Decaying dark matter may solve the mystery of how supermassive black holes formed in the early universe, giving them time to grow into the behemoths that they are now. If dark matter particles exist at the right masses, their decay could cause the black holes to be born big.
There are several ideas for how supermassive black holes form, but one of the simplest is direct collapse, in which a huge cloud of gas in the early universe becomes so dense that it crumples in on itself to create an enormous black hole. There is just one problem: this shouldn鈥檛 happen. As the cloud contracts, molecules of hydrogen form and radiate away heat very quickly, creating small cold patches that break up the larger cloud in a process called fragmentation. There isn鈥檛 enough matter left in the cloud to collapse into such a large black hole.
at the University of California, Los Angeles (UCLA), and his colleagues simulated how dark matter could solve this problem. This mysterious substance is thought to make up about 27 per cent of the universe, yet we know almost nothing else about it. But if dark matter particles decay into visible light particles, or photons 鈥� which would probably be a rare occurrence 鈥� they could prevent fragmentation and enable direct collapse to form supermassive black holes.
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鈥淎lmost all of the dark matter particles we鈥檝e studied have this bridge to regular matter,鈥� says Picker. 鈥淧retty much every dark matter candidate we鈥檙e looking for does have this decay into photons, because otherwise there鈥檚 really no way to look for it aside from its gravity.鈥�
Certain dark matter candidates that are very low-mass would decay into photons with just the right energy to hit hydrogen molecules in the cloud and smash them to pieces. And this could give us a way to spot them. 鈥淭he Hubble Space Telescope can still look for dark matter in this mass range by the light it produces when it decays,鈥� says at UCLA.
While these particles would be too light to meet the requirements for the most commonly accepted candidate to make up dark matter, called the weakly interacting massive particle or WIMP, several other candidates fit. One such particle is the axion, which has been proposed to solve several other problems in particle physics and cosmology.
Similarly, direct collapse is not the most widely preferred model for how to create early supermassive black holes 鈥� that would be formation from enormous primordial stars called Population III stars instead of directly forming from gas 鈥� but it is growing in popularity among astrophysicists due to recent observations of early supermassive black holes from the James Webb Space Telescope (JWST).
鈥淭he masses of supermassive black holes that we could form from Population III stars are much smaller than what we could form from direct collapse, and given that JWST is showing us very large black holes very early on, that鈥檚 an incentive to think about direct collapse,鈥� says at the University of Toronto in Canada, who was not involved in this research.
Observations of the amount of light in the universe over the next few decades, among other things, could help us determine whether decaying dark matter really does enable direct collapse and the early formation of the biggest black holes in the cosmos.
Physical Review Letters
