DEAD stars are exploding all around the universe and we aren’t sure why – but now a pair of researchers think that minuscule black holes made from dark matter might be to blame. Burnt out stars known as white dwarfs can ignite into a type Ia supernova when they gather matter from a nearby star or merge with other astronomical objects. How this works is still an open question. “The dirty secret of supernovae is that in the computer models, we can’t ever actually get them to do the final ignition. There always has to be an injected trigger,” says Ashley Pagnotta at the College of Charleston in South Carolina, who wasn’t involved in the work. Joseph Bramante and Javier Acevedo at Queen’s University in Canada say dark matter – the invisible stuff thought to make up 80 percent of the matter in the universe – could be that trigger. The pair modeled what could happen when dark matter meets a white dwarf that is between 1 and 1.4 times the mass of the sun.
White dwarfs are mostly made of electrons held apart by the rules of quantum physics, and a dwarf this large should have enough internal pressure that a black hole could form within it. Bramante and Acevedo suggest that when dark matter falls into the white dwarf at about 1 percent of the speed of light, it is much hotter than the material that makes up the star. As the dark matter cools, it gathers at the center enough of it clumps together, it will collapse under its own gravity into a tiny black hole nestled within the heart of the star. Depending on the size of the black hole and of the white dwarf, it could suck in the star’s material within a millisecond (Physical Review D, doi.org/dgkh). Or it could begin to evaporate and send out particles of Hawking radiation – energy thought to leak out of a black hole, making it slowly shrink. “It’s a competition between Hawking radiation and accretion. Which one wins out is a function of how big the black hole is,” says Bramante. If Hawking radiation wins, it could destroy the star.
As the black hole shrinks, it would heat up as emitted particles collide with the surrounding star’s matter. After about 3 billion years, fusion takes over and the white dwarf explodes. “Once it reaches a high enough temperature, we have no idea what it would do,” says Bramante. To work it out, we need to meld the rules of quantum physics and general relativity into a theory of quantum gravity – one of the biggest challenges in physics. “Although it would be hidden inside a white dwarf, this could be one probe of a quantum gravity process. Though, we would have to figure out what to observe in an exploding white dwarf that would come out as a result of this final ignition phase,” he says. Observing it would be hard, says Pagnotta. It would be interesting to put constraints on dark matter and begin to pin it down, she says, but the signature of dark matter may not be visible in the light we observe from supernovae.