Dark matter may not be found in primordial black holes after all. After the Laser Interferometer Gravitational-Wave Observatory (LIGO) watched a pair of black holes collide for the first time in 2015, speculation swirled that the black holes might be causing the strange gravitational effects that we attribute to mysterious dark matter. Now, that seems unlikely.
Primordial black holes – those that formed in the seething inferno just moments after the big bang – were proposed in 1971 by Stephen Hawking. Until LIGO there was no evidence for them, but the LIGO black holes had masses that meant they could plausibly be primordial black holes.
If all dark matter took the form of black holes about the size of the ones that LIGO has detected, we ought to see a few black hole collisions a year, which is the frequency with which we have detected them.
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and at the University of California, Berkeley, looked for the effects of primordial black holes on distant supernovae. A black hole between a supernova and Earth should warp and magnify the light that we see in a phenomenon called gravitational lensing – which should be relatively common if primordial black holes make up all of the dark matter in the universe.
But it is not, Zumalacárregui and Seljak found. They saw little sign of lensing, showing that there are not enough black holes out there to account for all the effects of dark matter. They found that primordial black holes cannot account for more than about 35 per cent of the dark matter in the universe.
All or nothing
Zumalacárregui and Seljak are assuming that dark matter takes one form only – either all black holes or some other form of matter altogether. Therefore, they say that if dark matter cannot be completely made up of these ancient behemoths, there is no reason to suspect any dark matter takes the form of these black holes. That would mean that LIGO did not detect dark matter.
LIGO team member at Carleton College in Minnesota disagrees. “I don’t think people who believe in primordial black holes are opening up bottles of whiskey and drowning their sorrows tonight,” he says. “Just because dark matter can’t be entirely composed of primordial black holes doesn’t mean they can’t exist.”
He says that LIGO is gearing up to look for small black holes with masses lighter than stars, which would have to be primordial because the conditions necessary for their birth only lasted for moments after the big bang.
Finding even a few such black holes could force us to rewrite our explanation of the early universe to include how exactly they were formed. “Even if primordial black holes weren’t all of the dark matter, even if they just existed, that would be a huge deal for explaining the beginning of the universe,” says Christensen.
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Read more: Earth has little to fear from a black hole attack
