In idea, supermassive black holes might grow to be much more supermassive

Our universe contains some huge black holes. The supermassive black hole in the center of our galaxy has a mass of 4 million suns, but is rather small in terms of galactic black holes. Many galactic black holes have a billion solar masses, and the most massive black hole known has an estimated mass of nearly 70 billion suns. But how big can a black hole get?

Artist’s view of an active black hole consuming matter. Photo credit: ESO / M. Kornmesser

For a black hole to become really massive, it has to use up a lot of matter at the beginning of its life. When it slowly consumes matter, its surrounding galaxy has stuck and the universe has expanded so that the black hole can no longer capture much matter. However, when a black hole consumes a lot of matter quickly, the matter becomes very hot and tends to push other matter away, making it difficult for the black hole to grow.

Based on observations of the largest black holes and computer simulations of the formation of black holes, it is assumed that the upper mass limit for galactic black holes is around 100 billion solar masses. However, new research suggests that the mass limit could be much higher.

The study notes that galactic black holes likely have a solar mass limit of 100 billion, but larger black holes could have formed independently in the early moments of the universe. These primordial black holes could have masses more than a million times larger than the largest galactic black holes. The research team calls them amazingly large black holes, or SLABs.

The idea of ​​primitive black holes has been around for a long time. They have been suggested as the solution to everything from dark matter to why we haven’t discovered the hypothetical ninth planet in our solar system yet. However, theoretical models suggest that primordial black holes are much smaller than even stellar-mass black holes, which are formed from tiny variations in density in the early Universe. However, this new study suggests that dark matter and other factors can cause some of them to grow amazingly large.

Hypothetical distribution of WIMPs in our galaxy. Photo credit: Davison Soper

If the early universe were rich in dark matter, particularly a form of dark matter known as WIMPs (Weakly Interacting Massive Particles), a primordial black hole could consume dark matter to grow quickly. Since dark matter does not interact strongly with light, the trapped dark matter would not give off much light or heat to dampen the rate of growth. As a result, these black holes could be huge even before the universe cooled down and galaxies formed. The upper mass limit of SLABs depends on how the dark matter of WIMP interacts with itself. So when we discover SLABs, it can help us understand dark matter.

We have not yet observed any surprisingly large black holes. They could hide in the hearts of distant galaxies, but they could also lurk in the vast space between galactic clusters. Or they may not exist. But they’re worth looking for because it would be a truly amazing discovery to find one.

Reference: Shemmer, O., et al. “Near-infrared spectroscopy of active galactic nuclei with high redshift. I. A relationship between metallicity and accretion rate. “The Astrophysical Journal 614.2 (2004): 547.

Reference: Carr, Bernard, Florian Kühnel and Luca Visinelli. “Limitations on Incredibly Large Black Holes.” Royal Astronomical Society monthly releases 501.2 (2021): 2029-2043.

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