The early moments of the universe were turbulent and filled with hot, dense matter. Fluctuations in the early universe could have been large enough that pockets of stellar-mass matter collapsed under their own weight to create primordial black holes. Although we never detected these small black holes, they may have played a vital role in cosmic evolution, possibly developing into the supermassive black holes we see today. A new study shows how it could work, but also finds the process complicated.
A popular model for primordial black holes is that they were seeds for galaxies and stars. Even a small black hole would attract matter there, forming a galactic nebula, and the denser gas around the black hole would trigger the formation of the first stars. This would explain why galaxies formed early in the universe, and also why most galaxies contain a supermassive black hole.
Some argue that the seeds of primordial black holes play an essential role in the formation of the first galaxies. Without black holes to trigger the process, galaxies would not have formed early. To answer this question, the team created a simulation on a massive supercomputer known as Stampede2. From their simulations, the team found that primordial black holes can encourage galaxy formation and star production, but they can also hinder them.
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Primordial black holes could have drawn matter towards it to trigger star formation, but matter consumed by a black hole also heats nearby gas, causing it to repel. Thus, primordial black holes turn out to be give and take. Gravitationally pulls matter into galactic clouds, but also heats the central region and hinders star production. Primordial black holes therefore do not play a conclusive role. The effects of seeding and heating nearly cancel each other out. The smallest changes in initial conditions can determine whether a primordial black hole helps or hinders the formation of early galaxies.
Of course, things may change significantly with the introduction of dark matter. Dark matter is gravitationally pulled towards a black hole but does not heat nearby materials like ordinary matter does. The primordial black holes and dark matter could have worked together in a way that surpasses any heating of the primordial black holes. If so, the interaction of dark matter and primordial black holes could have created gravitational waves. These waves are too weak for us to detect at present, but future gravitational wave telescopes might.
These detailed simulations show how subtle and complex the role of primordial black holes can be. As the team moves towards creating even more detailed simulations, they hope to see how dark matter, primordial black holes, and star production could lead to the formation of supermassive black holes. In time, they may be able to tell us how such large objects have such small beginnings.
Reference: Liu, Boyuan, Saiyang Zhang and Volker Bromm. “Effects of Primordial Stellar-Mass Black Holes on Early Star Formation.” Royal Astronomical Society Monthly Notices 514.2 (2022): 2376–2396.