Black hole 'superradiance' phenomenon may aid quest for dark matter

New research challenges what we thought we knew about what happens around black holes. 

We're used to thinking of black holes as the ultimate vacuums, capable of sucking in everything around them and refusing to let anything out again. That includes light, hence the "black" in their names. But over the past 50 years, physicists have realized that black holes affect their environments in interesting, complicated ways. One of those ways leads to a process called superradiance, in which a black hole boosts any nearby light into intense levels of energy.

For superradiance to work, the black hole has to spin. That's not a problem, as black holes are born from the deaths of massive stars, and those stars are already spinning. As black holes spin, they literally drag on space-time around them, creating a region around the event horizon — the point beyond which nothing can escape — known as the ergosphere. Inside the ergosphere, it's impossible to stay still. If you were to fall into a black hole, before you reached the event horizon, you would be pulled into its orbit, even if you tried really hard to stay still.

Related: What happens at the center of a black hole?

The spinning effect of the ergosphere gets stronger the closer you get to the black hole, and this is what creates the superradiance effect. Some photons — fundamental units of light — passing close to the black hole get trapped in the ergosphere, and as they get closer to the event horizon, they whip around the black hole faster and faster. With every loop, they gain more and more energy. Some of those photons fall to their doom, crossing the event horizon, never to be seen again. But some scatter off of other photons and escape, and are boosted to incredibly high energies in the process.

The superradiance process is unstable. Over incredibly long timescales, enough photons can get boosted to high enough energies that the entire surroundings of the black hole turn into a giant "bomb," with the trapped photons blasting away in a single gigantic burst. But this process happens slowly enough that we haven't seen it play out in the universe yet.

Superradiance can extend beyond photons; it can happen to any kind of boson, including all the force carriers of nature and the Higgs boson — and also, maybe, dark matter.

Dark matter is the dominant form of matter in the universe, making up over 80% of all the mass of every galaxy and cluster. Astronomers have troves of circumstantial evidence for the existence of dark matter but have yet to pin down its identity.  

Dark photons 

One possibility is that dark matter is a new kind of ultralight particle that shares a lot of characteristics with bosons but does not interact with all the normal particles in the universe. These "dark photons" would be incredibly light yet absolutely flood the cosmos. But because they would not interact with normal matter, they would be exceedingly difficult to observe directly.

That is, unless they collect around black holes. Superradiance can operate on dark photons just as well as it does on normal photons. When dark photons collect around black holes, they can get trapped and boosted to high energies, where they might transform into other particles (or even just normal photons).

For decades, physicists have been studying this strange phenomenon, especially because the superradiance process offers an avenue for potential direct observations of dark matter if it is made of some superlight boson. (If the dark matter bosons are too heavy, they will not collect around black holes in the same way, and the superradiance trick wouldn't work anymore.)

Indeed, astronomers have used actual observations of black holes to put limits on the number of dark photons in the universe. We have yet to observe superradiance around black holes, which means that dark photons may not exist. But a new research paper published to the preprint database arXiv challenges those results, saying the situation can be much more complex.

Most work on dark matter assumes it's made of a single new kind of particle. But there's no reason the world of dark matter can't be as complex and as rich as the world of normal matter. In contrast to previous work, the new research assumes the existence of two different "species" of dark matter: one that's similar to dark photons (a boson) and another that's similar to a new particle (like a dark matter version of an electron).

The researchers found that the interactions between the different kinds of dark matter can mess up the superradiance process, thus preventing the dark photons from getting a boost and blasting off. Instead, as they whip around the black hole, they might keep hitting the other species of dark matter particle, sapping their energy in the process.

This means we can't take the observed limits at face value. Just because astronomers haven't seen superradiance doesn't necessarily indicate that dark photons don't exist. Instead, it might mean the physics of dark matter is much more complicated than we thought.

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