The early moments of the universe were turbulent and filled with hot and dense matter. Fluctuations in the early universe may have been large enough for pockets of stellar-mass matter to collapse under their own weight to create primordial black holes. Although we have never detected these tiny black holes, they may have played a vital role in cosmic evolution, perhaps growing into the supermassive black holes we see today. A new study shows how this might work, but also finds that the process is 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 to itself, forming a galactic nebula, and the denser gas around the black hole would cause early stars to form. 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 early galaxies. Without black holes to trigger the process, galaxies would not have formed early. To look at 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 promote galaxy formation and star production, but they can also hinder it.
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Proto-black holes could pull matter towards them to trigger star formation, but matter swallowed by a black hole also heats nearby gas, causing it to repel. So primordial black holes turn out to be a give and take effect. It attracts the matter in the galactic clouds gravitationally, but also heats the central region and inhibits star production. So primordial black holes do not play a decisive role. The effects of seeding and heating almost cancel each other out. The smallest changes in initial conditions can determine whether a primordial black hole is a help or a hindrance in early galaxy formation.
The simulations show how different numbers of primordial black holes affect galaxy formation. Credit: Liu, et al
Of course, things could change significantly with the introduction of dark matter. Dark matter is attracted to a black hole gravitationally, but does not heat nearby material the way ordinary matter does. The primordial black holes and dark matter could work together in a way that overrides any heating from the primordial black holes. If so, the interaction of dark matter and primordial black holes may have created gravitational waves. These waves are too weak to detect right now, but future gravitational wave telescopes may be able to.
These detailed simulations show how subtle and complex the role of primordial black holes can be. As the team moves to create even more detailed simulations, they hope to see how dark matter, primordial black holes and star production can 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 Brom. “Effects of Stellar-Mass Protoblack Holes on the Formation of the First Star.” Monthly Notices of the Royal Astronomical Society 514.2 (2022): 2376–2396.
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