Our universe contains some enormous black holes. The supermassive black hole in the center of our galaxy has a mass of 4 million suns, but it’s quite small as galactic black holes are. Many galactic black holes have a billion solar masses, and the most massive known black hole is estimated to have a mass of nearly 70 billion suns. But how big can a black hole get?
To get a black hole really massive, it must consume a lot of matter early in its life. If it slowly digests matter, the surrounding galaxy will be in place and the universe will be expanded so that there is not much more matter that can trap the black hole. But when a black hole quickly consumes a lot of matter, the matter becomes very hot and tends to push away other material, making it difficult for the black hole to grow.
Based on observations of the largest black holes and computer simulations of how black holes form, the upper mass limit for galactic black holes is thought to be about 100 billion solar masses. But new research suggests that the mass restriction may be much higher.
The study noted that although galactic black holes probably have a hundred billion solar mass limit, larger black holes could have formed independently during the early moments of the universe. These primordial black holes can have masses more than a million times larger than the largest galactic black holes. The research team calls them Stupendously Large Black Holes or SLABs.
The idea of primeval black holes has been around for a long time. They have been proposed as a solution to everything, from dark matter to why we have not yet discovered the hypothetical ninth planet in our solar system. But theoretical models suggested that the original black holes would be much smaller than even black holes, formed from small density fluctuations in the early universe. But this new study suggests that dark matter and other factors can cause some of it to become enormous.
If the early universe was rich in dark matter, especially a form of dark matter known as WIMPs (Weakly Interacting Massive Particles), then an ancient black hole could digest dark matter to grow rapidly. Since dark matter does not interact strongly with light, the trapped dark matter will not emit much light or heat to dampen the growth rate. As a result, these black holes can be large even before the universe has cooled and formed galaxies. The upper mass limit of SLABs depends on how WIMP dark matter interacts with itself, so detecting SLABs can help us understand dark matter.
We have not yet observed any amazing large black holes. They can hide in the hearts of distant galaxies, but they can also hide in the vast space between galactic clusters. Or they may not exist. But it’s worth searching for them, because it’s a great discovery to find.
Reference: Shemmer, O., et al. ‘Near-infrared spectroscopy of active galactic nuclei with high redshift. I. A metal growth rate ratio. ” The Astrophysical Journal 614.2 (2004): 547.
Reference: Carr, Bernard, Florian Kühnel and Luca Visinelli. “Restrictions on incredibly large black holes.” Monthly notices from the Royal Astronomical Society 501.2 (2021): 2029-2043.