Dark matter radiation in the early universe may have kept hydrogen gas hot enough to condense into black holes.
- Supermassive black holes usually take billions of years to form. But the James Webb Space Telescope is not to find them so long after the Big Bang — before they had time to educate themselves.
- University of California Astrophysicists have discovered that when dark matter decays, the photons it emits keep the hydrogen gas hot enough for gravity to collect it in huge clouds and eventually form a supermassive black hole.
- The finding not only explains the existence of very early supermassive black holes, but also supports the assumption of a type of dark matter that can decay into particles such as photons.
Formation of supermassive black holes
It takes a long time for supermassive black holes like the one at the center of our Milky Way galaxy to form. Normally, the birth of a black hole requires that a giant star with the mass of at least 50 of our suns burn out – a process that can take a billion years – and that its core collapse in on itself.
Nevertheless, the resulting black hole, with only about 10 solar masses, is far from the 4 million solar mass black hole Sagittarius A* in our Milky Way or the billions of solar mass supermassive black holes in other galaxies. Such gigantic black holes can form from smaller black holes by accretion of gas and stars and by merging with other black holes, which takes billions of years.
The James Webb Space Telescope reveals secrets
So why did the James Webb Space Telescope discover supermassive black holes at the beginning of time, eons before they could have formed? Astrophysicists at the University of California, Los Angeles (UCLA) have an answer that is 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 finding was published August 27 in the journal Physical Examination Letters.
“It was really surprising to find a supermassive black hole with a billion times the mass of the sun when the universe itself is only half a billion years old,” said lead author Alexander Kusenko, 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.”
The challenge of gas cooling in space
Some astrophysicists have theorized that a large cloud of gas could collapse and form a supermassive black hole directly, without going through the long history of stellar combustion, accretion, and mergers. But there’s a catch: Gravity does indeed pull a large cloud of gas together, but not into a single big cloud. Instead, it collects bits of the gas into small halos that float next to each other but don’t form a black hole.
The reason for this is that the gas cloud cools too quickly. As long as the gas is hot, its pressure can counteract gravity. But as the gas cools, the pressure decreases and gravity can take over in many small areas, which collapse into dense objects before gravity has a chance to pull the entire cloud together into a single black hole.
“How quickly the gas cools depends strongly on the amount of molecular hydrogen,” said lead author and doctoral student Yifan Lu. “Hydrogen atoms that are bonded together in a molecule release energy when they encounter a loose hydrogen atom. atom. The hydrogen molecules become coolants as they absorb and radiate thermal energy. Hydrogen clouds in the early universe had too much molecular hydrogen, and the gas cooled quickly, forming small halos instead of large clouds.”
Lu and postdoctoral fellow Zachary Picker wrote code to calculate all the possible processes of this scenario and discovered that additional radiation can heat the gas and dissociate the 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 fragmentation of large clouds,” Lu said.
Role of dark matter in the formation of black holes
But where does the radiation come from?
Only a very small fraction of the matter in the universe is the kind that makes up our bodies, our planets, the stars, and everything else we can observe. The vast majority of the matter that has been detected by the gravitational effect on stellar objects and by the diffraction of light rays from distant sources is made up of some new particles that scientists have not yet identified.
The shapes and properties of dark matter are therefore a mystery that has yet to be solved. Although we do not know what dark matter is, particle theorists have long speculated that it may contain unstable particles that can decay into photons, the particles of light. Including this dark matter in the simulations provided the radiation needed to keep the gas in a large cloud as it collapses into a black hole.
Dark matter could consist of particles that slowly decay, or it could consist of more than one particle. species: some are stable and others decay at an early stage. In both cases, the decay product could be radiation in the form of photons, which split molecular hydrogen and prevent hydrogen clouds from cooling too quickly. Even very slight dark matter decay produces enough radiation to prevent cooling, creating large clouds and eventually supermassive black holes.
“This could explain why supermassive black holes are discovered very early,” said Picker. “If one is optimistic, one could also interpret this as positive evidence for some kind of dark matter. If these supermassive black holes were formed by the collapse of a gas cloud, perhaps the additional radiation required would have to come from the unknown physics of the dark sector.”
Reference: “Direct Collapse Supermassive Black Holes from Relic Particle Decay” by Yifan Lu, Zachary SC Picker and Alexander Kusenko, August 27, 2024, Physical Examination Letters.
DOI: 10.1103/PhysRevLett.133.091001
