Happy Dark Matter Day from the scientists at ICASU
Shining a light on dark matter, today and every day.
Scientists have long known that the matter we can see and interact with accounts for only about 15 percent of the total matter in the universe. The remaining 85 percent comprises dark matter. Scientists point to the overwhelming astrophysical evidence affirming dark matter’s existence, though it has never been detected or observed. With so many questions unanswered, Illinois Physics faculty must tackle this problem on multiple fronts.
Physicists at the University of Illinois at Urbana-Champaign have made significant contributions to our understanding of dark matter, through their work on multiple large-scale collaborative experiments. In the past two years, several new faculty hires at Illinois Physics have added their expertise and insight to the search for this elusive particle. And now a newly founded campus center, the Illinois Center for Advanced Studies of the Universe (ICASU), has taken on dark matter as a main research focus, synergizing efforts and supporting collaboration across scientific disciplines at Illinois and beyond.
Billiards, or ping pong?
One of the recently-hired faculty is theoretical particle physicist Professor Yonatan Kahn, who joined Illinois Physics in 2019. One of Professor Kahn’s primary research interests is low-mass dark matter candidates, which might be as light as the electron, or lighter.
According to Professor Kahn, “Historically, many experiments have treated dark matter like billiard balls. That is, when designing particle detectors that might detect dark matter, scientists assumed dark matter particles would have about the same mass as the atomic nucleus. Just as with billiard balls, when particles of similar mass collide, energy is efficiently transferred from one to the other. However, if dark matter is more like a ping pong ball, those detectors will never see it. It won’t have enough mass to transfer sufficient energy to the other particle.”
Professor Kahn helps design dark matter experiments to search for these lighter dark matter candidates, the “ping pong balls.” In his most recent work, Professor Kahn has also examined the data from several dark matter detectors, such as CDMS and EDELWEISS, that are designed specifically to look for these much lighter particles. He notes that “these detectors see frequent signals, but those signals do not match our current expectations for dark matter.”
If an individual detector sees an unexpected signal, scientists often disregard it as noise. However, Professor Kahn explains, “if multiple detectors see similar unexpected signals, that could be an indication that dark matter does not have the properties we expected.
“Occam’s razor says it’s probably not dark matter. However, we’re studying how these detectors work, and we’re eliminating assumptions that no longer hold for this lighter type of dark matter. That’s very important for understanding both the operation of current detectors and the design considerations for future detectors. Even if we're a little bit out on a limb, we will still learn important things about how these low-energy detectors behave.”
Above is a simulation showing the overdensities of dark matter, which form the seeds for galaxy formation. Credit: TNG Simulations
How clumpy is dark matter?
Professor Patrick Draper is a theorist who specializes in high energy physics and cosmology. He joined Illinois Physics in 2018. His dark matter research is focused primarily on axions and axion-like particles. In the past, he has worked on making precise theoretical predictions for the relic abundance, or the amount of axion dark matter in the universe today, in different possible cosmological histories. Professor Draper says, “Different models of the early universe – the first second after the big bang – lead to different amounts of dark matter. I work on making those predictions.”
More recently, he has turned his attention to the “clumpiness” of dark matter. Professor Draper explains, “Observationally, we don't know a great deal about what was happening in the universe when it was younger than about a second, or hotter than about ten billion degrees kelvin. How fast was it expanding? In what form was most of the energy? Different possibilities lead to different distributions of dark matter today. The question is not just how much is there, but how is it distributed?
“I’m interested in how different particle physics models and theories of the early universe imprint themselves onto the distributions of dark matter in the universe today. If we are able to map out the small-scale structure of dark matter, what do we learn about the universe just after the big bang?”
Black holes as (cosmic) dark matter detectors
Two new faculty specializing in gravitation are working to understand what we might learn about dark matter from black holes and gravitational waves. Professor Nicolás Yunes, who joined the faculty in 2019, and Professor Helvi Witek, who arrived in January of 2020, are collaborating with Professor Kahn to understand whether a dark matter candidate, ultralight axion fields, could be studied through gravitational wave observations of black holes.
Professor Witek provides an analogy: “Imagine your car is stuck in the mud. You press on the gas to try to get out, but as your wheels spin, they merely throw the mud, and your car doesn’t move. Some of the energy you put into your wheel is transferred to the mud.”
According to Professor Witek, in this analogy, the “mud” represents ultralight axion-like particles, and the “wheel” is a spinning black hole. As the particles approach the black hole, instead of being pulled in, they can actually be spun around and thrown out with more energy than they had before, a process called “superradiance.” They start to pile up, just like the mud, and they form giant clouds around the black hole. These clouds can then emit gravitational waves and disappear.
Gravitational waves emitted by merging black holes have been observed by the Laser Interferometer Gravitational Wave Observatory (LIGO). The Laser Interferometer Space Antenna (LISA) mission, a space-based gravitational wave detector, is scheduled for deployment in 2034. The combined data from these two experiments will provide crucial information about gravitational waves emitted by these axion clouds, because those waves would have distinctive properties.
For example, axion-cloud gravitational waves should be “monochromatic,” meaning they vibrate at a single frequency, unlike the “chirping” waves we hear from merging black holes. Professor Yunes says, “If no waves of this type are found, one could place stringent bounds on the existence of such particles in a given mass range.”
“We are trying to determine the telltale signs of gravitational waves emitted by axion clouds,” he adds. “Could the inspiral of black holes be affected by such clouds? Would the way black holes ‘ring down’ after a collision change and imprint into the gravitational waves emitted?” Such signatures, if they are detectable, could enhance our ability to either observe or rule out the existence of axions of a certain mass around black holes.
‘A wealth of expertise’ in dark matter
These new faculty members join a vibrant community of dark matter researchers who bring their diverse interests and expertise to one of the most vexing puzzles in modern physics.
Professors Gil Holder and Joaquin Vieira are using the South Pole Telescope (SPT) and supporting data to study dark matter on both the largest scales and smallest astrophysical scales. On large scales, they are mapping dark matter stretching to the cosmological horizon using the light of the cosmic microwave background. On small astrophysical scales, they are searching for the gravitational imprints of fine-scale dark matter structure in ALMA images of the galaxies that have been discovered with SPT.
Professor Ben Hooberman is an experimentalist who works with data from the ATLAS detector at the Large Hadron Collider (LHC). Dark matter weakly-interacting massive particles (WIMPs) may be produced in the high energy proton-proton collisions at the LHC, and Professor Hooberman’s research group searches the data for evidence of these particles. He says, “A discovery would confirm the WIMP nature of dark matter and allow us to measure its properties with the most sophisticated detectors ever built.”
Professor Jessie Shelton’s research is focused on the thermal evolution of “dark sectors”: proposed systems of new particles and forces—which may include dark matter—with vanishingly weak interactions with ordinary matter. She seeks both to understand what current and envisioned experiments can and cannot tell us about the cosmic history of our universe, and to find entirely new ways to detect the footprints of dark matter particle physics.
Professor Shelton recently collaborated with Professors Jeff Fillipini, Gordan Baym, and Doug Beck on a paper about detecting low-mass dark matter particles with superfluid helium. Regarding the recent growth of the department, she says, “It is exciting to have such a wealth of expertise right on hand in the department. I have already started several interesting and rewarding collaborations and look forward to many more!”
Professor Filippini is an experimentalist who has worked on dark matter detection with ultra-cold detectors. He shares Professor Shelton’s enthusiasm about the recent growth of the physics department, commenting, “Our recent hires have built up a rich environment in cosmology and astrophysics. This has already led to fresh ideas in dark matter detection and vibrant discussions that will take us new places in the future.”