Black holes could reveal the presence of dark matter in their immediate vicinity through their gravitational waves.
Gravitational waves, predicted by Einstein, are ripples in spacetime. They travel at the speed of light and arise from the most violent events in the universe, such as when two black holes merge. As these massive objects orbit each other and then collide, they release colossal energy that warps the cosmic fabric.
When two black holes collide and merge, they release gravitational waves. These waves are detected by the LIGO-Virgo-KAGRA observatories on Earth, allowing scientists to determine the mass and spin of the black holes.
Credit: Maggie Chiang for Simons Foundation
To capture these tiny vibrations, the LIGO and Virgo interferometers measure distance variations with unprecedented precision — on the order of a billionth of the width of an atom. Each black hole merger produces a characteristic chirp-like signal, whose shape depends on the masses, spins, and environment of the objects.
Scientists analyze this signal to deduce the properties of the merging objects. If the environment is not empty but contains dark matter, the wave will carry its imprint. Recent models allow distinguishing between the two cases, offering a tool to probe dark matter from a distance.
Invisible because it neither emits nor absorbs light, dark matter can only be observed through its gravitational pull on visible matter — for example, in galaxy rotation or light bending.
According to one theory, dark matter could consist of ultralight particles. Near a rotating black hole, these particles behave like waves. The black hole can then transfer some of its rotational energy to them via a mechanism called superradiance, which greatly increases the surrounding dark matter density.
When this density reaches a critical threshold, it leaves a signature in the gravitational waves emitted during a merger. Researchers have modeled this effect and are exploring observatory data to detect it. Superradiance thus provides a natural mechanism to concentrate dark matter around black holes, making its study possible.