A black hole is not a perfect vacuum cleaner. It is surrounded by a swarm of particles and fields, and this swarm alters its fate. In a new study, astrophysicists examined how a massive vector field, a hypothetical form of dark matter, behaves near a rotating black hole. Such a field resembles an ocean whose waves lap against the shore of the event horizon.
It turns out that a dense "spike" of the field forms around the hole, with density falling with distance as $r^{-3/2}$. This is akin to foam gathering at the edge of a whirlpool. The rotation of spacetime barely distorts this spike, except near the ergosphere — a region where space itself rotates faster than light. But the biggest surprise came in the superradiance regime. When a field wave rotates in the same direction as the hole, it begins to extract energy from it.
For very light field particles, the effect weakens, and the hole grows as usual — up to a hundred solar masses per year. These numbers matter: they show that the dance between a black hole and dark matter can be not just absorption but a struggle, where the hole sometimes loses mass. In the future, this could help explain why some supermassive black holes appear "hungry."
The ergosphere is a region around a rotating black hole where nothing can remain stationary: space itself drags everything along, like a whirlpool carrying a twig.
In the movie Interstellar, the heroes use the black hole's rotation for a maneuver — superradiance, described in the paper, works on a similar principle, tapping rotational energy.