RIVERSIDE, Calif. -- A growing mystery in astronomy is the presence of gargantuan black holes — some weighing as much as a billion suns — existing less than a billion years after the Big Bang. According to the standard theory of black hole formation, these black holes simply should not have had enough time to grow so large.
A study led by University of California, Riverside graduate student Yash Aggarwal shows that dark matter decays could be the key to understanding the origin of these cosmic behemoths. Published in the Journal of Cosmology and Astroparticle Physics, the research shows that the energy released from dark matter decay could alter the chemistry of early galaxies enough to cause some of them to directly collapse into black holes rather than forming stars.
The result is timely since NASA's James Webb Space Telescope continues to observe unusually large black holes in the early universe that could have formed by direct collapse. Astronomers had believed this process requires a coincidence of nearby stars shining onto pre-stellar gas and so expected it to be rare.
Aggarwal's team goes beyond the standard approach by using dark matter — the unknown 85% of the matter in the universe that helps form galaxies. They show that if dark matter decays, it can leak a small amount of its energy into the gas and supercharge the direct collapse rate. Each decaying dark matter particle would only need to inject an amount of energy that is a billion trillionth the energy of a single AA battery.
"Our study suggests that decaying dark matter could profoundly reshape the evolution of the first stars and galaxies, with widespread effects across the universe," Aggarwal said. "With the James Webb Space Telescope now revealing more supermassive black holes in the early universe, this mechanism may help bridge the gap between theory and observation."
Flip Tanedo , associate professor of physics and astronomy at UCR and Aggarwal's doctoral co-advisor, said ideas related to this work had been bouncing around his group since 2018.
"The first galaxies are essentially balls of pristine hydrogen gas whose chemistry is incredibly sensitive to atomic-scale energy injection," said Tanedo, a coauthor on the paper. "These are the properties that we want for a dark matter detector — the signature of these 'detectors' might be the supermassive black holes that we see today."
The research team, which included James Dent of Sam Houston State University in Texas and Tao Xu of the University of Oklahoma, modeled the thermo-chemical dynamics of the gas in the presence of decaying axions and found that a window of dark matter masses between 24 and 27 electronvolts could produce the conditions to seed direct collapse black holes.
Tanedo pointed out that the work stemmed from a series of coincidences that brought the right people together at the right time, including a series of workshops that connected particle physicists, cosmologists, and astrophysicists to discuss the big questions in their field.
"We showed that the right dark matter environment can help make the 'coincidence' of direct collapse black holes much more likely," he said. "In the same way, the support for interdisciplinary work helped make the 'coincidence' leading to this work possible."
The research was supported by the National Science Foundation and a UCR Hellman Fellowship.
The title of the research paper is "Direct collapse black hole candidates from decaying dark matter."
The University of California, Riverside is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment is more than 26,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual impact of more than $2.7 billion on the U.S. economy. To learn more, visit www.ucr.edu .
