How did black holes get so big and so fast? The answer lies in the darkness

Radiation from dark matter in the early universe may have helped keep hydrogen gas hot enough to condense into black holes.

• Supermassive black holes typically take billions of years to form. But James Webb Space Telescope Are they found after not long? Big Bang – before they have enough time to form.
• University of California Astrophysicists have discovered that if dark matter decays, the photons it emits keep hydrogen gas hot enough for gravity to gather it into giant clouds and eventually condense it into a massive mass. Black hole.
• In addition to explaining the existence of supermassive black holes so early, this discovery supports the existence of a type of dark matter capable of decaying into particles such as photons.

Supermassive black holes form

It takes a long time for supermassive black holes, like the one at the center of our galaxy, to form. milky way Typically, the birth of a black hole requires the burning of a giant star with a mass of at least fifty solar masses—a process that can take a billion years—and the collapse of its core in on itself.

However, the resulting black hole, which is only about 10 times the mass of the Sun, is a far cry from the 4-million-solar-mass black hole known as Sagittarius A* found in our own Milky Way galaxy, or the billion-solar-mass supermassive black holes found in other galaxies. Such giant black holes can form from smaller black holes by accreting gas and stars, and by merging with other black holes, which takes billions of years.

Secrets revealed by the James Webb Space Telescope

But why would the James Webb Space Telescope discover supermassive black holes so close to the beginning of time itself, thousands of years before they could have formed? Astrophysicists at the University of California, Los Angeles, have come up with an answer as mysterious as the black holes themselves: Dark matter has prevented hydrogen from cooling long enough for gravity to condense it into clouds large and dense enough to become black holes instead of stars. The results were published Aug. 27 in the journal Nature Communications. Physical Review Letters.

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

Gas cooling challenge in space

Some astrophysicists have hypothesized that a huge cloud of gas could collapse directly into a supermassive black hole, bypassing the long history of stars burning, accreting and merging. But there’s one problem: Gravity will pull a huge cloud of gas together, but not into one massive cloud. Instead, it will gather bits of gas into little halos that float close together but don’t form a black hole.

The reason for this is that the gas cloud cools very quickly. As long as the gas is hot, its pressure can resist gravity. But if the gas cools, the pressure drops, and gravity may prevail in many small regions, which collapse into dense bodies before gravity has a chance to pull the entire cloud into a single black hole.

“The cooling speed of the gas has a lot to do with the amount of molecular hydrogen,” said first author and doctoral student Yifan Lu. “Hydrogen atoms bound to each other in a molecule dissipate energy when they encounter a loose hydrogen atom.” corn“Hydrogen molecules act as cooling agents because they absorb heat energy and radiate it away. Hydrogen clouds in the early universe had a lot of molecular hydrogen, and the gas cooled quickly and formed small halos instead of large clouds.”

Lu and postdoctoral researcher Zachary Baker wrote code to calculate all the possible processes for this scenario and discovered that the extra radiation could heat the gas and split off hydrogen molecules, changing the way the gas cools.

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

The role of dark matter in the formation of black holes

But where does radiation come from?

Only a tiny fraction of the matter in the universe is the kind that makes up our bodies, our planets, our stars, and everything else we can observe. In fact, the vast majority of the matter, which can be detected through its gravitational effects on stellar bodies and by the bending of light rays coming from distant sources, is made up of some new particles that scientists have not yet been able to identify.

The shapes and properties of dark matter are a mystery that remains to be solved. Although we don’t know what dark matter is, particle scientists have long suspected that it might contain unstable particles that can decay into photons, particles of light. Including such dark matter in simulations would provide the radiation needed to keep gas in a large cloud afloat as it collapses into a black hole.

Dark matter may be made up of particles that decay slowly, or it may be made up of more than one particle. ClassSome are stable and some decay early. In either case, the decay product can be radiation in the form of photons, which break apart molecular hydrogen and prevent hydrogen clouds from cooling too quickly. Even very mild decays of dark matter produce enough radiation to prevent cooling, forming large clouds, and eventually supermassive black holes.

“This could be the answer to why supermassive black holes were discovered so early,” Baker said. “If you’re optimistic, you could also read this as positive evidence for one type of dark matter. If these supermassive black holes formed as a result of a collapsing gas cloud, perhaps the extra radiation needed must come from the unknown physics of the dark sector.”

Reference: “Direct Collapse of Supermassive Black Holes from Residual Particle Decay” by Yifan Lu, Zachary S.C. Baker, and Alexander Kosenko, August 27, 2024, Physical Review Letters.
doi: 10.1103/PhysRevLett.133.091001