Dark matter may have contributed to the formation of supermassive black holes in the early universe

The formation of a supermassive black hole, like the one at the center of our Milky Way galaxy, takes a long time. Typically, the birth of a black hole requires a giant star with the mass of at least 50 of our suns to burn up (a process that can take a billion years) and its core to collapse in on itself.

Still, at only about 10 solar masses, the resulting black hole is a far cry from the 4-million-solar-mass Sagittarius A* black hole found in our Milky Way galaxy, or the billion-solar-mass supermassive black holes found in other galaxies. Such gigantic black holes can form from smaller black holes through accretion of gas and stars, and mergers with other black holes, which take billions of years.

So why did the James Webb Space Telescope discover supermassive black holes at the dawn of time, eons before they could have formed? UCLA astrophysicists have an answer as mysterious as the black holes themselves: Dark matter prevented hydrogen from cooling long enough for gravity to condense it into clouds large and dense enough to turn into black holes rather than stars. The discovery is published in the journal Physical Exam Letters.

“It’s surprising to find a supermassive black hole with a billion solar masses when the universe itself is only half a billion years old,” said Alexander Kusenko, lead author of the study and a professor of physics and astronomy at UCLA. “It’s like finding a modern car among dinosaur bones and wondering who built that car in prehistoric times.”

Some astrophysicists have proposed that a large gas cloud could collapse to form a supermassive black hole directly, bypassing the long history of stars burning, accreting, and merging. But there’s a catch: gravity will actually pull a large gas cloud together, but not into a single large cloud. Instead, it pulls sections of gas together into small halos that float next to each other but don’t form a black hole.

The reason is that the gas cloud cools too quickly. As long as the gas is hot, its pressure can counteract gravity. However, if the gas cools, the pressure decreases and gravity can take over in many small regions, which collapse into dense objects before gravity has a chance to pull the entire cloud into a single black hole.

“The rate at which the gas cools depends largely on the amount of molecular hydrogen,” says Yifan Lu, first author and a doctoral student. “Hydrogen atoms bonded together in a molecule dissipate energy when they encounter a free hydrogen atom. Hydrogen molecules become cooling agents because they absorb thermal energy and release it. Hydrogen clouds in the early universe contained too much molecular hydrogen, and the gas cooled quickly and formed small halos instead of large clouds.”

Lu and postdoctoral researcher Zachary Picker wrote code to calculate all the possible processes in this scenario and found that additional radiation can heat the gas and dissociate hydrogen molecules, changing how the gas cools.

“If you add radiation in a certain energy range, it destroys molecular hydrogen and creates conditions that prevent the fragmentation of large clouds,” Lu said.

But where does the radiation come from?

Only a tiny fraction of the matter in the universe is that which makes up our bodies, our planet, the stars and everything else we can observe. The vast majority of matter, detected by its gravitational effects on stellar objects and by the bending of light rays from distant sources, is made up of new particles, which scientists have not yet identified.

The shapes and properties of dark matter are thus a mystery that remains to be solved. Although we don’t know what dark matter is, particle theorists have long speculated that it might contain unstable particles that can decay into photons, the particles of light. Including this dark matter in the simulations provided the radiation needed for the gas to remain in a large cloud as it collapsed into a black hole.

Dark matter could be made up of particles that decay slowly, or it could be made up of several kinds of particles: some stable and others that decay sooner. In either case, the product of the decay could be radiation in the form of photons, which break down molecular hydrogen and prevent the hydrogen clouds from cooling too quickly. Even very slight decay of dark matter produced enough radiation to prevent cooling, forming large clouds and, eventually, supermassive black holes.

“This could explain why supermassive black holes are being discovered so early,” Picker said. “If you’re optimistic, you could also read this as positive evidence for the existence of some type of dark matter. If these supermassive black holes formed from the collapse of a gas cloud, perhaps the extra radiation needed would have to come from the unknown physics of the dark sector.”