presence and distribution of Dark Matter -- and, as demonstrated in this study, the nature of Dark Matter itself.

When the foreground lensing object and the background lensed object -- both constituting individual galaxies in the illustration -- are closely aligned, multiple images of the same background object can be seen in the sky. The positions and brightness of the multiply-lensed images depend on the distribution of Dark Matter in the foreground lensing object, thus providing an especially powerful probe of Dark Matter.

Another assumption of the nature of Dark Matter

In the 1970s, after the existence of Dark Matter was firmly established, hypothetical particles referred to as Weakly Interacting Massive Particles (WIMPs) were proposed as candidates for Dark Matter. These WIMPs were thought to be ultramassive -- more than at least ten times as massive as a proton -- and interact with other matter only through the weak nuclear force. These particles emerge from Supersymmetry theories, developed to fill deficiencies in the Standard Model, and have since been widely advocated as the most likely candidate for Dark Matter. However, for the past two decades, adopting ultramassive particles for Dark Matter, astrophysicists have struggled to correctly reproduce the positions and brightness of multiply-lensed images. In these studies, the density of Dark Matter is assumed to decrease smoothly outwards from the centres of galaxies in accordance with theoretical simulations employing ultramassive particles.

Beginning also in the 1970s, but in dramatic contrast to WIMPs, versions of theories that seek to rectify deficiencies in the Standard Model, or those (e.g., String Theory) that seek to unify the four fundamental forces of nature (the three in the Standard Model, along with gravity), advocate the existence of ultralight particles. Referred to as axions, these hypothetical particles are predicted to be far less massive than even the lightest particles in the Standard Model and constitute an alternative candidate for Dark Matter.

According to the theory of Quantum Mechanics, ultralight particles travel through space as waves, interfering with each other in such large numbers as to create random fluctuations in density. These random density fluctuations in Dark Matter give rise to crinkles in spacetime. As might be expected, the different patterns of spacetime around galaxies depending on whether Dark Matter constitutes ultramassive or ultralight particles -- smooth versus crinkly -- ought to give rise to different positions and brightness for multiply-lensed images of background galaxies.

In work led by Alfred AMRUTH, a PhD student in Dr Jeremy LIM's team at HKU, astrophysicists have for the first time computed how gravitationally-lensed images generated by galaxies incorporating ultralight Dark Matter particles differ from those incorporating ultramassive Dark Matter particles.

Their research has shown that the general level of disagreement found between the observed and predicted positions as well as the brightness of multiply-lensed images generated by models incorporating ultramassive Dark Matter can be resolved by adopting models incorporating ultralight Dark Matter particles. Moreover, they demonstrate that models incorporating ultralight Dark Matter particles can reproduce the observed positions and brightness of multiply-lensed galaxy images, an important achievement that reveals the crinkly rather than smooth nature of spacetime around galaxies.

'The possibility that Dark Matter does not comprise ultramassive particles, as has long been advocated by the scientific community, alleviates other problems in both laboratory experiments and astronomical observations,' explains Dr Lim. 'Laboratory experiments have been singularly unsuccessful at finding WIMPs, the long-favoured candidate for Dark Matter. Such experiments are in their final stretch, culminating in the planned DARWIN experiment, leaving WIMPs with no place to hide if not found (see remark 2).'

Professor Tom BROADHURST, an Ikerbasque Professor at the University of the Basque Country, a Visiting Professor at HKU, and a co-author of the paper adds, 'If Dark Matter comprises ultramassive particles, then according to cosmological simulations, there should be hundreds of satellite galaxies surrounding the Milky Way. However, despite intensive searches, only around fifty have been discovered so far. On the other hand, if Dark Matter comprises ultralight particles instead, then the theory of Quantum Mechanics predicts that galaxies below a certain mass simply cannot form owing to the wave interference of these particles, explaining why we observe a lack of small satellite galaxies around the Milky Way.'

'Incorporating ultralight rather than ultramassive particles for Dark Matter resolve several longstanding problems simultaneously in both particle physics and astrophysics,' said Amruth Alfred, 'We have reached a point where the existing paradigm of Dark Matter needs to be reconsidered. Waving goodbye to ultramassive particles, which have long been heralded as the favoured candidate for Dark Matter, may not come easily, but the evidence accumulates in favour of Dark Matter having wave-like properties as possessed by ultralight particles.' The pioneering work used the supercomputing facilities at HKU, without which this work would not have been possible.

The co-author Professor George SMOOT added, 'Understanding the nature of particles that constitute Dark Matter is the first step towards New Physics. This work paves the way for future tests of Wave-like Dark Matter in situations involving gravitational lensing. The James Webb Space Telescope should discover many more gravitationally-lensed systems, allowing us to make even more exacting tests of the nature of Dark Matter.'

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