Bernard Carr and colleagues recently considered the observational constraints on stupendously large black holes (SLABs) in the mass range M≳10¹¹ M, i.e. about the size of 100 billion suns or more. Discovering such gargantuan black holes may shed light on the nature of a significant fraction of the mysterious dark matter that makes up four-fifths of the matter in the universe.
Currently the largest known black hole, powering the quasar TON 618, has a mass of 66 billion solar masses. TON 618’s enormous bulk led scientists to speculate whether or not even larger black holes exist, and if there is any upper limit to their sizes.
In the new study, the researchers dubbed black holes 100 billion solar masses in size or larger — bigger than any currently seen — “stupendously large black holes,” or SLABs. Although they noted there is currently no evidence that stupendously large black holes are real, they noted that supermassive black holes almost that size do exist.
A key question when it comes to stupendously large black holes is whether they could form in the first place. However, much remains uncertain about how even regular supermassive black holes are born.
The conventional assumption is that the supermassive black holes at the hearts of galaxies formed as smaller black holes merged and gobbled up matter around them. However, previous research found this model faced challenges when it comes to explaining how black holes could have reached supermassive sizes when the universe was only a few billion years old.
Another way to explain how both regular supermassive black holes and possibly stupendously large black holes formed hinges on so-called primordial black holes. Prior work speculated that within a second after the Big Bang, random fluctuations of density in the hot, rapidly expanding newborn universe might have concentrated pockets of matter enough for them to collapse into black holes. These primordial black holes could have served as seeds for larger black holes to form later on.
If primordial black holes do exist, they might help explain what dark matter is. Although dark matter is thought to make up most of the matter in the universe, scientists don’t know what this strange stuff is made of, as researchers still have not seen it; it can currently be studied only through its gravitational effects on normal matter. The nature of dark matter is currently one of the greatest mysteries in science.
One way to detect stupendously large black holes is through gravitational lensing.
Another way to detect stupendously large black holes is through the effects they would have on their environment, such as gravitationally distorting galaxies. These black holes could also generate heat, light and other radiation as they consume matter that astronomers could detect.
Aside from primordial black holes, another potential candidate for dark matter are so-called weakly interacting massive particles (WIMPs). If WIMPs exist, they would be invisible and largely intangible, but previous research suggested that if two WIMPs ever collided, they would annihilate one another and generate gamma rays, providing a way for scientists to spot them indirectly. The powerful gravitational pulls of stupendously large black holes would gather a halo of WIMPs around them, and the high-energy gamma rays that could result from WIMP annihilation might help scientists discover stupendously large black holes.
They also commented on the constraints on the mass of ultra-light bosons from future measurements of the mass and spin of SLABs.
References: Bernard Carr, Florian Kuhnel, Luca Visinelli, “Constraints on Stupendously Large Black Holes”, ArXiv, 2020. Doi: arXiv:2008.08077v2 link: https://arxiv.org/abs/2008.08077