Effects of dark matter on black hole dynamics and gravitational wave signals.Black hole dark matter interaction refers to the various ways dark matter can influence the behavior of black holes, and vice versa. Dark matter, which makes up about 27% of the universe, does not emit light but interacts gravitationally. When a black hole is embedded in a dark matter halo, the dark matter can affect the black hole's spin, mass, and the gravitational waves it emits during mergers. Conversely, black holes can alter the distribution of dark matter around them, creating dense spikes or depletion zones. These interactions provide a unique observational window into dark matter's properties, which remain largely unknown.
The study of dark matter's effect on black holes gained momentum in the 2000s–2010s, with the advent of gravitational wave astronomy and improved dark matter models. Earlier work focused on dark matter density profiles around black holes, but the connection to gravitational wave signals is a recent development.
Dark matter particles passing near a black hole can be captured or scattered, transferring energy and angular momentum. This can cause the black hole to spin down or up, depending on the geometry. During black hole mergers, the presence of dark matter can modify the inspiral phase, leaving a distinct imprint on the gravitational wave signal—such as a phase shift or a change in frequency evolution. Additionally, dark matter can form a dense 'spike' around a black hole, which can affect the black hole's shadow and lensing of light.
By analyzing gravitational wave signals from black hole mergers, scientists can test dark matter models—for example, by looking for the characteristic 'dark matter drag' that would slow the inspiral. This turns LIGO and Virgo into dark matter detectors.
Dark matter can act like a 'cosmic brake' on merging black holes, slightly delaying the final collision and altering the gravitational wave chirp in a way that could be measured.
