Category: Dark Matter and Dark Energy

Explaining A Universe Composed Of Matter

The universe consists of a massive imbalance between matter and antimatter. Antimatter and matter are actually the same, but have opposite charges, but there’s hardly any antimatter in the observable universe, including the stars and other galaxies. In theory, there should be large amounts of antimatter, but the observable universe is mostly matter.

“We’re here because there’s more matter than antimatter in the universe,” says Professor Jens Oluf Andersen at the Norwegian University of Science and Technology’s (NTNU)Department of Physics. This great imbalance between matter and antimatter is all tangible matter, including life forms, exists, but scientists don’t understand why.

Physics uses a standard model to explain and understand how the world is connected. The standard model is a theory that describes all the particles scientists are familiar with. It accounts for quarks, electrons, the Higgs boson particle and how they all interact with each other. But the standard model cannot explain the fact that the world consists almost exclusively of matter. So there must be something we don’t yet understand.

When antimatter and matter meet, they annihilate, and the result is light and nothing else. Given equal amounts of matter and antimatter, nothing would remain once the reaction was completed. As long as we don’t know why more matter exists, we can’t know why the building blocks of anything else exist, either. “This is one of the biggest unsolved problems in physics,” says Andersen.

Researchers call this the “baryon asymmetry” problem. Baryons are subatomic particles, including protons and neutrons. All baryons have a corresponding antibaryon, which is mysteriously rare. The standard model of physics explains several aspects of the forces of nature. It explains how atoms become molecules, and it explains the particles that make up atoms.

“The standard model of physics includes all the particles we know about. The newest particle, the Higgs boson, was discovered in 2012 at CERN, says Andersen. With this discovery, an important piece fell into place. But not the final one. The standard model works perfectly to explain large parts of the universe, so researchers are intrigued when something doesn’t fit. Baryon asymmetry belongs in this category.

Physicists do have their theories as to why there is more matter, and thus why we undeniably exist. “One theory is that it’s been this way since the Big Bang,” says Andersen. In other words, the imbalance between matter and antimatter is a basic precondition that has existed more or less from the beginning.

Quarks are among nature’s smallest building blocks. An early surplus of quarks relative to antiquarks was propagated as larger units formed. But Andersen doesn’t care for this explanation. “We’re still not happy with that idea, because it doesn’t tell us much,” he says.

So why was this imbalance present from the beginning? Why did quarks initially outnumber antiquarks? “In principle, it’s possible to generate asymmetry within the standard model of physics—that is, the difference between the amount of matter and antimatter. But we run into two problems,” says Andersen.

First of all, scientists have to go way back in time, to just after the Big Bang when everything started—we’re talking about 10 picoseconds, or 10-11 seconds after the Big Bang.

The second problem is that temperatures have to be around 1 trillion degrees Kelvin, or 1015 degrees. That’s scorching—consider that the sun’s surface is only about 5700 degrees. Regardless, it is not sufficient to explain baryonic matter. “It can’t work. In the standard model, we don’t have enough matter,” Andersen says. “The problem is that the jump in the expectation value of the Higgs field is too small,” he adds for the benefit those with only a minimum grasp of physics.

“It’s probably not just our imagination that’s imposing limits, but lots of possibilities exist,” says Andersen. These possibilities therefore need to work together with the standard model. “What we’re really looking for is an extension of the standard model. Something that fits into it.”

Neither he nor other physicists doubt that the standard model is right. The model is continuously tested at CERN and other particle accelerators. It’s just that the model isn’t yet complete. Andersen and his colleagues are investigating various possibilities for the model to fit with the imbalance between matter and antimatter. The latest results were recently published in Physical Review Letters.

“Actually, we’re talking about phase transitions,” says Andersen. His group is considering processes of change in matter, like water turning into steam or ice under changing conditions. They’re also considering whether matter came about as a result of an electroweak phase transition (EWPT) and formed a surplus of baryons just after the Big Bang. The electroweak phase transition occurs by the formation of bubbles. The new phase expands, a bit like water bubbles, and takes over the entire universe.

Andersen and his colleagues tested the so-called “two Higgs doublet” model (2HDM), one of the simplest extensions of the standard model. They searched for possible areas where the right conditions are present to create matter. “Several scenarios exist for how the baryon asymmetry was created. We studied the electroweak phase transition using the 2HDM model. This phase transition takes place in the early stage of our universe,” says Andersen.

The process is comparable to boiling water. When water reaches 100 degrees Celsius, gas bubbles form and rise up. These gas bubbles contain water vapour which is the gas phase. Water is a liquid. When it transitions from the gas phase to the liquid phase in the early universe during a process in which the universe expands and is cooled, a surplus of quarks is produced compared to antiquarks, generating the baryon asymmetry.

Last but not least, the researchers are also doing mathematics. In order for the models to work in sync, parameters or numerical values have to fit so that both models are right at the same time. So the work is about finding these parameters. In the most recent article in Physical Review Letters, Andersen and his colleagues narrowed down the mathematical area in which matter can be created and at the same time correspond to both models. They have now narrowed the possibilities.

“For the new model (2HDM) to match what we already know from CERN, for example, the parameters in the model can’t be just anything. On the other hand, to be able to produce enough baryon asymmetry, the parameters also have to be within a certain range. So that’s why we’re trying to narrow the parameter range. But that’s still a long way off,” says Andersen. In any case, the researchers have made a bit of headway on the road to understanding why we and everything else are here.

X-Ray Technology Reveals Never-Before-Seen Matter Around Black Hole

In an international collaboration between Japan and Sweden, scientists clarified how gravity affects the shape of matter near the black hole in binary system Cygnus X-1. Their findings, which were published in Nature Astronomy this month, may help scientists further understand the physics of strong gravity and the evolution of black holes and galaxies.

Near the center of the constellation of Cygnus is a star orbiting the first black hole discovered in the universe. Together, they form a binary system known as Cygnus X-1. This black hole is also one of the brightest sources of X-rays in the sky. However, the geometry of matter that gives rise to this light was uncertain. The research team revealed this information from a new technique called X-ray polarimetry.

Taking a picture of a black hole is not easy. For one thing, it is not yet possible to observe a black hole because light cannot escape it. Rather, instead of observing the black hole itself, scientists can observe light coming from matter close to the black hole. In the case of Cygnus X-1, this matter comes from the star that closely orbits the black hole.

Most light that we see, like from the sun, vibrates in many directions. Polarization filters light so that it vibrates in one direction. It is how snow goggles with polarized lenses let skiers see more easily where they are going down the mountain — they work because the filter cuts light reflecting off of the snow.

“It’s the same situation with hard X-rays around a black hole,” Hiroshima University Assistant Professor and study coauthor Hiromitsu Takahashi said. “However, hard X-rays and gamma rays coming from near the black hole penetrate this filter. There are no such ‘goggles’ for these rays, so we need another special kind of treatment to direct and measure this scattering of light.”

The team needed to figure out where the light was coming from and where it scattered. In order to make both of these measurements, they launched an X-ray polarimeter on a balloon called PoGO+. From there, the team could piece together what fraction of hard X-rays reflected off the accretion disk and identify the matter shape.

Two competing models describe how matter near a black hole can look in a binary system such as Cygnus X-1: the lamp-post and extended model. In the lamp-post model, the corona is compact and bound closely to the black hole. Photons bend toward the accretion disk, resulting in more reflected light. In the extended model, the corona is larger and spread around the vicinity of the black hole. In this case, the reflected light by the disk is weaker.

Since light did not bend that much under the strong gravity of the black hole, the team concluded that the black hole fit the extended corona model.

With this information, the researchers can uncover more characteristics about black holes. One example is its spin. The effects of spin can modify the space-time surrounding the black hole. Spin could also provide clues into the evolution of the black hole. It could be slowing down in speed since the beginning of the universe, or it could be accumulating matter and spinning faster.

“The black hole in Cygnus is one of many,” Takahashi said. “We would like to study more black holes using X-ray polarimetry, like those closer to the center of galaxies. Maybe we better understand black hole evolution, as well as galaxy evolution.”

How Might Dark Matter Interact With Ordinary Matter?

An international team of scientists that includes University of California, Riverside, physicist Hai-Bo Yu has imposed conditions on how dark matter may interact with ordinary matter — constraints that can help identify the elusive dark matter particle and detect it on Earth.

Dark matter — nonluminous material in space — is understood to constitute 85 percent of the matter in the universe. Unlike normal matter, it does not absorb, reflect, or emit light, making it difficult to detect.

Physicists are certain dark matter exists, having inferred this existence from the gravitational effect dark matter has on visible matter. What they are less certain of is how dark matter interacts with ordinary matter — or even if it does.

In the search for direct detection of dark matter, the experimental focus has been on WIMPs, or weakly interacting massive particles, the hypothetical particles thought to make up dark matter.

But Yu’s international research team invokes a different theory to challenge the WIMP paradigm: the self-interacting dark matter model, or SIDM, a well-motivated framework that can explain the full range of diversity observed in the galactic rotation curves. First proposed in 2000 by a pair of eminent astrophysicists, SIDM has regained popularity in both the particle physics and the astrophysics communities since around 2009, aided, in part, by work Yu and his collaborators did.

Yu, a theorist in the Department of Physics and Astronomy at UCR, and Yong Yang, an experimentalist at Shanghai Jiaotong University in China, co-led the team analyzing and interpreting the latest data collected in 2016 and 2017 at PandaX-II, a xenon-based dark matter direct detection experiment in China (PandaX refers to Particle and Astrophysical Xenon Detector; PandaX-II refers to the experiment). Should a dark matter particle collide with PandaX-II’s liquefied xenon, the result would be two simultaneous signals: one of photons and the other of electrons.

Yu explained that PandaX-II assumes dark matter “talks to” normal matter — that is, interacts with protons and neutrons — by means other than gravitational interaction (just gravitational interaction is not enough). The researchers then search for a signal that identifies this interaction. In addition, the PandaX-II collaboration assumes the “mediator particle,” mediating interactions between dark matter and normal matter, has far less mass than the mediator particle in the WIMP paradigm.

“The WIMP paradigm assumes this mediator particle is very heavy — 100 to 1000 times the mass of a proton — or about the mass of the dark matter particle,” Yu said. “This paradigm has dominated the field for more than 30 years. In astrophysical observations, we don’t, however, see all its predictions. The SIDM model, on the other hand, assumes the mediator particle is about 0.001 times the mass of the dark matter particle, inferred from astrophysical observations from dwarf galaxies to galaxy clusters. The presence of such a light mediator could lead to smoking-gun signatures of SIDM in dark matter direct detection, as we suggested in an earlier theory paper. Now, we believe PandaX-II, one of the world’s most sensitive direct detection experiments, is poised to validate the SIDM model when a dark matter particle is detected.”

The international team of researchers reports July 12 in Physical Review Letters the strongest limit on the interaction strength between dark matter and visible matter with a light mediator. The journal has selected the research paper as a highlight, a significant honor.

“This is a particle physics constraint on a theory that has been used to understand astrophysical properties of dark matter,” said Flip Tanedo, a dark matter expert at UCR, who was not involved in the research. “The study highlights the complementary ways in which very different experiments are needed to search for dark matter. It also shows why theoretical physics plays a critical role to translate between these different kinds of searches. The study by Hai-Bo Yu and his colleagues interprets new experimental data in terms of a framework that makes it easy to connect to other types of experiments, especially astrophysical observations, and a much broader range of theories.”

PandaX-II is located at the China Jinping Underground Laboratory, Sichuan Province, where pandas are abundant. The laboratory is the deepest underground laboratory in the world. PandaX-II had generated the largest dataset for dark matter detection when the analysis was performed. One of only three xenon-based dark matter direct detection experiments in the world, PandaX-II is one of the frontier facilities to search for extremely rare events where scientists hope to observe a dark matter particle interacting with ordinary matter and thus better understand the fundamental particle properties of dark matter.

Particle physicists’ attempts to understand dark matter have yet to yield definitive evidence for dark matter in the lab.

“The discovery of a dark matter particle interacting with ordinary matter is one of the holy grails of modern physics and represents the best hope to understand the fundamental, particle properties of dark matter,” Tanedo said.

For the past decade, Yu, a world expert on SIDM, has led an effort to bridge particle physics and cosmology by looking for ways to understand dark matter’s particle properties from astrophysical data. He and his collaborators have discovered a class of dark matter theories with a new dark force that may explain unexpected features seen in the systems across a wide range, from dwarf galaxies to galaxy clusters. More importantly, this new SIDM framework serves as a crutch for particle physicists to convert astronomical data into particle physics parameters of dark matter models. In this way, the SIDM framework is a translator for two different scientific communities to understand each other’s results.

Now with the PandaX-II experimental collaboration, Yu has shown how self-interacting dark matter theories may be distinguished at the PandaX-II experiment.

“Prior to this line of work, these types of laboratory-based dark matter experiments primarily focused on dark matter candidates that did not have self-interactions,” Tanedo said. “This work has shown how dark forces affect the laboratory signals of dark matter.”

Yu noted that this is the first direct detection result for SIDM reported by an experimental collaboration.

“With more data, we will continue to probe the dark matter interactions with a light mediator and the self-interacting nature of dark matter,” he said.

The Gaia Sausage: The Major Collision That Changed The Milky Way Galaxy

An international team of astronomers has discovered an ancient and dramatic head-on collision between the Milky Way and a smaller object, dubbed the “Sausage” galaxy. The cosmic crash was a defining event in the early history of the Milky Way and reshaped the structure of our galaxy, fashioning both its inner bulge and its outer halo, the astronomers report in a series of new papers.

The astronomers propose that around 8 billion to 10 billion years ago, an unknown dwarf galaxy smashed into our own Milky Way. The dwarf did not survive the impact: It quickly fell apart, and the wreckage is now all around us.

“The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits” that are long and narrow like needles, said Vasily Belokurov of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute in New York City. The stars’ paths take them “very close to the centre of our galaxy. This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed.”

The new papers in the Monthly Notices of the Royal Astronomical Society, The Astrophysical Journal Letters and arXiv.org outline the salient features of this extraordinary event. Several of the papers were led by Cambridge graduate student GyuChul Myeong. He and colleagues used data from the European Space Agency’s Gaia satellite. This spacecraft has been mapping the stellar content of our galaxy, recording the journeys of stars as they travel through the Milky Way. Thanks to Gaia, astronomers now know the positions and trajectories of our celestial neighbours with unprecedented accuracy.

The paths of the stars from the galactic merger earned them the moniker “the Gaia Sausage,” explained Wyn Evans of Cambridge. “We plotted the velocities of the stars, and the sausage shape just jumped out at us. As the smaller galaxy broke up, its stars were thrown onto very radial orbits. These Sausage stars are what’s left of the last major merger of the Milky Way.”

The Milky Way continues to collide with other galaxies, such as the puny Sagittarius dwarf galaxy. However, the Sausage galaxy was much more massive. Its total mass in gas, stars and dark matter was more than 10 billion times the mass of our sun. When the Sausage crashed into the young Milky Way, its piercing trajectory caused a lot of mayhem. The Milky Way’s disk was probably puffed up or even fractured following the impact and would have needed to regrow. And Sausage debris was scattered all around the inner parts of the Milky Way, creating the ‘bulge’ at the galaxy’s centre and the surrounding ‘stellar halo.’

Numerical simulations of the galactic mashup can reproduce these features, said Denis Erkal of the University of Surrey. In simulations run by Erkal and colleagues, stars from the Sausage galaxy enter stretched-out orbits. The orbits are further elongated by the growing Milky Way disk, which swells and becomes thicker following the collision.

Evidence of this galactic remodelling is seen in the paths of stars inherited from the dwarf galaxy, said Alis Deason of Durham University. “The Sausage stars are all turning around at about the same distance from the centre of the galaxy.” These U-turns cause the density in the Milky Way’s stellar halo to decrease dramatically where the stars flip directions. This discovery was especially pleasing for Deason, who predicted this orbital pileup almost five years ago. The new work explains how the stars fell into such narrow orbits in the first place.

The new research also identified at least eight large, spherical clumps of stars called globular clusters that were brought into the Milky Way by the Sausage galaxy. Small galaxies generally do not have globular clusters of their own, so the Sausage galaxy must have been big enough to host a collection of clusters.

“While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all,” said Sergey Koposov of Carnegie Mellon University, who has studied the kinematics of the Sausage stars and globular clusters in detail.

Does Some Dark Matter Carry An Electric Charge?

Astronomers have proposed a new model for the invisible material that makes up most of the matter in the Universe. They have studied whether a fraction of dark matter particles may have a tiny electrical charge.

“You’ve heard of electric cars and e-books, but now we are talking about electric dark matter,” said Julian Munoz of Harvard University in Cambridge, Mass., who led the study that has been published in the journal Nature. “However, this electric charge is on the very smallest of scales.”

Munoz and his collaborator, Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., explore the possibility that these charged dark matter particles interact with normal matter by the electromagnetic force.

Their new work dovetails with a recently announced result from the Experiment to Detect the Global EoR (Epoch of Reionization) Signature (EDGES) collaboration. In February, scientists from this project said they had detected the radio signature from the first generation of stars, and possible evidence for interaction between dark matter and normal matter. Some astronomers quickly challenged the EDGES claim. Meanwhile, Munoz and Loeb were already looking at the theoretical basis underlying it.

“We’re able to tell a fundamental physics story with our research no matter how you interpret the EDGES result,” said Loeb, who is the chair of the Harvard astronomy department. “The nature of dark matter is one of the biggest mysteries in science and we need to use any related new data to tackle it.”

The story begins with the first stars, which emitted ultraviolet (UV) light. According to the commonly accepted scenario, this UV light interacted with cold hydrogen atoms in gas lying between the stars and enabled them to absorb the cosmic microwave background (CMB) radiation, the leftover radiation from the Big Bang.

This absorption should have led to a drop in intensity of the CMB during this period, which occurs less than 200 million years after the Big Bang. The EDGES team claimed to detect evidence for this absorption of CMB light, though this has yet to be independently verified by other scientists. However, the temperature of the hydrogen gas in the EDGES data is about half of the expected value.

“If EDGES has detected cooler than expected hydrogen gas during this period, what could explain it?” said Munoz. “One possibility is that hydrogen was cooled by the dark matter.”

At the time when CMB radiation is being absorbed, the any free electrons or protons associated with ordinary matter would have been moving at their slowest possible speeds (since later on they were heated by X-rays from the first black holes). Scattering of charged particles is most effective at low speeds. Therefore, any interactions between normal matter and dark matter during this time would have been the strongest if some of the dark matter particles are charged. This interaction would cause the hydrogen gas to cool because the dark matter is cold, potentially leaving an observational signature like that claimed by the EDGES project.

“We are constraining the possibility that dark matter particles carry a tiny electrical charge – equal to one millionth that of an electron – through measurable signals from the cosmic dawn,” said Loeb. “Such tiny charges are impossible to observe even with the largest particle accelerators.”

Only small amounts of dark matter with weak electrical charge can both explain the EDGES data and avoid disagreement with other observations. If most of the dark matter is charged, then these particles would have been deflected away from regions close to the disk of our own Galaxy, and prevented from reentering. This conflicts with observations showing that large amounts of dark matter are located close to the disk of the Milky Way.

Scientists know from observations of the CMB that protons and electrons combined in the early Universe to form neutral atoms. Only a small fraction of these charged particles, about one in a few thousand, remained free. Munoz and Loeb are considering the possibility that dark matter may have acted in a similar way. The data from EDGES, and similar experiments, might be the only way to detect the few remaining charged particles, as most of the dark matter would be neutral.

“The viable parameter space for this scenario is quite constrained, but if confirmed by future observations, of course we would be learning something fundamental about the nature of dark matter, one of the biggest puzzles that we have in physics today,” said Harvard’s Cora Dvorkin who was not involved with the new study.

Lincoln Greenhill also from the CfA is currently testing the observational claim by the EDGES team. He leads the Large Aperture Experiment to Detect the Dark Ages (LEDA) project, which uses the Long Wavelength Array in Owen’s Valley California and Socorro, New Mexico.

A paper describing these results appear in the May 31, 2018 issue of the journal Nature.

Dark Matter Goes Missing In Oddball Galaxy

Galaxies and dark matter go together like peanut butter and jelly. You typically don’t find one without the other.

Therefore, researchers were surprised when they uncovered a galaxy that is missing most, if not all, of its dark matter. An invisible substance, dark matter is the underlying scaffolding upon which galaxies are built. It’s the glue that holds the visible matter in galaxies — stars and gas — together.

“We thought that every galaxy had dark matter and that dark matter is how a galaxy begins,” said Pieter van Dokkum of Yale University in New Haven, Connecticut, lead researcher of the Hubble observations. “This invisible, mysterious substance is the most dominant aspect of any galaxy. So finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real: it has its own separate existence apart from other components of galaxies. This result also suggests that there may be more than one way to form a galaxy.”

The unique galaxy, called NGC 1052-DF2, contains at most 1/400th the amount of dark matter that astronomers had expected. The galaxy is as large as our Milky Way, but it had escaped attention because it contains only 1/200th the number of stars. Given the object’s large size and faint appearance, astronomers classify NGC 1052-DF2 as an ultra-diffuse galaxy. A 2015 survey of the Coma galaxy cluster showed these large, faint objects to be surprisingly common.

But none of the ultra-diffuse galaxies discovered so far have been found to be lacking in dark matter. So even among this unusual class of galaxy, NGC 1052-DF2 is an oddball.

Van Dokkum and his team spotted the galaxy with the Dragonfly Telephoto Array, a custom-built telescope in New Mexico they designed to find these ghostly galaxies. They then used the W.M. Keck Observatory in Hawaii to measure the motions of 10 giant groupings of stars called globular clusters in the galaxy. Keck revealed that the globular clusters were moving at relatively low speeds, less than 23,000 miles per hour. Stars and clusters in the outskirts of galaxies containing dark matter move at least three times faster. From those measurements, the team calculated the galaxy’s mass. “If there is any dark matter at all, it’s very little,” van Dokkum explained. “The stars in the galaxy can account for all the mass, and there doesn’t seem to be any room for dark matter.”

The researchers next used NASA’s Hubble Space Telescope and the Gemini Observatory in Hawaii to uncover more details about the unique galaxy. Gemini revealed that the galaxy does not show signs of an interaction with another galaxy. Hubble helped them better identify the globular clusters and measure an accurate distance to the galaxy.

The Hubble images also revealed the galaxy’s unusual appearance. “I spent an hour just staring at the Hubble image,” van Dokkum recalled. “It’s so rare, particularly these days after so many years of Hubble, that you get an image of something and you say, ‘I’ve never seen that before.’ This thing is astonishing: a gigantic blob that you can look through. It’s so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy.”

The ghostly galaxy doesn’t have a noticeable central region, or even spiral arms and a disk, typical features of a spiral galaxy. But it doesn’t look like an elliptical galaxy, either. The galaxy also shows no evidence that it houses a central black hole. Based on the colors of its globular clusters, the galaxy is about 10 billion years old. Even the globular clusters are oddballs: they are twice as large as typical stellar groupings seen in other galaxies.

“It’s like you take a galaxy and you only have the stellar halo and globular clusters, and it somehow forgot to make everything else,” van Dokkum said. “There is no theory that predicted these types of galaxies. The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is completely unknown.”

But the researchers do have some ideas. NGC 1052-DF2 resides about 65 million light-years away in a collection of galaxies that is dominated by the giant elliptical galaxy NGC 1052. Galaxy formation is turbulent and violent, and van Dokkum suggests that the growth of the fledgling massive galaxy billions of years ago perhaps played a role in NGC 1052-DF2’s dark-matter deficiency.

Another idea is that gas moving toward the giant elliptical NGC 1052 may have fragmented and formed NGC 1052-DF2. The formation of NGC 1052-DF2 may have been helped by powerful winds emanating from the young black hole that was growing in the center of NGC 1052. These possibilities are speculative, however, and don’t explain all of the characteristics of the observed galaxy, the researchers said.

The team is already hunting for more dark-matter deficient galaxies. They are analyzing Hubble images of 23 other diffuse galaxies. Three of them appear similar to NGC 1052-DF2.

“Every galaxy we knew about before has dark matter, and they all fall in familiar categories like spiral or elliptical galaxies,” van Dokkum said. “But what would you get if there were no dark matter at all? Maybe this is what you would get.”

Chasing Dark Matter With Oldest Stars In The Milky Way

Just how quickly is the dark matter near Earth zipping around? The speed of dark matter has far-reaching consequences for modern astrophysical research, but this fundamental property has eluded researchers for years.

In a paper published Jan. 22 in the journal Physical Review Letters, an international team of astrophysicists provided the first clue: The solution to this mystery, it turns out, lies among some of the oldest stars in the galaxy.

“Essentially, these old stars act as visible speedometers for the invisible dark matter, measuring its speed distribution near Earth,” said Mariangela Lisanti, an assistant professor of physics at Princeton University. “You can think of the oldest stars as a luminous tracer for the dark matter. The dark matter itself we’ll never see, because it’s not emitting light to any observable degree — it’s just invisible to us, which is why it’s been so hard to say anything concrete about it.”

In order to determine which stars behave like the invisible and undetectable dark matter particles, Lisanti and her colleagues turned to a computer simulation, Eris, which uses supercomputers to replicate the physics of the Milky Way galaxy, including dark matter.

“Our hypothesis was that there’s some subset of stars that, for some reason, will match the movements of the dark matter,” said Jonah Herzog-Arbeitman, an undergraduate and a co-author on the paper. His work with Lisanti and her colleagues the summer after his first year at Princeton turned into one of his junior papers and contributed to this journal article.

Herzog-Arbeitman and Lina Necib at the California Institute of Technology, another co-author on the paper, generated numerous plots from Eris data that compared various properties of dark matter to properties of different subsets of stars.

Their big breakthrough came when they compared the velocity of dark matter to that of stars with different “metallicities,” or ratios of heavy metals to lighter elements.

The curve representing dark matter matched up beautifully with the stars that have the least heavy metals: “We saw everything line up,” Lisanti said.

“It was one of those great examples of a pretty reasonable idea working pretty darn well,” Herzog-Arbeitman said.

Astronomers have known for decades that metallicity can serve as a proxy for a star’s age, since metals and other heavy elements are formed in supernovas and the mergers of neutron stars. The small galaxies that merged with the Milky Way typically have comparatively less of these heavy elements.

In retrospect, the correlation between dark matter and the oldest stars shouldn’t be surprising, said Necib. “The dark matter and these old stars have the same initial conditions: they started in the same place and they have the same properties … so at the end of the day, it makes sense that they’re both acted on only through gravity,” she said.

Why it matters

Since 2009, researchers have been trying to observe dark matter directly, by putting very dense material — often xenon — deep underground and waiting for the dark matter that flows through the planet to interact with it.

Lisanti compared these “direct detection” experiments to a game of billiards: “When a dark matter particle scatters off a nucleus in an atom, the collision is similar to two billiard balls hitting each other. If the dark matter particle is much less massive than the nucleus, then the nucleus won’t move much after the collision, which makes it really hard to notice that anything happened.”

That’s why constraining the speed of dark matter is so important, she explained. If dark matter particles are both slow and light, they might not have enough kinetic energy to move the nuclear “billiard balls” at all, even if they smack right into one.

“But if the dark matter comes in moving faster, it’s going to have more kinetic energy. That can increase the chance that in that collision, the recoil of the nucleus is going to be greater, so you’d be able to see it,” Lisanti said.

Originally, scientists had expected to see enough particle interactions — enough moving billiard balls — to be able to derive the mass and velocity of the dark matter particles. But, Lisanti said, “we haven’t seen anything yet.”

So instead of using the interactions to determine the speed, researchers like Lisanti and her colleagues are hoping to flip the script, and use the speed to explain why the direct detection experiments haven’t detected anything yet.

The failure — at least so far — of the direct detection experiments leads to two questions, Lisanti said. “How am I ever going to figure out what the speeds of these things are?” and “Have we not seen anything because there’s something different in the speed distribution than we expected?”

Having a completely independent way to work out the speed of dark matter could help shed light on that, she said. But so far, it’s only theoretical. Real-world astronomy hasn’t caught up to the wealth of data produced by the Eris simulation, so Lisanti and her colleagues don’t yet know how fast our galaxy’s oldest stars are moving.

Fortunately, that information is being assembled right now by the European Space Agency’s Gaia telescope, which has been scanning the Milky Way since July 2014. So far, information on only a small subset of stars has been released, but the full dataset will include far more data on nearly a billion stars.

“The wealth of data on the horizon from current and upcoming stellar surveys will provide a unique opportunity to understand this fundamental property of dark matter,” Lisanti said.