Mind the environment

Next generation of gravitational-wave detectors will observe signals from small compact objects orbiting supermassive black holes inside galactic cores. Modeling such extreme-mass ratio systems is a daunting task on its own, made even more challenging by the recent black hole images of M87 and Sagittarius A* which indicate highly complex descriptions of galactic centers.

Accretion disks, dark matter halos, and tertiary companions are all expected to affect the binary evolution. But how important are astrophysical environments? Can they undermine tests of General Relativity? Can they mimic signatures of new physics? Are their effects strong enough to facilitate the study of their properties using gravitational waves?

Accurate gravitational-waveform models for the radiation emitted by these binaries are key to answer these questions. However, a comprehensive description of such binaries in non-vacuum environments was still missing. In a Physical Review Letters, Vitor Cardoso and collaborators employed relativistic perturbation techniques to study black holes surrounded by generic matter distributions. The outcome of this study is the first fully-relativistic formalism handling gravitational-wave emission in spherically symmetric, but otherwise generic, spacetimes, without weak-field and slow-motion approximations.

The formalism was applied to a solution of Einstein's equations modeling a black hole surrounded by a dark matter halo. This is the typical situation in galaxies like our Milky Way, where black holes with more than one million solar masses inhabit the galactic core and a dark matter structure extends way beyond the luminous part of the galaxy. The results are striking: the waveform contains clear imprints of the environment, and the energy emitted through gravitational waves by the binary can significantly differ from its vacuum counterpart. The environmental effects yield an unconventional inspiral, emitting a unique signal to be observed by future spaceborne detectors.

With much to understand, the approach can become the benchmarking tool to study environmental effects in gravitational waves. We plan to build synergies with the community specialized in modeling astrophysical environments and apply the formalism to other exciting systems, such as active galactic nuclei disks.