Black holes from the cosmos’s infancy could expose invisible matter

Primordial black holes could one day help scientists locate invisible dark matter, suggests new theoretical research.

New theoretical research suggests that dark matter could actually produce faint flashes of light when interacting with tiny black holes. These flashes could someday help researchers locate and study the enigmatic matter, which has thus far remained invisible. Exact nature of dark matter still eludes scientists, despite the fact that it makes up the vast majority of the mass of almost every galaxy in the cosmos. It has gravity, but does not interact with light or yield light of its own, thus we only have circumstantial evidence of its existence owing to its gravitational interactions with everything else.

In these circumstances, scientists are desperate to cook up any setting that might make dark matter more visible. So why not put to use black holes? It sounds like a ludicrous question: How could black holes, which consume any light that gets too close to them, make dark matter shine? But scientists have put together a scenario that might in fact make it possible. They made their findings public in September this year.

The scientists assume that dark matter can, in principle, interact with regular matter, but, for some inexplicable reason, usually doesn’t. Possibly the interaction requires a certain amount of energy that just isn’t available or is forbidden without a mediator particle doing the work. Black holes could offer the avenue required to overcome these barriers and get the dark matter to light up.

But just any black hole will not do the trick, only ultra-tiny primordial black holes will help. These black holes aren’t the remnants of giant stars but the leftovers of the chaotic eras of the very early cosmos, where matter and energy compressed spontaneously to make them. Primordial black holes were first conjectured by Stephen Hawking, but observations have thus far failed to find any. If they in fact do exist, they are very uncommon.

Similar to all black holes, primordial black holes would evaporate Hawking radiation, a weird quantum effect discovered by Stephen Hawking in which virtual particles pop up near a black hole’s edge and some are able to escape. The smaller the black hole, the more radiation it discharges. So primordial black holes roughly the mass of an asteroid would be emitting plenty of radiation in the present-day cosmos.

This radiation discharged by black holes isn’t just packets of light, or photons. It’s virtually every kind of particle, including dark matter particles. As the primordial black holes decay, they discharge dark matter particles that then energize any ambient dark matter particles in their proximity, causing a cascade that can briefly release visible light.

The scientists predict that these signals will characteristically be in the form of gamma ray flashes. They are far too weak for present experiments to detect, but future observatories, like NASA’s proposed AMEGO-X (All-sky Medium Energy Gamma-ray Observatory eXplorer), might have the resolution and sensitivity needed to find these kinds of signals.