Just how big can a supermassive black hole (SMBH) get? A new hypothesis suggests absolutely freaking enormous – or their more high-brow phrasing, “stupendously large.” The work sought to understand – and place some constraints – on how big these celestial bodies can be.
In a paper published in the Monthly Notices of the Royal Astronomical Society, researchers investigated the possible properties, formation, and effect of these stupendously large black holes, or SLABs. The researchers started by stressing that there is no evidence for the existence of black holes more massive than what we see at the center of the most massive galaxies.
That said, we don’t know how SMBHs form and there is speculation around a physical upper limit to their size, but without a strong limit, it is interesting to see what could be beyond. The mass range considered for SLABs would go from the size of the Milky Way to a million times the size of our galaxy. The lower limit is 25 times bigger than the biggest SMBH that we have ever discovered.
“We already know that black holes exist over a vast range of masses, with a SMBH of four million solar masses residing at the centre of our own galaxy. 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. However, surprisingly, the idea of SLABs has largely been neglected until now,” lead author Professor Bernard Carr from Queen Mary University of London said in a statement.
“We’ve proposed options for how these SLABs might form, and hope that our work will begin to motivate discussions amongst the community.”
We know that it is unlikely that these objects would grow so large over the age of the universe, so one possible consideration is that they have always been here. There are concepts that in the instants right after the Big Bang, there were conditions that would facilitate the formation of black holes.
These primordial black holes have been invoked to explain dark matter, the rapid formation of supermassive black holes, and other questions we have about the universe. But what limits are there we can put in place? An interesting one is imagining that these objects formed everywhere and they are well spread out across the universe. These massive objects would influence the motion of galaxies significantly. Based on these ideas and the motion of the Milky Way, the team was able to estimate that if only one of these SLABs existed, it would be the biggest ever – about one billion times the mass of the Milky Way.
Another constraint is the effect of these primordial SLABs onto the cosmic microwave background, the first light shone in the universe. Anything 100 times bigger than the Milky Way would leave a clear mark, so maybe that is a more reasonable ranger to consider. Black holes are not easy to find, but these SLABs would have a huge gravitational influence. Nothing like that has been reported yet, so maybe there is an upper limit to how big black holes can get and we haven't found it yet.