A black hole isn't just a cosmic vacuum cleaner. If it rotates, it can amplify waves around it, much like a mountain echo repeats sound. For ultralight dark matter (particles billions of times lighter than an electron), this effect, called superradiance, can create a huge cloud. Physicists have suggested that such a cloud resembles a dilute gas where all particles move in concert, like dancers following a single rhythm.
But what if the particles also repel each other slightly? Previous calculations either ignored this or neglected the black hole's rotation. The authors of the new work combined both approaches. They solved equations for the dark matter field in the curved spacetime of a rotating black hole, step by step achieving consistency. The main result: rotation changes the strength of the interaction. Near the horizon, where spacetime is twisted most strongly, the mutual repulsion of particles effectively switches off.
The calculations confirmed: the total angular momentum of the cloud equals simply the number of particles times Planck's constant. No corrections from interactions. So the black hole conducts its invisible orchestra: while 𝒩 is small, the music is pure; as the number of particles grows, interactions distort the cloud's shape but not its rhythm.
The growth rate of an ultralight dark matter cloud depends on the black hole's spin raised to the fifth, seventh, or even ninth power — a tiny change in rotation accelerates the process hundreds of times.
In Robert Wilson's novel 'Spin', a hypothetical form of matter envelops Earth, altering the passage of time. Here, a dark matter cloud around a black hole can 'steal' rotational energy, gradually slowing the hole down.