A spinning top whirls so fast that the air around it starts to rotate in unison. A black hole behaves in much the same way: if it spins, it can transfer its energy to particles that happen to be nearby. This process is called superradiance: it's like an echo in the mountains, but instead of sound, new particles are born.
In this work, physicists considered what would happen if hypothetical dark photons, invisible carriers of interactions in the dark matter sector, were caught in such an 'echo.' They found that under certain conditions, these dark photons could decay into pairs of neutrinos and antineutrinos. The key to this decay is that neutrinos possess unusual electromagnetic properties that go beyond the Standard Model. It's like a neutral object suddenly showing an ability to be attracted to a magnet.
But so far, no such signals have been seen. And that in itself is a result. The scientists used the silence in the detectors to constrain the possible number of primordial black holes: those born in the early Universe that could have set this whole mechanism in motion. It turns out that their share of dark matter cannot be too large, otherwise the neutrino background would be noticeable. So the absence of a signal becomes a tool that outlines the boundaries of the invisible world.
Superradiance from black holes was predicted back in the 1970s, but its 'dark' variant, involving hypothetical particles like dark photons, has only become a hot topic now, with the rise of multi-messenger astronomy.
In Stanisław Lem's novel 'Solaris,' the planet's ocean spawned mysterious 'visitors' from the depths of the subconscious — here, a black hole, like a cosmic ocean, gives birth to particles from the vacuum, and we try to understand their nature.