News Archives

Massive halo finally explains stream of gas swirling around the Milky Way

a starscape showing the milky way in the distance and a rendering of the gases surrounding the large magellenic cloud
a starscape showing the milky way in the distance and a rendering of the gases surrounding the large magellenic cloud
The Large and Small Magellanic Clouds as they would appear if the gas around them was visible to the naked eye. | Credits: Scott Lucchini (simulation), Colin Legg (background)

The Large and Small Magellanic Clouds are satellite galaxies of the Milky Way. They are surrounded by a high-velocity gaseous structure called the Magellanic Stream, which consists of gas stripped from both clouds. So far, simulations have been unable to reconcile observations with a complete picture of how the stream was formed. In this Nature week’s issue, numerical simulations carried out at by Scott Lucchini, graduate student at the Physics Department working with Elena D’Onghia, present a model that potentially resolves this conundrum. By embedding the Large Magellanic Cloud in a corona of ionized gas, the researchers were able to simulate the Magellanic Stream accurately and explain its structure. Ellen Zweibel and Chad Bustard are also co-authors of the article.

Read the full UW news story | Read the Nature article

 

WQI team named winners in international quantum research competition

a blue-laser-hued image of a trapped ball of strontium ions in an optical lattice clock

A WQI faculty team was one of 18 winners in the Innovare Advancement Center’s “Million Dollar International Quantum U Tech Accelerator” competition, which awarded a total of $1.35 million last week. The winning teams, including UW­–Madison physics professors Shimon Kolkowitz and Mark Saffman, each earned $75,000 toward their proposed research.

The competition attracted nearly 250 proposals from teams across the world in the areas of quantum timing, sensing, computing and communications, and 36 teams were invited to present at the live virtual event.

Full story

Prof. Brian Rebel promoted to Senior Scientist at Fermilab

Brian Rebel

Yesterday, Fermilab promoted Prof. Brian Rebel to Senior Scientist. He has a joint appointment there, and his new title at Fermilab is the closest equivalent to full professor for which scientific staff are eligible. Congrats, Brian!

Q-NEXT collaboration awarded National Quantum Initiative funding

the tip of a scanning electron microscope is poised over a setup with metal clips pointing out

The University of Wisconsin–Madison solidified its standing as a leader in the field of quantum information science when the U.S. Department of Energy (DOE) and the White House announced the Q-NEXT collaboration as a funded Quantum Information Science Research Center through the National Quantum Initiative Act. The five-year, $115 million collaboration was one of five Centers announced today.

Q-NEXT, a next-generation quantum science and engineering collaboration led by the DOE’s Argonne National Laboratory, brings together nearly 100 world-class researchers from three national laboratories, 10 universities including UW–Madison, and 10 leading U.S. technology companies to develop the science and technology to control and distribute quantum information.

“The main goals for Q-NEXT are first to deliver quantum interconnects — to find ways to quantum mechanically connect distant objects,” says Mark Eriksson, the John Bardeen Professor of Physics at UW–Madison and a Q-NEXT thrust lead. “And next, to establish a national resource to both develop and provide pristine materials for quantum science and technology.”

profile photo of Mark Eriksson
Mark Eriksson

Q-NEXT will focus on three core quantum technologies:

  • Communication for the transmission of quantum information across long distances using quantum repeaters, enabling the establishment of “unhackable” networks for information transfer
  • Sensors that achieve unprecedented sensitivities with transformational applications in physics, materials, and life sciences
  • Processing and utilizing “test beds” both for quantum simulators and future full-stack universal quantum computers with applications in quantum simulations, cryptanalysis, and logistics optimization.

Eriksson is leading the Materials and Integration thrust, one of six Q-NEXT focus areas that features researchers from across the collaboration. This thrust aims to: develop high-coherence materials, including for silicon and superconducting qubits, which is an essential component of preserving entanglement; develop a silicon-based optical quantum memory, which is important in developing a quantum repeater; and improve color-center quantum bits, which are used in both communication and sensing.

“One of the key goals in Materials and Integration is to not just improve the materials but also to improve how you integrate those materials together so that in the end, quantum devices maintain coherence and preserve entanglement,” Eriksson says. “The integration part of the name is really important. You may have a material that on its own is really good at preserving coherence, yet you only make something useful when you integrate materials together.”

Six other UW­–Madison and Wisconsin Quantum Institute faculty members are Q-NEXT investigators: physics professors Victor Brar, Shimon Kolkowitz, Robert McDermott, and Mark Saffman, electrical and computer engineering professor Mikhail Kats, and chemistry professor Randall Goldsmith. UW–Madison researchers are involved in five of the six research thrusts.

“I’m excited about Q-NEXT because of the connections and collaborations it provides to national labs, other universities, and industry partners,” Eriksson says. “When you’re talking about research, it’s those connections that often lead to the breakthroughs.

The potential impacts of Q-NEXT research include the creation of a first-ever National Quantum Devices Database that will promote the development and fabrication of next generation quantum devices as well as the development of the components and systems that enable quantum communications across distances ranging from microns to kilometers.

“This funding helps ensure that the Q-NEXT collaboration will lead the way in future developments in quantum science and engineering,” says Steve Ackerman, UW–Madison vice chancellor for research and graduate education. “Q-NEXT is the epitome of the Wisconsin Idea as we work together to transfer new quantum technologies to the marketplace and support U.S. economic competitiveness in this growing field.”

infographic of all q-next partner national labs, universities, and industry
The Q-NEXT partners

New study expands types of physics, engineering problems that can be solved by quantum computers

A well-known quantum algorithm that is useful in studying and solving problems in quantum physics can be applied to problems in classical physics, according to a new study in the journal Physical Review A from University of Wisconsin–Madison assistant professor of physics Jeff Parker.

Quantum algorithms – a set of calculations that are run on a quantum computer as opposed to a classical computer – used for solving problems in physics have mainly focused on questions in quantum physics. The new applications include a range of problems common to physics and engineering, and expands on the types of questions that can be asked in those fields.

profile photo of Jeff Parker
Jeff Parker

“The reason we like quantum computers is that we think there are quantum algorithms that can solve certain kinds of problems very efficiently in ways that classical computers cannot,” Parker says. “This paper presents a new idea for a type of problem that has not been addressed directly in the literature before, but it can be solved efficiently using these same quantum computer types of algorithms.”

The type of problem Parker was investigating is known as generalized eigenvalue problems, which broadly describe trying to find the fundamental frequencies or modes of a system. Solving them is crucial to understanding common physics and engineering questions, such as the stability of a bridge’s design or, more in line with Parker’s research interests, the stability and efficiency of nuclear fusion reactors.

As the system being studied becomes more and more complex — more components moving throughout three-dimensional space — so does the numerical matrix that describes the problem. A simple eigenvalue problem can be solved with a pencil and paper, but researchers have developed computer algorithms to tackle increasingly complex ones. With the supercomputers available today, more and more difficult physics problems are finding solutions.

“If you want to solve a three-dimensional problem, it can be very complex, with a very complicated geometry,” Parker says. “You can do a lot on today’s supercomputers, but there tends to be a limit. Quantum algorithms may be able to break that limit.”

The specific quantum algorithm that Parker studied in this paper, known as quantum phase estimation, had been previously applied to so-called standard eigenvalue problems. However, no one had shown that they could be applied to the generalized eigenvalue problems that are also common in physics. Generalized eigenvalue problems introduce a second matrix that ups the mathematical complexity.

Parker took the quantum algorithm and extended it to generalized eigenvalue problems. He then looked to see what types of matrices could be used in this problem. If the matrix is sparse ­— meaning, if most of the numerical components that make it up are zero — it means this problem could be solved efficiently on a quantum computer.

The study shows that quantum algorithms could be applied to classical physics problems, such as nuclear fusion mirror machines. | Credit: Cary Forest

“What I showed is that there are certain types of generalized eigenvalue problems that do lead to a sparse matrix and therefore could be efficiently solved on a quantum computer,” Parker says. “This type includes the very natural problems that often occur in physics and engineering, so this study provides motivation for applying these quantum algorithms more to generalized eigenvalue problems, because it hasn’t been a big focus so far.”

Parker emphasizes that quantum computers are in their infancy, and these classical physics problems are still best approached through classical computer algorithms.

“This study provides a step in showing that the application of a quantum algorithm to classical physics problems can be useful in the future, and the main advance here is it shows very clearly another type of problem to which quantum algorithms can be applied,” Parker says.

The study was completed in collaboration with Ilon Joseph at Lawrence Livermore National Laboratory. Funding support was provided by the U.S. Department of Energy to Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344 and U.S. DOE Office of Fusion Energy Sciences “Quantum Leap for Fusion Energy Sciences” (FWP SCW1680).

A somber remembrance marks the 50th anniversary of the Sterling Hall bombing

black and white photo of Sterling Hall after the bombing shows a building with windows blown out and obvious damage.

By Doug Erickson

On an August afternoon 50 years ago, graduate student Bill Evans bumped into Robert Fassnacht, a postdoctoral researcher, in Sterling Hall at the University of Wisconsin­–Madison.

The two didn’t know each other well, but they had talked before. Both were conducting physics experiments in Sterling Hall.

profile photo of Robert Fassnacht
33-year-old Robert Fassnacht, a postdoctoral researcher in physics and father of three young children, was killed in the bombinb. It injured four others. | UW ARCHIVES

Fassnacht mentioned he’d be working through the night. Evans planned to do the same, so he made a mental note to walk over and chat with Fassnacht at some point.

The conversation never happened. At 3:42 a.m. that morning — August 24, 1970 — a bomb tore through a wing of Sterling Hall, killing Fassnacht. Evans, whose lab was much farther from the blast, felt the building shake but was uninjured.

A short time later, Evans says he and another physics researcher, John Lynch, came upon Fassnacht’s lifeless body.

“That’s the part I’m trying to forget and the reason I haven’t talked about it in all these years,” says Evans, 78, by phone from his home in California. “I still have flashbacks.”

The target of the explosion was the Army Mathematics Research Center, housed on multiple upper floors of Sterling Hall. Four young men orchestrated the bombing as a protest against the center’s research connections with the U.S. military during the Vietnam War.

Fassnacht, 33, working in a basement lab in the Physics Department below the Army Mathematics Research Center, was an unintended victim. His research had no connection to the center. Four others — three in Sterling Hall and one across the street at University Hospital — were injured.

black and white photo of Sterling Hall after the bombing shows a building with windows blown out and obvious damage.
The bombing of Sterling Hall on August 24, 1970, was the shocking culmination of years of dissent and despair over the Vietnam War. | UW ARCHIVES

Three of the four bombers — David Fine and brothers Karl and Dwight Armstrong — eventually served prison time. The fourth, Leo Burt, remains at large. Burt and Fine were UW–Madison students at the time.

Evans was pursuing a Ph.D. in atomic physics. He remembers feeling the building shudder, then seeing a wave of dirt and dust blow by a lab door.

He immediately called the university’s overnight phone desk and reported that something terrible had happened at Sterling Hall.

Stepping into the hallway, he tried to head toward the blast’s origin, but thick dust forced him back. He called the UW operator again: “You better get someone over here.”

Evans then went down a basement hallway in the other direction.

“I came upon a night watchman, dazed and covered all over with what looked like pieces of insulation,” he says. “I got him out of the building. There were two policemen nearby, and I yelled, ‘This guy needs help.’”

The night watchman, UW security officer Norbert Sutter, suffered memory impairment, disc problems, and permanent loss of some hearing and vision. The officers who helped Sutter insisted that Evans go with them to University Hospital to be checked for possible injuries. At the time, the hospital was located across Charter Street from Sterling Hall. Evans, certain he was not injured, protested but gave in, then quickly slipped out of the hospital before being evaluated.

Returning to Sterling Hall, Evans says he ran into Lynch. Today, the two differ on the sequence of events that led them to Fassnacht’s body. Both say it’s hard to remember events from so far back — some details remain vivid to them; others have become hazy with time.

The blast had awakened Lynch at his apartment just a few blocks away. He remembers racing to Sterling Hall and entering the building alone. He says he saw Fassnacht’s body, then went looking for others dead or alive inside the building.

“There were no policemen, no firemen yet,” says Lynch, 82, who is retired from the National Science Foundation and lives in Florida. “I’m running around looking for anybody alive. The person I found was Bill Evans.”

Evans thinks he ran into Lynch in the crowd that was forming outside Sterling Hall. He recalls the two of them entering the building together and finding Fassnacht’s body.

“He was face down, with a large piece of concrete on him, and his nose and mouth were under water,” Evans says. “There was no question he was dead. The water (due to broken water pipes) was fairly deep by then.”

The two alerted rescuers to Fassnacht’s body. The pair also helped emergency workers find and shut off a gas leak that had led to a fire, Lynch says.

Later that same day, Lynch recounted the story to a reporter for The Capital Times, the city’s afternoon newspaper. The article’s large headline reads, “I Found Bob Under a Foot of Water.”

Given the era’s anti-war fervor, Lynch says it did not surprise him to look out his bedroom window and see Sterling Hall with a cloud of smoke above it. He had stopped spending evenings at Sterling Hall after a conversation with strangers in a Madison bar a few months earlier.

“One guy said to me, ‘Don’t hang around that place at night. Bad things are going to happen there,’” Lynch says. “I didn’t go to the police because people were saying all sorts of crazy things back then. But I felt I had been forewarned.”

Lynch provided prosecutors with a deposition in the case. Following an esteemed career, he received a Distinguished Alumni Fellow Award in 2003 from UW–Madison’s Physics Department. The department recognized him in large part for his early and sustained support at the federal level of the IceCube Neutrino Observatory, a project at the South Pole managed and operated by UW–Madison.

Evans told his story to law enforcement officers but says he otherwise has rarely discussed his involvement with anyone.

In the spring of 1972, Evans married fellow UW–Madison student Gertrude “Kim” Miller. A few months later, the couple moved to Washington, D.C., where Evans began a job at the U.S. Naval Research Laboratory. He returned to Madison in 1975 to defend his doctoral dissertation but otherwise has not been back to campus.

Evans retired in 2012 following a long career as a research physicist and software architect. He says he’s spent five decades avoiding anything that might trigger a memory of Sterling Hall.

“I tried not to think of it for obvious reasons,” he says. “I guess you could say I disappeared for a good while.”

The experience remains upsetting.

“What happens is it pops up in your memory and then takes about a week to disappear,” Evans says. “I guess it would be like what they talk about with PTSD (post-traumatic stress disorder). If so, I can understand why these people have troubles.”

Late last year, in anticipation of the 50th anniversary of the Sterling Hall bombing this August, UW–Madison issued a call to alumni for memories related to the bombing. Hundreds responded. A sample can be found in the summer 2020 issue of On Wisconsin, the university’s alumni magazine.

Evans was not among those who submitted memories. He says he read the magazine article and found it interesting how the bombing impacted other people. For him, though, it is something he prefers not to reflect on.

“It was so long ago,” he says. “Strange things happen.”

black and white photo of a blown-out building due to bomb damage
Damage as seen from inside the building. | UW ARCHIVES

NSF Physics Frontier Center for neutron star modeling to include UW–Madison

A green, egg-shaped density in the middle has two cones of dark blue representing the gravitational waves projecting perpendicularly out either side of the green density

A group of universities, including the University of Wisconsin–Madison, has been named the newest Physics Frontier Center, the National Science Foundation announced Aug. 17. The center expands the reach and depth of existing capabilities in modeling some of the most violent events known in the universe: the mergers of neutron stars and their explosive aftermath.

The Network for Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) is already an established hub of eight institutions, including UW–Madison, that uses the most extreme environments found in astrophysics — the Big Bang, supernovae, and neutron star and black hole mergers — as laboratories for testing fundamental physics under conditions beyond the reach of Earth-based labs. The upgrade to a Physics Frontier Center adds five institutions, provides $10.9 million in funding for postdoctoral fellowships and allows members to cover an expanded scope of research.

“For 20 years, we’ve expected that the growing precision of astrophysical and cosmological measurements would make this field an increasingly important part of fundamental physics. Indeed, four monumental discoveries — neutrino masses, dark matter, the accelerating universe, and gravitational waves — have confirmed this prediction,” says A. Baha Balantekin, a professor of physics at UW–Madison and one of the principal investigators for N3AS.

Read the full story 

Welcome, Assistant Professor Jeff Parker!

profile photo of Jeff Parker

Have you heard the joke about the lawyer who became a physics professor? Jeff Parker has, but rather than be the punchline, he was always in on the joke. After earning his Ph.D. in plasma physics from Princeton in 2014, Parker enrolled at Stanford Law School to pursue a career in energy and climate policy. “I lasted one year in law school, decided I really didn’t like it and just loved physics, and I wanted to get back to physics research,” Parker says.

After that one year, Parker accepted a postdoctoral fellowship at Lawrence Livermore National Lab, and two years later became a staff scientist there. On July 1, 2020, Parker joined the UW–Madison Physics Department as its newest assistant professor. Here, he will focus his research interests in theoretical plasma astrophysics. To welcome Professor Parker, we sat down for a (virtual) Q+A with him.

What are the main topics or projects that you will focus your research on?

My immediate research program has two main directions.

One area of research is going to be in plasma astrophysics and astrophysical fluid dynamics. This concerns plasmas in space or in the universe, like in the sun, or the origin of magnetic fields in the cosmos and how they shape what we see in the universe.  I will be investigating angular momentum transport by magnetic fields, which can occur in stars, accretion disks around black holes, and planetary interiors.

Another area is topological phases of matter in plasma physics, related to the 2016 Nobel prize on topological insulators, which came out of condensed matter physics. I am applying these ideas for the first time to plasma physics and plasma waves. This is a brand-new field in plasmas and I’m just getting into it, but I think it’s really, really interesting.

You’re in Madison now, and you’re getting started with your research. What is the first thing you’re doing?

One particular project I’m very interested in is the astrophysical fluid dynamics involving angular momentum transport due to magnetic fields. I have developed theory on something that I call magnetic eddy viscosity, which could be important where there are magnetic fields and rotation. This can occur in astrophysical objects like stars or accretion discs or planets. And so where I studied this was in a pretty idealized system, and I’d really like to extend this into more realistic models that are closer to reality that would help us say something more about real object like stars or accretion discs, or potentially could be measured in the laboratory. So, there are these experiments, Prof. Forest has one, and there are other experiments throughout the country or the world that have rotating plasmas or liquid metals. This effect could potentially be seen in those experiments as well, and that is something I’d love to do right away.

Your work is primarily theory and computation. Do you see your work as predicting ideas that would be tested with collaborators in the department?

That is one thing I do hope to do. But I do also enjoy developing theory to better understand plasmas, even if those theories cannot be tested immediately in an experiment. I’m a theoretical physicist at heart, but there are so many great plasma physics experiments at Madison, which enable a close collaboration of theory and experiment. Progress is truly made when you can measure, observe, analyze, and use theory to understand what you see.

What’s one thing you hope students who take a class with you will come away with?

I want students to take away how plasma physics is everywhere, how most of the universe is plasma, and so if we want to understand the universe, we need to understand plasma physics.

What is your favorite element and/or elementary particle?

For elementary particle, I’ll say the neutrino because it’s so mysterious, and mysterious is good for physics. For favorite element, hydrogen and its isotopes because they’re what’s important for fusion.

What hobbies/other interests do you have?

I like to hike, run, and travel.

 

First-year physics grad student uses her disrupted summer – and her science training – to research N95 safety

profile photo of Winnie Wang

Shortly after incoming physics graduate student Winnie Wang attended a UW­–Madison campus visit weekend in February, her plans took an abrupt change due to COVID-19. The University of Massachusetts, where she was studying, closed right before spring break, and she decided to go to Taipei to be with extended family. But first, she needed to follow the regulations in Taiwan and self-isolate for 14 days.

“I chose to be quarantined in a hotel, so I was by myself for two weeks. It was honestly kind of brutal, and for the first five days I was feeling pretty miserable,” Wang recalls. “I’m putting it bluntly, because that misery was what inspired me to do something about it. I was like, ‘Okay, well, why don’t I proactively use some of my free time.’”

profile photo of Winnie Wang
Winnie Wang

Wang, who is from Canada and attended school in the U.S., watched what was happening to the case numbers in those two countries, especially compared to the relatively lower numbers in Taiwan, and started looking for ways to get involved. She posted on Facebook asking if anyone knew groups she could volunteer with, and eventually landed on a group called N95DECON.

According to the group’s website, N95DECON is a volunteer collective of scientists, engineers, clinicians, and students from universities across the U.S. as well as other professionals in the private sector. N95DECON seeks to review, collate, publish, and disseminate scientific information about N95 decontamination to help inform decisions about N95 decontamination and reuse.

“Hospitals use a lot of N95s, and you’ve probably heard of things where people have put masks in microwaves or rice cookers to decontaminate them. And basically, you don’t want to do that,” Wang says. “We looked at the research that’s already out there, looked at what the CDC recommends, and we culminated our findings into papers and seminars for hospitals to use around the world.”

Wang serves as a communications volunteer for the group, meaning she responds to emails and proofreads and edits the group’s publications. She says that when she first started, N95DECON did not have much in the way of formal documentation, so much of her early efforts were spent answering emails from the public asking about reuse procedures. But knowing that N95s were in short supply and time was of the essence, N95DECON worked quickly to put together online seminars that could be viewed by anyone.

“After we organized and recorded the seminars in May and put them on our website so that anyone can watch them, the email team received less email from the general public,” Wang says. “And I’ve moved on now to more literature review.”

N95DECON shared their work largely through the hospital networks of the health professionals that volunteer with the group, as well as through social media and other word-of-mouth. The group will continue to monitor research on best practices for decontaminating and reusing N95 masks and update their recommendations accordingly. Much of their current efforts are focused on translating their papers and seminars.

“We’d have people from all over the world join our seminars and talk about their experiences,” Wang says. “So, another aspect of our outreach is that we do translations. Our goal is to disperse this information around the world, and we’ve translated it into seven languages now.”

Wang plans to continue volunteering with N95DECON after the UW­–Madison academic year begins. She is interested in studying experimental high energy physics for her doctorate.

Dark Energy Survey census of the smallest galaxies hones the search for dark matter

two circles with clusters of stars in them, showing predictions of warm dark matter (fewer stars visible) on the left and cold dark matter (far more stars) on the right

This story is adapted from one originally published by Fermilab

Today, scientists in the Dark Energy Survey — including UW–Madison assistant professor of physics Keith Bechtol and his research group — released results that have been five years in the making. Researchers used the world’s most complete census of dwarf galaxies around our Milky Way galaxy to probe the nature of dark matter, an invisible form of matter that dominates the universe. These new measurements provide information about what dark matter can and cannot be made of.

In particular, the new results constrain the minimum mass of the dark matter particles, as well as the strength of interactions between dark matter and normal matter.

profile photo of keith bechtol
Keith Bechtol

According to these new results, a dark matter particle must be heavier than a zeptoelectronvolt, which is 10-21 electronvolts. That’s one trillionth of a trillionth of the mass of an electron. This study also shows that dark matter’s interactions with normal matter must be roughly 1,000 times weaker than the weak nuclear force. Of the known forces, only gravity is weaker.

These novel measurements used data from the Dark Energy Survey, a cosmological survey designed to study dark energy, the mysterious force driving the accelerated expansion of the universe. In contrast, dark matter is gravitationally attractive, resisting the expansion of the universe and gravitationally binding astronomical systems such as galaxies. The smallest “dwarf” galaxies can have hundreds to thousands of times more dark matter than normal matter. Over the past five years, the Dark Energy Survey has combined with other surveys to more than double the known population of these tiny galaxies. The current total is now over 50.

“The large number of dwarf galaxies that we found orbiting the Milky Way is consistent with expectations from the simplest picture of dark matter — that is, comprising slow-moving particles that interact only through gravity,” Bechtol explained. “In this new paper, we rule out several alternative possibilities for the nature of dark matter.”

profile photo of Mitch McNanna
Mitch McNanna

Dark matter makes up 85% of the matter in the universe, but we have yet to detect it directly in the laboratory. The gravitational effects of dark matter are clearly visible in the motions of stars in galaxies, the clumpy distribution of galaxies in the universe, and even in the amount of lightweight elements. The robust astronomical evidence for the existence of dark matter has motivated many experimental searches here on Earth, using instruments ranging from cryogenic detectors buried deep underground to energetic particle colliders.

“The faintest galaxies are among the most valuable tools we have to learn about dark matter because they are sensitive to several of its fundamental properties all at once,” said Ethan Nadler, the study’s lead author and graduate student at Stanford University and SLAC.

In these multi-year, multi-telescope sky surveys, the raw data comes in the form of tens of thousands high-resolution digital images. But identifying these ultrafaint galaxies, as their description implies, is not as simple as looking at an image and seeing a faint smudge of light. Bechtol and his group, including physics grad student Mitch McNanna, designed the search algorithms needed to identify, with some statistical assurance, which individual stars are part of a dwarf galaxy.

“We worked closely with experts in galaxy formation and particle physics theory to compare the Dark Energy Survey observations with predictions,” Bechtol said. “Part of our job was to determine the sensitivity of our search — how far away from the Earth could we spot a galaxy with only a few hundred stars?”

By combining the observed census of dwarf galaxies with advanced cosmological simulations of the distribution of dark matter around the Milky Way, scientists were able to predict how the physical properties of dark matter would affect the number of small galaxies. Small galaxies form in regions where the dark matter density in the early universe is very slightly above average. Physical processes that smooth out these regions of higher density (if dark matter moves too quickly or gains energy due to interactions with normal matter) or prevent density variations from collapsing to form galaxies (thanks to quantum interference effects) would reduce the number of galaxies observed by the Dark Energy Survey.

“Astrophysical observations provide unique information about the fundamental nature of dark matter, and are complementary to searches for dark matter particles in terrestrial experiments.” Bechtol said. “With the Dark Energy Survey, we continue to learn about the deep connection between particle physics and the growth of cosmic structure, ranging from the vast network of galaxies in the cosmic web, down to smallest individual galaxies.”

two circles with clusters of stars in them, showing predictions of warm dark matter (fewer stars visible) on the left and cold dark matter (far more stars) on the right
This shows the result of two numerical simulations predicting the distribution of dark matter around a galaxy similar to our Milky Way. The left panel assumes that dark matter particles were moving fast in the early universe (warm dark matter), while the right panel assumes that dark matter particles were moving slowly (cold dark matter). The warm dark matter model predicts many fewer small clumps of dark matter surrounding our galaxy and thus many fewer satellite galaxies that inhabit these small clumps of dark matter. By measuring the number of satellite galaxies, scientists can distinguish between these models of dark matter. | Image: Bullock and Boylan-Kolchin (2017); simulations by V. Robles, T. Kelley and B. Bozek, in collaboration with Bullock and Boylan-Kolchin

The Dark Energy Survey is a collaboration of more than 300 scientists from 25 institutions in six countries. For more information about the survey, please visit the experiment’s website.

Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago Funding Authority for Funding and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science and Technology, the German Research Foundation and the collaborating institutions in the Dark Energy Survey.