Hunting for dark matter — inside the Earth


By Paul Sutter - Astrophysicist

The answer to the dark-matter mystery may be under our feet.


The Cryogenic Dark Matter Search is one of the most sensitive efforts to track down dark matter particles. But the best dark-matter detector may be Earth itself, a new study suggests.(Image: © SuperCDMS/Slac National Accelerator Laboratory)


Dark matter is a hypothetical component to our universe, used to explain many strange behaviors of stars and galaxies.

Despite the almost overwhelming evidence that dark matter does indeed exist, we still don't know what it's made of. Detectors scattered around the world have been operating for decades, trying to catch the faint trace of a passing dark matter particle, but to no avail. A new paper offers an alternative approach: dig deep.

Related: The 11 biggest unanswered questions about dark matter

We know that dark matter exists through a variety of astronomical observations. Stars are orbiting the centers of their galaxies too fast. Galaxies are whizzing around inside clusters too quickly. Massive structures in the universe are appearing too early.

As far as we can tell, there is much more to the cosmos than meets the eye — there is some form of matter that is entirely invisible to us. Whatever the dark matter is, it's a new kind of particle that doesn't interact with light, which means it doesn't emit, absorb, reflect or refract electromagnetic radiation. Which means we can't see it. Which makes it dark.

So far, the only way we know dark matter exists is through gravity. Despite its invisibility superpower, dark matter still has mass, which means it can tug and shape on the biggest objects in the universe, revealing its presence through the motion of the more luminous stars and galaxies.

On the other end of the scale, particle physicists have been concocting new particles as consequences for new theories of physics, and some of them fit the bill for what the dark matter could be. The most promising candidate is a particle known as a WIMP: a weakly interacting massive particle.

The "weakly interacting" part doesn't just mean the particle is feeble: it means that the dark matter does occasionally interact with normal matter through the weak nuclear force. But as the name suggests, the weak nuclear force isn't the strongest, and it has very short range, making these interactions incredibly rare.

Buried clues

But "rare" doesn't mean "never." It's thought that billions — even trillions — of dark matter particles are swimming through you right now. But since the dark matter hardly notices normal matter, and vice versa, you simply don't feel it. You have to go out to big scales before you start to see its gravitational effects.

Still, rarely (exactly how rarely is not known yet), a dark matter particle goes rogue and interacts with a particle of normal matter through the weak nuclear force. This involves a transfer of energy (i.e., the dark matter particle kicks the normal particle), sending the normal matter flying, something that we can, in principle at least, detect.

But since it's so rare and so weak, our detection attempts haven't proven fruitful. We need big detectors that take up a lot of volume (since the interactions are so rare, it's either build a giant detector or wait for hundreds of years to get lucky). What's more, we have to bury these detectors deep underground, the deepest going 1.2 miles (2 kilometers) below the surface. This is because there's a lot of subatomic nuisance going on: other high-energy particles, like neutrinos and cosmic rays, cause similar kicks, and we need to use lots of rock to absorb them before they hit the detector, ensuring that if we do see a signal, it's more likely to be caused by dark matter.

And so far, after decades of building ever-larger detectors and watching carefully, we haven't found squat.

Read more: "Searching for Dark Matter with Paleo-Detectors"

Fossil evidence

There's a limit to how big we can make a dark matter detector, based solely on engineering and cost constraints. But thankfully, according to a new paper recently appearing on the online preprint site arXiv, there's a gigantic dark matter detector that's been collecting data for millions of years.

And it's right under our feet.

The crust of the Earth itself serves as a massive dark matter detector. When stray dark matter particles interact with normal matter inside a rock, a proton or neutron can get knocked loose, changing the chemical composition of the rock in the vicinity of the impact site. This can potentially even send the particle flying, leaving behind a microscopic scar.

Even better, deep digs have access to portions of the Earth's crust over twice as deep as our current dark matter detectors, promising results even freer of confusion from cosmic rays and other nuisance particles. And since rocks stay as rocks for millions, and even hundreds of millions, of years, they've been recording dark matter interactions for all that time, far longer than we can ever hope to access in the lifetimes of our experiments.

So it's pretty simple: dig up a bunch of rock (preferably something pure, so it's easy to analyze) and look it over with a fine-tooth microscopic comb, looking for any signs of subatomic violence.

There is one catch, however. Earth rocks naturally contain some radioactive elements, and radioactive decays will give rise to similar features. To solve this, the researchers suggest digging into oceanic crust, which is much more pure than the stuff that builds continents. With this in hand, the researchers predict that we could have a super-detector within easy reach: even a mere kilogram of rock would beat the sensitivity of the world's current best detectors.

We just have to dig in.
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Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

Dark Matter and Dark Energy: The Mystery Explained (Infographic)

By Karl Tate April 03, 2013

Astronomers know more about what dark matter is not than what it actually is.

(Image: © Karl Tate, SPACE.com Infographics Artist)

Most of the universe is made up of dark energy, a mysterious force that drives the accelerating expansion of the universe. The next largest ingredient is dark matter, which only interacts with the rest of the universe through its gravity. Normal matter, including all the visible stars, planets and galaxies, makes up less than 5 percent of the total mass of the universe.

Astronomers cannot see dark matter directly, but can study its effects. They can see light bent from the gravity of invisible objects (called gravitational lensing). They can also measure that stars are orbiting around in their galaxies faster than they should be.

This can all be accounted for if there were a large amount of invisible matter tied up in each galaxy, contributing to its overall mass and rotation rate.

Astronomers know more about what dark matter is not than what it is.

Dark matter is dark: It emits no light and cannot be seen directly, so it cannot be stars or planets.

Dark matter is not clouds of normal matter: Normal matter particles are called baryons. If dark matter were composed of baryons it would be detectable through reflected light. [Gallery: Dark Matter Throughout the Universe]

Dark matter is not antimatter: Antimatter annihilates matter on contact, producing gamma rays. Astronomers do not detect them.

Dark matter is not black holes: Black holes are gravity lenses that bend light. Astronomers do not see enough lensing events to account for the amount of dark matter that must exist.

Structure in the universe formed on the smallest scales first. It is believed that dark matter condensed first to form a “scaffolding,” with normal matter in the form of galaxies and clusters following the dark matter concentrations.

Scientists are using a variety of techniques across the disciplines of astronomy and physics to hunt for dark matter:

• Particle colliders such as the Large Hadron Collider.
• Cosmology instruments such as WMAP and Planck.
• Direct detection experiments including CDMS, XENON, Zeplin, WARP, ArDM and others.
• Indirect detection experiments including: Gamma ray detectors (Fermi from space and Cherenkov Telescopes from the ground); neutrino telescopes (IceCube, Antares); antimatter detectors (Pamela, AMS-02) and X-ray and radio facilities.

Dark matter travelling through stars could produce potentially detectable shock waves

by Ingrid Fadelli , Phys.org

Illustration of the shock formation. A dark asteroid traveling supersonically through a star 

creates a strong shock wave near it. The shock wave travels to the surface of the star, 

where it releases its energy as heat. Credit: Das et al

Dark matter, a hypothetical material that does not absorb, emit or reflect light, is thought to account for over 80 percent of the matter in the universe. While many studies have indirectly hinted at its existence, so far, physicists have been unable to directly detect dark matter and thus to confidently determine what it consists of.

One factor that makes searching for dark matter particularly challenging is that very little is known about its possible mass and composition. This means that dark matter searches are based on great part on hypotheses and theoretical assumptions.

Researchers at SLAC National Accelerator Laboratory and Université Paris Saclay have recently carried out a theoretical study that could introduce a new way of searching for dark matter. Their paper, published in Physical Review Letters, shows that when macroscopic dark matter travels through a star, it could produce shock waves that might reach the star's surface. These waves could in turn lead to distinctive and transient optical, UV and X-ray emissions that might be detectable by sophisticated telescopes.

"Most experiments have searched for dark matter made of separate particles, each about as heavy as an atomic nucleus, or clumps about as massive as planets or stars," Kevin Zhou, one of the researchers who carried out the study, told Phys.org. "We were interested in the intermediate case of asteroid-sized dark matter, which had been thought to be hard to test experimentally, since dark asteroids would be too rare to impact Earth, but too small to see in space."

Initially, Zhou and his colleagues started exploring the possibility that the heat produced during the impact between a dark matter asteroid and an ordinary star could result in the star exploding. This hypothesis was based on past studies suggesting that energy deposition can sometimes trigger supernova in white dwarfs. After a few weeks of calculations and discussions, however, the team realized that the impact between a dark matter asteroid and an ordinary star would most likely not lead to an explosion, as ordinary stars are more stable than white dwarfs.

"We had a hunch that the energy produced by such a collision should be visible somehow, so we brainstormed for a few months, trying and tossing out idea after idea," Zhou explained. "Finally, we realized that the shock waves generated by the dark asteroid's travel through the star were the most promising signature."

Illustration of the detection method. In a traditional search for particle dark matter (left), 

individual dark matter particles collide with nuclei in a detector on Earth. The resulting 

recoil energy can be seen by sensitive detectors. Analogously, dark asteroids can collide 

with stars (right), leading to shock waves that heat up their surfaces. The resulting UV

 emission can be seen by telescopes on Earth. Credit: Das et al

Shock waves are sharp signals that are produced when an object is moving faster than the speed of sound. For instance, a supersonic aircraft produces a sonic boom, which can be heard from the Earth's surface even when it is flying miles above it.

Similarly, Zhou and his colleagues predicted that the shock waves produced by dark asteroids deep inside a star could reach a star's surface. This would in turn result in a short-lived hot spot that could be detected using telescopes that can examine the UV spectrum.

"We're excited that we identified a powerful new way to search for a kind of dark matter thought to be hard to test, using telescopes that we already have in an unexpected way," Zhou said. "The most powerful UV telescope is the Hubble space telescope, but since stellar shock events are transients, it helps to be able to monitor more of the sky at once."

The recent study follows a growing trend within the astrophysics community to use astronomical objects as enormous dark matter detectors. This promising approach to searching for dark matter unites the fields of particle physics and astrophysics, bringing these two communities closer together.

In the future, the recent work by this team of researchers could inspire engineers to build new and smaller UV telescopes that can observe wider parts of the universe. A similar telescope, dubbed ULTRASAT, is already set to be released in 2024. Using this telescope, physicists could try searching for dark matter by examining stellar surfaces. In their next works, the researchers themselves plan to try to detect potential dark asteroid impact events using UV telescope data.

"The ideal case would be to use the Hubble space telescope to monitor a large globular cluster in the UV," Zhou said. "It would also be interesting to consider dark asteroids impacting other astronomical objects. Since our work, there have been papers by others considering impacts on neutron stars and red giants, but there are probably even more promising ideas in this direction that nobody has thought of yet."Physicist seeks to understand dark matter with Webb Telescope

More information: Anirban Das et al, Stellar Shocks from Dark Matter Asteroid Impacts, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.021101

Journal information: Physical Review Letters 

© 2022 Science X Network

NASA Proposes a Way That Dark Matter’s Influence Could Be Directly Observed

By ELIZABETH LANDAU, NASA FEBRUARY 4, 2022

This artist’s rendering shows a view of our own Milky Way Galaxy and its central bar as it might appear if viewed from above. Credit: NASA/JPL-Caltech/R. Hurt (SSC)

How Dark Matter Could Be Measured in the Solar System

Pictures of the Milky Way show billions of stars arranged in a spiral pattern radiating out from the center, with illuminated gas in between. But our eyes can only glimpse the surface of what holds our galaxy together. About 95 percent of the mass of our galaxy is invisible and does not interact with light. It is made of a mysterious substance called dark matter, which has never been directly measured.

Now, a new study calculates how dark matter’s gravity affects objects in our solar system, including spacecraft and distant comets. It also proposes a way that dark matter’s influence could be directly observed with a future experiment. The article is published in the Monthly Notices of the Royal Astronomical Society.

“We’re predicting that if you get out far enough in the solar system, you actually have the opportunity to start measuring the dark matter force,” said Jim Green, study co-author and advisor to NASA’s Office of the Chief Scientist. “This is the first idea of how to do it and where we would do it.”

Dark matter in our backyard

Here on Earth, our planet’s gravity keeps us from flying out of our chairs, and the Sun’s gravity keeps our planet orbiting on a 365-day schedule. But the farther from the Sun a spacecraft flies, the less it feels the Sun’s gravity, and the more it feels a different source of gravity: that of the matter from the rest of the galaxy, which is mostly dark matter. The mass of our galaxy’s 100 billion stars is minuscule compared to estimates of the Milky Way’s dark matter content.

To understand the influence of dark matter in the solar system, lead study author Edward Belbruno calculated the “galactic force,” the overall gravitational force of normal matter combined with dark matter from the entire galaxy. He found that in the solar system, about 45 percent of this force is from dark matter and 55 percent is from normal, so-called “baryonic matter.” This suggests a roughly half-and-half split between the mass of dark matter and normal matter in the solar system.

“I was a bit surprised by the relatively small contribution of the galactic force due to dark matter felt in our solar system as compared to the force due to the normal matter,” said Belbruno, mathematician and astrophysicist at Princeton University and Yeshiva University. “This is explained by the fact most of dark matter is in the outer parts of our galaxy, far from our solar system.”

A large region called a “halo” of dark matter encircles the Milky Way and represents the greatest concentration of the dark matter of the galaxy. There is little to no normal matter in the halo. If the solar system were located at a greater distance from the center of the galaxy, it would feel the effects of a larger proportion of dark matter in the galactic force because it would be closer to the dark matter halo, the authors said.

In this artist’s conception, NASA’s Voyager 1 spacecraft has a bird’s-eye view of the solar system. The circles represent the orbits of the major outer planets: Jupiter, Saturn, Uranus, and Neptune. Launched in 1977, Voyager 1 visited the planets Jupiter and Saturn. The spacecraft is now more than 14 billion miles from Earth, making it the farthest human-made object ever built. In fact, Voyager 1 is now zooming through interstellar space, the region between the stars that is filled with gas, dust, and material recycled from dying stars. Credit: NASA, ESA, and G. Bacon (STScI)

How dark matter may influence spacecraft

Green and Belbruno predict that dark matter’s gravity ever so slightly interacts with all of the spacecraft that NASA has sent on paths that lead out of the solar system, according to the new study.

“If spacecraft move through the dark matter long enough, their trajectories are changed, and this is important to take into consideration for mission planning for certain future missions,” Belbruno said.

Such spacecraft may include the retired Pioneer 10 and 11 probes that launched in 1972 and 1973, respectively; the Voyager 1 and 2 probes that have been exploring for more than 40 years and have entered interstellar space; and the New Horizons spacecraft that has flown by Pluto and Arrokoth in the Kuiper Belt.

But it’s a tiny effect. After traveling billions of miles, the path of a spacecraft like Pioneer 10 would only deviate by about 5 feet (1.6 meters) due to the influence of dark matter. “They do feel the effect of dark matter, but it’s so small, we can’t measure it,” Green said.

Where does the galactic force take over?

At a certain distance from the Sun, the galactic force becomes more powerful than the pull of the Sun, which is made of normal matter. Belbruno and Green calculated that this transition happens at around 30,000 astronomical units, or 30,000 times the distance from Earth to the Sun. That is well beyond the distance of Pluto, but still inside the Oort Cloud, a swarm of millions of comets that surrounds the solar system and extends out to 100,000 astronomical units.

This means that dark matter’s gravity could have played a role in the trajectory of objects like ‘Oumuamua, the cigar-shaped comet or asteroid that came from another star system and passed through the inner solar system in 2017. Its unusually fast speed could be explained by dark matter’s gravity pushing on it for millions of years, the authors say.

If there is a giant planet in the outer reaches of the solar system, a hypothetical object called Planet 9 or Planet X that scientists have been searching for in recent years, dark matter would also influence its orbit. If this planet exists, dark matter could perhaps even push it away from the area where scientists are currently looking for it, Green and Belbruno write. Dark matter may have also caused some of the Oort Cloud comets to escape the orbit of the Sun altogether.

Could dark matter’s gravity be measured?

To measure the effects of dark matter in the solar system, a spacecraft wouldn’t necessarily have to travel that far. At a distance of 100 astronomical units, a spacecraft with the right experiment could help astronomers measure the influence of dark matter directly, Green and Belbruno said.

Specifically, a spacecraft equipped with radioisotope power, a technology that has allowed Pioneer 10 and 11, the Voyagers, and New Horizon to fly very far from the Sun, may be able to make this measurement. Such a spacecraft could carry a reflective ball and drop it at an appropriate distance. The ball would feel only galactic forces, while the spacecraft would experience a thermal force from the decaying radioactive element in its power system, in addition to the galactic forces. Subtracting out the thermal force, researchers could then look at how the galactic force relates to deviations in the respective trajectories of the ball and the spacecraft. Those deviations would be measured with a laser as the two objects fly parallel to one another.

A proposed mission concept called Interstellar Probe, which aims to travel to about 500 astronomical units from the Sun to explore that uncharted environment, is one possibility for such an experiment.

Two views from Hubble of the massive galaxy cluster Cl 0024+17 (ZwCl 0024+1652) are shown. To the left is the view in visible-light with odd-looking blue arcs appearing among the yellowish galaxies. These are the magnified and distorted images of galaxies located far behind the cluster. Their light is bent and amplified by the immense gravity of the cluster in a process called gravitational lensing. To the right, a blue shading has been added to indicate the location of invisible material called dark matter that is mathematically required to account for the nature and placement of the gravitationally lensed galaxies that are seen. Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)

More about dark matter

Dark matter as a hidden mass in galaxies was first proposed in the 1930s by Fritz Zwicky. But the idea remained controversial until the 1960s and 1970s, when Vera C. Rubin and colleagues confirmed that the motions of stars around their galactic centers would not follow the laws of physics if only normal matter were involved. Only a gigantic hidden source of mass can explain why stars at the outskirts of spiral galaxies like ours move as quickly as they do.

Today, the nature of dark matter is one of the biggest mysteries in all of astrophysics. Powerful observatories like the Hubble Space Telescope and the Chandra X-Ray Observatory have helped scientists begin to understand the influence and distribution of dark matter in the universe at large. Hubble has explored many galaxies whose dark matter contributes to an effect called “lensing,” where gravity bends space itself and magnifies images of more distant galaxies.

Astronomers will learn more about dark matter in the cosmos with the newest set of state-of-the-art telescopes. NASA’s James Webb Space Telescope, which launched Dec. 25, 2021, will contribute to our understanding of dark matter by taking images and other data of galaxies and observing their lensing effects. NASA’s Nancy Grace Roman Space Telescope, set to launch in the mid-2020s, will conduct surveys of more than a billion galaxies to look at the influence of dark matter on their shapes and distributions.

The European Space Agency’s forthcoming Euclid mission, which has a NASA contribution, will also target dark matter and dark energy, looking back in time about 10 billion years to a period when dark energy began hastening the universe’s expansion. And the Vera C. Rubin Observatory, a collaboration of the National Science Foundation, the Department of Energy, and others, which is under construction in Chile, will add valuable data to this puzzle of dark matter’s true essence.

But these powerful tools are designed to look for dark matter’s strong effects across large distances, and much farther afield than in our solar system, where dark matter’s influence is so much weaker.

“If you could send a spacecraft out there to detect it, that would be a huge discovery,” Belbruno said.

Reference: “When leaving the Solar system: Dark matter makes a difference” by Edward Belbruno and James Green, 4 January 2022, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stab3781

 SPACE/COSMOLOGY

Combination and summary of ATLAS dark matter searches in 2HDM+a

Peer-Reviewed Publication

Science China Press

In the 1930s, Swiss astronomer Fritz Zwicky observed that the velocities of galaxies in the Coma Cluster were too high to be maintained solely by the gravitational pull of luminous matter. He proposed the existence of some non-luminous matter within the galaxy cluster, which he called dark matter. This discovery marked the beginning of humanity's understanding and study of dark matter.

Today, the most precise measurements of dark matter in the universe come from observations of the cosmic microwave background. The latest results from the Planck satellite indicate that about 5% of the mass in our universe comes from visible matter (mainly baryonic matter), approximately 27% comes from dark matter, and the rest from dark energy.

Despite extensive astronomical observations confirming the existence of dark matter, we have limited knowledge about the properties of dark matter particles. From a microscopic perspective, the Standard Model of particle physics, established in the mid-20th century, has been hugely successful and confirmed by numerous experiments. However, the Standard Model cannot explain the existence of dark matter in the universe, indicating the need for new physics beyond the Standard Model to account for dark matter candidate particles, and the urgent need to find experimental evidence of these candidates.

Consequently, dark matter research is not only a hot topic in astronomy but also at the forefront of particle physics research. Searching for dark matter particles in colliders is one of the three major experimental approaches to detect the interaction between dark matter and regular matter, complementing other types of dark matter detection experiments such as underground direct detection experiments and space-based indirect detection experiments.

Recently, the ATLAS collaboration searched for dark matter using the 139 fb-1 of proton-proton collision data accumulated during LHC's Run 2, within the 2HDM+a dark matter theoretical framework. The search utilized a variety of dark matter production processes and experimental signatures, including some not considered in traditional dark matter models. To further enhance the sensitivity of the dark matter search, this work statistically combined the three most sensitive experimental signatures: the process involving a Z boson decaying into leptons with large missing transverse momentum, the process involving a Higgs boson decaying into bottom quarks with large missing transverse momentum, and the process involving a charged Higgs boson with top and bottom quark final states.

This is the first time ATLAS has conducted a combined statistical analysis of final states including dark matter particles and intermediate states decaying directly into Standard Model particles. This innovation has significantly enhanced the constraint on the model parameter space and the sensitivity to new physics.

"This work is one of the largest projects in the search for new physics at the LHC, involving nearly 20 different analysis channels. The complementary nature of different analysis channels to constrain the parameter space of new physics highlights the unique advantages of collider experiments," said Zirui Wang, a postdoctoral researcher at the University of Michigan.

This work has provided strong experimental constraints on multiple new benchmark parameter models within the 2HDM+a theoretical framework, including some parameter spaces never explored by previous experiments. This represents the most comprehensive experimental result from the ATLAS collaboration for the 2HDM+a dark matter model.

Lailin Xu, a professor at the University of Science and Technology of China stated, "2HDM+a is one of the mainstream new physics theoretical frameworks for dark matter in the world today. It has significant advantages in predicting dark matter phenomena and compatibility with current experimental constraints, predicting a rich variety of dark matter production processes in LHC experiments. This work systematically carried out multi-process searches and combined statistical analysis based on the 2HDM+a model framework, providing results that exclude a large portion of the possible parameter space for dark matter, offering important guidance for future dark matter searches."

Vu Ngoc Khanh, a postdoctoral researcher at Tsung-Dao Lee institute, stated: “Although we have not yet found dark matter particles at the LHC, compared to before the LHC’s operation, we have put stringent constraints on the parameter space where dark matter might exist, including the mass of the dark matter particles and their interaction strengths with other particles, further narrowing the search scope.” Tsung Dao Lee Fellow Li Shu, added: “So far, the data collected by the LHC only accounts for about 7% of the total data the experiment will record. The data that the LHC will generate over the next 20 years presents a tremendous opportunity to discover dark matter. Our past experiences have shown us that dark matter might be different from what we initially thought, which motivates us to use more innovative experimental methods and techniques in our search.”

ATLAS is one of the four large experiments at CERN's Large Hadron Collider (LHC). The ATLAS experiment is a multipurpose particle detector with a forward–backward symmetric cylindrical geometry and nearly 4π coverage in solid angle. It consists of an inner tracking detector surrounded by a thin superconducting solenoid, high-granularity sampling electromagnetic and hadronic calorimeters, and a muon spectrometer with three superconducting air-core toroidal magnets. The ATLAS Collaboration consists of more than 5900 members from 253 institutes in 42 countries on 6 continents, including physicists, engineers, students, and technical staff.

---

Journal

Science Bulletin

DOI

10.1016/j.scib.2024.06.003 

This faraway galaxy may be completely devoid of dark matter


By Mara Johnson-Groh

published 1 day ago

A galaxy without dark matter would upend fundamental theories about galaxy formation.

The galaxy AGC 114905 seems to be devoid of dark matter. In this image, the stellar emission is shown in blue; and green clouds show the neutral hydrogen gas. (Image credit: Javier Román & Pavel Mancera Piña, CC BY 4.0)

On the surface, a galaxy 250 million light-years from Earth seems like any other, but a deeper look reveals a puzzling quirk: It seems to have no dark matter.

If these galaxies are ultimately confirmed to be devoid of dark matter, it could upend fundamental theories about the making of galaxies (dark matter is considered essential to this process). And that, in turn, could rule out a leading candidate for the mysterious substance, called cold dark matter.

"In principle, galaxies like this shouldn't exist," said Pavel Mancera Piña, a doctoral candidate in astronomy at the University of Groningen in the Netherlands and an astronomer at ASTRON (the Netherlands Institute for Radio Astronomy), referring to the fact that dark matter is thought to be the glue that holds a galaxy's stars, gas and dust together. "We cannot effectively explain them with any existing theory," said Mancera Piña, who is the lead author of a new paper describing the findings.

The galaxy, called AGC 114905, is an ultradiffuse galaxy (UDG). These galaxies are faint; AGC 114905 is about the same size as the Milky Way but has 1,000 times fewer stars.

When Mancera Piña and his colleagues first looked at AGC 114905 in 2019 , they suspected it might not have dark matter because of how fast it was rotating. The speed at which a galaxy rotates reveals how much stuff it contains; the more massive a galaxy, the stronger its gravity and the faster it spins. By comparing the speed with how much stuff can be seen — the amount of stars, gas and dust — astronomers can work backward to figure out how much extra invisible stuff — dark matter — must be present to account for the speed of the galaxy.

But because the galaxy in question is so faint, they didn't have enough data initially to fully resolve the rotation speed to tell if it was totally devoid of dark matter. So they went back for a second look, compiling 40 hours of observations with the Very Large Array, a radio observatory in New Mexico.

Through their observations, which mapped the gas in the galaxy, the astronomers figured out how fast the gas was moving. This allowed them to figure out the galaxy's rotation speed and thus how much dark matter is present in the galaxy. But ultimately the researchers concluded that there doesn't seem to be any room for dark matter.

"That's what we were expecting, of course, but you never know," Mancera Piña said. "It was still a bit of a surprise."

Missing dark matter

In the past, astronomers have discovered some UDGs that are rich in dark matter and others lacking it, Live Science previously reported. Some of the latter type are found near more massive galaxies; this finding suggests they may have lost their dark matter through interactions with their larger nearby galaxies, whose gravity may have whisked the smaller galaxies' dark matter away. But considering AGC 114905 doesn't have any massive galaxies nearby, that explanation is unlikely, Mancera Piña said.

This poses a challenge to theories of galaxy formation, because dark matter is thought to be essential for their formation, as its gravity helps pull the relatively rarer normal material together. If there are galaxies with no dark matter, that implies dark matter may not be needed to form galaxies.

Studying AGC 114905 also gives astronomers a new way to test the nature of dark matter. Current theories of galaxy formation rely on a special type of dark matter called cold dark matter, but if those theories don't explain weird galaxies like AGC 114905, then maybe cold dark matter isn't workable either.

"We have been trying to understand what dark matter is for the last 50 years, but we seem to have reached some kind of dead end," Mancera Piña said. By carefully studying this galaxy and others like it, the astronomers might be able to provide constraints on what dark matter might be like, if it's not cold dark matter. In the study, the researchers also applied models of an alternative theory to dark matter called Modified Newtonian dynamics, or MOND, to see if that could explain the galaxy's unique characteristics. However, this theory wasn't able to reconcile the speed of the galaxy either.

Pieter van Dokkum, an astronomer atYale University who has studied dark matter-deficient UDGs but was not involved with the new work, said he thinks the new findings are promising and significant, but that more research will be required for astronomers to be sure the galaxy is truly devoid of dark matter.

"There will be a lot of discussion," van Dokkum said. "Extraordinary claims require extraordinary evidence."

The study authors plan to study AGC 114905 further and are gathering observations of other UDGs that might be free of dark matter.

The findings were published on Nov. 30 to the preprint server arXiv and have been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

Originally published on Live Science.

SEE 

LA REVUE GAUCHE - Left Comment: Search results for AETHYR 

LA REVUE GAUCHE - Left Comment: Search results for ETHER 

Scientists make shocking claim dark matter may really be an alternate shadow universe

Dark matter has proved an elusive concept to scientists, with some claiming it does not even exist. Now, researchers think the phenomenon could actually be part of an alternate universe.

By JOHN MAC GHLIONN
 Sun, Mar 10, 2024

The behaviour of galaxies would be inexplicable without the existence of dark matter 

(Image: Getty)

Scientists believe that dark matter could be viewed as an alternative universe following a breakthrough study.

The new paper by Dr. Arushi Bodas, a postdoctoral fellow at the University of Chicago’s Enrico Fermi Institute, and his colleagues, states that dark matter could and possibly should be viewed as a distorted alternate universe that never fully developed.

But before discussing the paper, it’s important to understand just how mysterious dark matter really is.

That's easier said than done, however.

Despite it constituting more than 80 percent of all matter in the universe, scientists have yet to observe dark matter, Its existence is inferred because the behavior of stars, planets, and galaxies would be wholly inexplicable without its presence.

Dark matter is difficult to observe; in fact, it’s completely imperceptible. It emits zero light or energy, making it undetectable by conventional sensors and detectors.

Scientists believe its composition is the key to understanding its mysterious nature. Visible matter, also known as baryonic matter, is composed of subatomic particles called baryons, which consist of protons, neutrons, and electrons. The composition of dark matter, on the other hand, remains speculative.

Potentially, it could consist of baryons, but it could also be composed of non-baryonic matter, which refers to different types of particles. The prevailing belief among scientists is that dark matter is primarily composed of non-baryonic matter. Another potential candidate is neutralinos, hypothetical particles that are heavier and slower than neutrinos, although they have yet to be observed.

Sterile neutrinos are also considered as a candidate for dark matter. Neutrinos are particles that do not contribute to regular matter. While a stream of neutrinos emanates from the sun, they rarely interact with normal matter and pass through the Earth and billions of inhabitants. Among the three known types of neutrinos, the sterile neutrino is proposed as a potential dark matter candidate. It would only interact with regular matter through gravity.

The most recent hypothesis proposes that dark matter exists in a distorted parallel universe within our own, where atoms are unable to come together. In the realm of ordinary matter, protons and neutrons possess almost identical masses, creating the necessary conditions for the formation of stable atoms.

The recent study proposes the existence of a potential shadow universe where protons and neutrons have asymmetrical masses, resulting in a chaotic mix of subatomic particles that rarely interact. In other words, the polar opposite of how conventional matter operates. This phenomenon could also clarify why dark matter does not aggregate.

Ever since astronomers initially suspected the presence of dark matter in the 1930s, debates surrounding what it is (and isn’t) have raged. Observations indicate that it surpasses ordinary matter by a ratio of 6 to 1. Galaxies and galaxy clusters are surrounded by massive spheres, known as "halos," of dark matter.

To remain undetected, astronomers theorize that this substantial amount of material must be composed of particles that have minimal interaction with ordinary matter or even with each other. Their primary function is to provide the gravitational framework for luminous matter. Astronomers believe that these halos were created in the early stages of cosmic history and subsequently attracted ordinary matter, which, due to its diverse range of behaviors, evolved into complex structures, while dark matter, being inert, remained unchanged.

Dark energy, on the other hand, seems to only serve the purpose of accelerating cosmic expansion, and the existing evidence suggests that it has remained constant throughout the existence of the universe.

Although a minority of scientists reject the idea of dark matter, there is now a plethora of evidence supporting its existence, with one of the most straightforward explanations involving the rotation of galaxies.

As Dr. Don Lincoln, a senior scientist at Fermilab, America’s leading particle physics laboratory, has noted, despite the gravitational pull towards the Sun, the planets' velocities result in nearly circular orbits.

The balance between velocity and gravity dictates that planets farther from the Sun move at a slower pace compared to those in closer proximity. Similarly, in galaxies, stars follow a similar pattern, with the laws of physics making analogous predictions.

Specifically, stars located further from the galactic center should move at a slower pace than those nearer to it.

However, observations by astronomers reveal that stars in the outer regions of galaxies move faster than anticipated. If the laws of gravity and motion hold true, the only plausible explanation is the presence of additional, unseen matter intensifying the gravitational force experienced by these rapidly moving stars.

The new paper by Dr. Bodas and his colleagues is just the latest to solidify the "dark matter really does exist' thesis.

Controversial new theory of gravity rules out need for dark matter


Exclusive: Paper by UCL professor says ‘wobbly’ space-time could instead explain expansion of universe and galactic rotation



Hannah Devlin 

THE GUARDIAN

Science correspondent

Sat 9 Mar 2024 

Dark matter is supposed to account for 85% of the mass in the universe, according to conventional scientific wisdom. But proponents of a radical new theory of gravity, in which space-time is “wobbly”, say their approach could render the elusive substance obsolete.

The proposition, outlined in a new paper, raises the controversial possibility that dark matter, which has never been directly observed, is a mirage that a substantial portion of the physics community has been chasing for several decades. The theory is viewed as quite left-field and is yet to be thoroughly tested, but the latest claims are creating a stir in the world of physics.

Announcing the paper on X, Prof Jonathan Oppenheim, of University College London, said: “Folks, something seems to be happening. We show that our theory of gravity … can explain the expansion of the universe and galactic rotation without dark matter or dark energy.”

There are multiple lines of evidence for dark matter, but its nature has remained mysterious and searches by the Large Hadron Collider have come up empty-handed. Last year, the European Space Agency launched a mission, Euclid, aiming to produce a cosmic map of dark matter.

The latest paper, published on the Arxiv website and yet to be peer-reviewed, raises the question of whether it even exists, drawing parallels between dark matter and flawed concepts of the past, such as “the ether”, an invisible substance that was thought to permeate all of space.

“In the absence of any direct evidence for dark energy or dark matter it is natural to wonder whether they may be unnecessary scientific constructs like celestial spheres, ether, or the planet Vulcan, all of which were superseded by simpler explanations,” it states. “Gravity has a long history of being a trickster.”

In this case, the simpler explanation being proposed is Oppenheim’s “postquantum theory of classical gravity”. The UCL professor has spent the past five years developing the approach, which aims to unite the two pillars of modern physics: quantum theory and Einstein’s general relativity, which are fundamentally incompatible.

Oppenheim’s theory envisages the fabric of space-time as smooth and continuous (classical), but inherently wobbly. The rate at which time flows would randomly fluctuate, like a burbling stream, space would be haphazardly warped and time would diverge in different patches of the universe. The theory also envisions an intrinsic breakdown in predictability.


The paper, by Oppenheim and Andrea Russo, a PhD candidate at UCL, claims this take on the universe could explain landmark observations of rotating galaxies that led to the “discovery” of dark matter. Stars at the edges of galaxies, where gravity is expected to be weakest based on visible matter, ought to be rotating more slowly than stars at the centre. But in reality, the orbital motion of stars does not drop off. From this, astronomers inferred the presence of a halo of unseen (dark) matter exerting a gravitational pull.

In Oppenheim’s approach the additional energy required to keep the stars locked in orbit is provided by the random fluctuations in spacetime, which in effect add in a background hum of gravitation. This would be negligible in a high gravity interaction, such as the Earth orbiting the Sun. But in low gravity situations, such as the fringes of a galaxy, the phenomenon would dominate – and cumulatively could account for the majority of the energy in the universe.

“We show that it can explain the expansion of the universe and galactic rotation curves without the need for dark matter or dark energy,” Oppenheim said on X. “We do urge caution, however, since there is other indirect evidence for dark matter, so further calculations and comparison with data are needed. But if it holds, it would appear that 95% of the energy in the universe is due to the erratic nature of spacetime, signalling either a fundamental breakdown in predictability of physics, or we are immersed in an environment which does not obey the laws of classical or quantum theory.”

Not everyone is convinced, including the well-known theorists Prof Carlo Rovelli and Prof Geoff Penington, who have signed a 5,000:1 odds bet with Oppenheim against his theory being proven correct.

“I think it’s good that physicists explore a wide variety of approaches to very difficult problems like combining quantum mechanics with gravity,” said Penington.

“Personally, I don’t think this particular approach is likely to be the correct one. I’ve obviously put my money where my mouth is on that front and there is nothing new in the recent papers that would make me change that assessment.”

Others are more enthusiastic. “I think the authors are on to something really interesting here, exploring some beautiful and novel ideas,” said Prof Andrew Pontzen, a cosmologist at University College London. “However, the challenge for replacing dark matter is that there are so many different lines of evidence that suggest its presence. So far they have only addressed one of these lines. Only time will tell whether the new ideas can truly explain the huge variety of phenomena that point towards dark matter.”

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