Ever since astronomers reached a consensus in the 1980s that invisible “dark matter” binds galaxies together and gravitationally shapes the cosmos — experimentalists have hunted for the elusive nonluminous particles. Without luck. “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,” says astronomer Avi Loeb at the Harvard-Smithsonian Center for Astrophysics.
“Approaching Science Fiction”
Today, reports Charlie Wood for Quanta, physicists are contemplating a broader spectrum of possibilities, some approaching science fiction –ranging from the invisible matter clumping into black holes as heavy as stars to dark matter that could spread out in a fine mist of particles thousands of trillions of trillions of times lighter than electrons to dark matter particles the size of galaxies.
We know dark matter must make up about 85 percent of the total mass of the universe because of its massive gravitational footprint, but we don’t yet know what it’s made of, but if it has anything to do with any scalar particles, it may be older than the Big Bang, suggest physicists, who, sounding like creators of science-fiction, have imagined new kinds of dark matter ranging from planet-sized particles to hyper-speculative dark-matter life.
“I think there’s going to be a substantial part of the field that’s going to shift into these new kinds of experiments,” said Kathryn Zurek, a theoretical physicist at the California Institute of Technology, if current WIMP experiments fail to detect any signals.
“While experimentalists,” reports Wood about current dark-matter detectors, “prepare the next generation of apparatuses seeking direct contact with dark matter, others plan to scour the heavens for indirect signposts.”
Enormous Clouds of Dark Matter
“Enormous clouds of dark matter.” writes Wood, “are thought to create galaxies and stars by gravitationally drawing in visible matter. But any smaller dark matter clusters that might exist wouldn’t do this. These modest blobs would be completely dark, but they should still gravitationally bend passing starlight. One group of researchers is searching for this “lensing” of starlight by dark matter blobs in data from the ongoing GAIA space observatory survey.”
Dark Structures Moving Throughout the Milky Way
“Dark structures are moving throughout our galaxy,” said Anna-Maria Taki, a physicist from the University of Oregon and a member of the team. “As they move, they distort the positions and the proper motions and trajectories of luminous sources.”
But early results published in September did not locate any such structures heavier than about 100 million suns. “With larger future data sets, the researchers hope to discern the possible outlines of wispier dark clouds. From the shapes and sizes of these hypothetical structures, the scientists could infer whether, and in what manner, dark matter particles interact with themselves.”
Beyond Standard-Model Physics–Dark-Matter Planets
“We have billions of these things just sitting out there,” said Rebecca Leane, an astroparticle physicist at the SLAC National Accelerator Laboratory who looks for new particles, including dark matter or other Beyond the Standard Model physics. Leane isco-author of a proposal about planets that may collect dark matter in its core as it sweeps through the Milky Way.
“As those dark matter particles annihilate with their antiparticles, they heat up the planet,” she says. “Exoplanets closer to the galactic center pass through denser dark matter, so they should glow hotter with infrared light. If the upcoming James Webb Space Telescope can take the temperature of a few thousand exoplanets,” Leane and a colleague calculated, “that data set could bear the fingerprints of annihilating dark particles in the electron-to-proton mass range.”
“Anything can be a dark matter detector,” Leane said about whether ghostly dark matter “tickles different types of detectors, or whether it nudges starlight, warms planetary cores or even lodges in rocks — its ghostly influence could show up anywhere. You just have to be creative enough to think of how to use it.”
In 2019, while at MIT, Leane suggested that a “strange glow” coming from the Milky Way’s center that was thought to be due to ordinary pulsars may in fact be dark matter, “There’s something happening in the data we don’t understand,” said Leane, reopening the possibility that space-based instruments have found the first direct evidence of the elusive “dark matter” thought to pervade the universe. Data from NASA’s Fermi Gamma-ray Space Telescope suggested it appeared to be picking up too many gamma rays.
Dan Hooper, an astrophysicist at the University of Chicago, suggested “the anomaly could originate from a theoretical jumbling of dark matter particles in the galaxy’s center. While dark matter doesn’t shine or fraternize with known particles, in the right sort of collision these particles could annihilate in a shower of familiar matter and antimatter — a process called “annihilation”–that would then go out with a puff of gamma rays in their wake. A measurement of these offshoots would represent the first evidence of dark matter that wasn’t exclusively gravitational in nature.”
Hooper’s analysis was never going to find dark matter, commented Leane regardless of whether it’s there. “Something about our understanding of the gamma rays is missing at this stage,” Leane said. “It’s possible to hide a dark matter signal, if it were really there.”
In the 2019 study, Leane said she discovered a loophole in the model that was used in the 2015 research. By running a simulated dark matter signal through the model, Leane found that the dark matter mimicked a clumpy signal.
“We discover striking behavior consistent with a mismodeling effect in the real Fermi data, finding that large artificial injected dark matter signals are completely misattributed to point sources,” the study said. “Consequently, we conclude that dark matter may provide a dominant contribution to the [galactic center energy] after all.”
“The Sci-Fi Solution”-A Parallel Universe
Moving far beyond the galactic center, one new theory, says physicist Michio Kaku, is that the as yet undetected dark matter of the universe may be ordinary baryonic matter that makes up stars, planets and galaxies in a parallel universe. “If a galaxy is hovering above in another dimension,’ says Kaku, “we would not be able to see it. It would be invisible, yet we would feel its gravity. Hence, it might explain dark matter.”
A less radical finding by scientists at the Institut d’Astrophysique Spatiale (CNRS/Université Paris-Saclay), suggests that dark matter remains undetected, concealed in the form of a hot gas in the complex cosmic web. For the first time, the possible signal of the hidden matter has been detected in the filaments of the cosmic web buried 20-year-old spacecraft data through an innovative statistical analysis.
Theoretical physicist Asimina Arvanitaki, at Canada’s Perimeter Institute for Theoretical Physics, proposes that black holes can be thought of as nature’s “particle accelerators,” and that we may be able to discover new particles through detection of the gravitational waves black holes create. “I’m not surprised,” she says. “How else could you respond to the idea that black holes generate swirling clouds of planet-sized particles that could be the dark matter thought to hold galaxies together? We tend to think about particles as being tiny but, theoretically, there is no reason they can’t be as big as a galaxy.”
LIGO gravitational-wave detectors, suggests Arvanitaki, could discern a signal from a cloud of dark-matter particles feeding off the spin of a black hole, that would be more like a steady hum rather than a merger collision.
Superradiance–Dark Matter on a Massive Scale
“So now the [axion] waves scatter from the spinning black hole, but then keep bouncing back and forth, and eventually the amplification becomes exponential,” says Arvanitaki. In this picture of superradiance, a cloud of gazillions of axions — a hypothetical particle that has long been suggested as a dark-matter candidate that has gained traction because of the success of LIGO, the Nobel Prize-winning experiment that detected gravitational waves caused by the ancient collisions of black holes–.would be created, which would arrange themselves in an orderly fashion, she adds, “a lot like those pictures of atomic orbitals, only on a massive scale”.
A dark matter galaxy? Markarian 1216 (abbreviated as Mrk 1216) contains stars that are within 10% the age of the universe—that is, almost as old as the universe itself. Isolated for billions of years, Mrk 1216 with more dark matter packed into its core than expected has been identified by astronomers using data from NASA’s Chandra X-ray Observatory.
Mrk 1216 an elliptically shaped galaxy densely packed with stars at its centers descended from reddish, compact galaxies called “red nuggets” that formed about a billion years after the big bang, but then stalled in their growth about 10 billion years ago. “In the future we hope to go a step further and study the nature of dark matter,” said David Buote of the University of California at Irvine. “The dense accumulation of dark matter in the middle of Mrk 1216 may provide an interesting test for non-standard theories that predict less centrally concentrated dark matter, such as for dark matter particles that interact with each other by an additional means other than gravity.
Buote and colleagues interpreted the Chandra data using both standard, “Newtonian” models of gravity and an alternative theory known as modified Newtonian dynamics, or “MOND” designed to remove the need for dark matter in typical galaxies. The results showed that both theories of gravity required about the same extraordinary amount of dark matter in the center of Mrk 1216, effectively removing the need for the MOND explanation.
“When we compared the Chandra data to our computer models, we found a much stronger concentration of dark matter was required than we find in other galaxies of similar total mass,” said Buote. “This tells us the history of Mrk 1216 is very different from the typical galaxy. Essentially all of its stars and dark matter was assembled long ago with little added in the past 10 billion years.”
No one sums up the elusive, frustrating, ongoing search for the dominant dark sector of our universe than the director of CERN, particle physicist, Fabiaola Gianotti –“Dark matter is everywhere. In the room where you are reading this. Everywhere.”
Image credit top of page: New Hubble data, reports NASA, have been used to explain the reason behind the missing dark matter in NGC 1052-DF4, which resides 45 million light-years away. Mireia Montes of the University of New South Wales in Australia led an international team of astronomers to study the galaxy using deep optical imaging. They discovered that the missing dark matter can be explained by the effects of tidal disruption. The gravity forces of the neighboring massive galaxy NGC 1035 are tearing NGC 1052-DF4 apart. During this process, the dark matter is removed, while the stars feel the effects of the interaction with another galaxy at a later stage.