In theory, supermassive black holes could become even more supermassive

Our universe contains huge black holes. The supermassive black hole at the center of our galaxy has a mass of 4 million suns, but it is rather small like galactic black holes. Many galactic black holes have a billion solar masses, and the most massive black hole known is estimated to have a mass of almost 70 billion suns. But how big can a black hole reach?

For a black hole to become truly massive, it must consume a lot of material early in its life. If it is slowly consuming matter, then its surrounding galaxy will have settled in and the universe will have expanded so that there isn’t much more matter for the black hole to capture. But when a black hole quickly consumes a lot of matter, the matter becomes very hot and tends to repel other matter, making it more difficult for the black hole to grow.

Based on observations of the largest black holes and computer simulations of black hole formation, the upper mass limit of galactic black holes is believed to be around 100 billion solar masses. But new research suggests the mass limit could be much higher.

The study notes that while galactic black holes likely have a solar mass limit of one hundred billion, larger black holes could have formed independently during the early moments of the universe. These primordial black holes could have masses more than a million times the size of the largest galactic black holes. The research team calls them insanely large black holes, or SLABs.

The idea of ​​primordial black holes has been around for a long time. They have been offered as a solution to everything from dark matter to why we have yet to discover the hypothetical ninth planet in our solar system. But theoretical models have suggested that primordial black holes would be much smaller than even stellar mass black holes, formed from tiny fluctuations in density in the early universe. But this new study suggests that dark matter and other factors could cause some of them to grow insanely large.

If the early universe was rich in dark matter, particularly a form of dark matter known as massive weak interaction particles (WIMP), then a primordial black hole could consume dark matter to grow rapidly. Since dark matter does not interact strongly with light, the captured dark matter would not emit much light or heat to dampen the rate of growth. As a result, these black holes could be huge even before the universe cools and galaxies form. The upper mass limit of SLABs would depend on how the dark matter in WIMP interacts with itself, so if we find SLABs, it could help us understand dark matter.

We have yet to observe any insanely large black holes. They might be hiding in the hearts of distant galaxies, but they could also be hiding in the vast space between galactic clusters. Or they may not exist. But it is worth looking for them, because finding one would be a truly extraordinary find.

Reference: Shemmer, O., et al. “Near infrared spectroscopy of active galactic nuclei at high redshift. I. A relation between the rate of metallicity and accretion. ” The astrophysical journal 614.2 (2004): 547.

Reference: Carr, Bernard, Florian Kühnel and Luca Visinelli. “Stresses on Incredibly Large Black Holes.” Monthly notices from the Royal Astronomical Society 501.2 (2021): 2029-2043.