Stupendously Large Black Holes Could Be Hiding in Universe

If ‘stupendously large’ black holes, those with masses more than 100 billion times that of the Sun, exist in the Universe, they would provide a powerful tool for cosmological tests due to their unique imprints, according to a paper published in the Monthly Notices of the Royal Astronomical Society.

Carr et al. investigated how stupendously large black holes could form and potential limits to their size. Image credit: NASA’s Goddard Space Flight Center.

“Black holes are a key prediction of general relativity,” said Queen Mary Emeritus Professor Bernard Carr and colleagues.

“There are a plethora of observations indicating their existence in the solar or intermediate-mass range. In particular, the existence of binary black holes in the mass range between 10 and 50 solar masses has been demonstrated by the detection of gravitational waves from inspiralling binaries.”

“There is also evidence for supermassive black holes at the centers of galaxies, including Sagittarius A* at the center of our own Milky Way Galaxy, with a mass of 4 million solar masses.”

“Recently, the imaging of the shadow created by M87*, the supermassive black hole at the center of the giant elliptical galaxy M87 with a mass of 6.5 billion solar masses, has been reported by the Event Horizon Telescope.”

“The supermassive black holes in galactic nuclei span a huge mass range, extending up to nearly 100 billion solar masses,” they said.

“The current heaviest black hole is associated with the quasar TON 618 and has a mass of 70 billion solar masses, while the second heaviest, at the center of the galaxy IC 1101, has a mass inferred from its radio emission of 40 billion solar masses.”

“This raises the issue of whether there could be even larger black holes in galactic nuclei and whether indeed there is any natural upper limit to the mass of a supermassive black hole.”

In their new paper, Professor Carr, Dr. Florian Kühnel from the Ludwig-Maximilians-Universitat and Dr. Luca Visinelli from the University of Amsterdam suggest that stupendously large black holes (SLABs) could be primordial, forming in the early Universe, and well before galaxies.

As primordial black holes don’t form from a collapsing star, they could have a wide range of masses, including very small and stupendously large ones.

“Whilst there isn’t currently evidence for the existence of SLABs, it’s conceivable that they could exist and they might also reside outside galaxies in intergalactic space, with interesting observational consequences,” Professor Carr said.

“However, surprisingly, the idea of SLABs has largely been neglected until now.”

“We’ve proposed options for how these black holes might form, and hope that our work will begin to motivate discussions amongst the community.”

If SLABs are of primordial origin, this raises an interesting link with the suggestion that primordial black holes could provide the dark matter.

Although SLABs themselves clearly cannot do this, since they are too large to reside in galactic halos, it is possible that primordial black holes provide the dark matter in a much lower mass range.

“SLABs themselves could not provide the dark matter,” Professor Carr said.

“But if they exist at all, it would have important implications for the early Universe and would make it plausible that lighter primordial black holes might do so.”