Many galaxies have “supermassive” black holes at their centre – vast monsters with masses of up to 10 billion times that of our Sun.
But there could also be far bigger black holes out there, scientists believe – so big they merit their own category, “stupendously large black holes”, or SLABs.
These vast objects won’t live in the centre of galaxies, and instead lurk in the space between the stars, said researchers from the Queen Mary University of London.
Normal supermassive black holes (SMBHs) form within a galaxy and grow to their enormous size by swallowing stars and gas – or merging with other black holes.
The research was published in Monthly Notices of the Royal Astronomical Society.
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But “primordial” black holes, which formed before the galaxies, could exist at far larger sizes, the researchers say.
Professor Bernard Carr said: “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.
“We’ve proposed options for how these SLABs might form, and hope that our work will begin to motivate discussions amongst the community.”
The research investigated how these SLABs could form and potential limits to their size.
The existence of SLABS even larger than this could provide researchers with a powerful tool for understanding the early universe.
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The idea of primordial black holes can be traced back to the 1970s, when Professor Carr and Professor Stephen Hawking suggested that in the first moments of the universe fluctuations in its density could have resulted in some regions collapsing into black holes.
The objects could throw light on one of the great mysteries of the universe – dark matter.
Dark matter is thought to make up around 80% of the ordinary mass of the universe.
Whilst we can’t see it, researchers think dark matter exists because of its gravitational effects on visible matter, such as stars and galaxies.
However, we still don’t know what the dark matter is.
“SLABs themselves could not provide the dark matter,“ said Professor Carr, “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.”
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