Dark matter might be made of black holes – and gravitational waves could help us find out.
Last month, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that they had detected gravitational waves radiating from two black holes that crashed together about 1.3 billion years ago – a Nobel-worthy discovery. Now a team suggests that those black holes could have been among the first to form in the early universe.
Such so-called primordial black holes would have spawned soon after the big bang, when sound waves radiated throughout the seething hot ball of plasma that filled the universe. Areas where those waves are densest could have collapsed to form them.
If these black holes exist today in great enough numbers, they could make up dark matter, the mysterious substance that comprises 85 per cent of the universe’s mass yet gives no sign of its presence except through gravity. But so far there is no firm evidence for their existence.
Simeon Bird at Johns Hopkins University and his colleagues suggest that LIGO may have caught a pair of primordial black holes, rather than the astrophysical black holes that form when massive stars collapse at the end of their lives.
Their first line of reasoning comes from the mass of the LIGO black holes: each is around 30 times the mass of the sun. Theoretically, primordial black holes can come in an array of masses, but previous observations ruled out objects above and below certain thresholds.
“There’s actually a window where they’re allowed, and it just happens to go straight through where the LIGO detection was,” says Bird.
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