As scientists gain greater ability to peer deeper and deeper into the Universe, we've been finding something very surprising: Supermassive black holes millions to billions of times the mass of the Sun, before the Universe was even 10 % of its current age.
Given what astronomers know about the growth rate of black holes, there oughtn't have been enough time since the Big Bang for them to grow so huge. But their presence is undeniable – so something strange must be afoot.
According to new research, that something might be one of the strangest things in the Universe: dark matter.
These results came by monitoring and following up a portion of the "baryonic dark matter".
In the university press release, astrophysicist Hai-Bo Yu of the University of California Riverside says that thinking about the reasons why black holes were so huge in the early universe raises the question of what the physical mechanisms are to produce a sufficiently large black hole, or Achieving a fast enough growth rate?
Baryonic dark matter
Dark matter is one of the Universe's greatest mysteries. We don't know what it is, or what it's made of. The only way it interacts with the normal baryonic matter in the Universe – that's the stuff that all the stuff we can see is made of – is gravitationally.
This part consists of a baryon, a complex atomic particle consisting of heavy subatomic particles such as protons and neutrons, and a mixture of both, including ordinary, non-radiative atoms.
Baryonic dark matter may also exist in non-luminous gas, or in condensed objects such as black holes, neutron stars, white dwarfs, and very faint stars, or non-luminous objects such as planets and brown dwarfs.
Why were black holes so huge?
The only way that baryonic dark matter interacts in the universe is through its interaction with gravity, so we can observe gravitational effects in the Universe, such as the rotation of galaxies and the way light curves along a strong gravitational field.
Most galaxies reside in a halo of dark matter; it's thought to be vital for their formation. One model for the formation of supermassive black holes is the direct collapse of a dense cloud of gas. Yu and his colleagues wondered if there might be another contribution.
In this regard, astrophysicist Yu said "This mechanism ... cannot produce a massive enough seed black hole to accommodate newly observed supermassive black holes – unless the seed black hole experienced an extremely fast growth rate. "
"Our work provides an alternative explanation: A self-interacting dark matter halo experiences gravothermal instability and its central region collapses into a seed black hole, and thus baryonic dark matter interacts with gravity, but may be able to interact with itself."
“The advantage of our scenario is that the mass of the seed black hole can be high since it is produced by the collapse of a dark matter halo,” Yu said. “Thus, it can grow into a supermassive black hole in a relatively short timescale.”
The inward pull of gravity would compete against the outward push of heat and pressure; for non-self-interacting dark matter, particles condensing towards the center of the halo would speed up under the increasing gravity, and recoil under the higher pressure, because they were unable to transfer their energy to other particles.
Therefore, Self-interacting dark matter particles, however, would be able to transfer energy to other particles, introducing friction to the rotating dark matter fluid. This would cause the particles to slow, reducing angular momentum and shrinking the central halo so that, eventually, it would collapse under its own mass to form the seed of a black hole.
From this point on, the seed could grow by accumulating baryonic matter, the researchers said, though the dark matter "seed" could have a mass high enough to allow the black hole to grow rapidly.
Based on this analysis, the influence of dark matter and its responsibility for the formation of massive black holes in the early universe is evident.
Sources:
https://iopscience.iop.org/article/10.3847/2041-8213/ac04b0
https://news.ucr.edu/articles/2021/06/16/how-supermassive-black-hole-originates
