Extreme mass ratio inspiral

In astrophysics, an extreme mass ratio inspiral (EMRI) is the orbit of a relatively light object around a much heavier (by a factor 10,000 or more) object, that gradually spirals in due to the emission of gravitational waves. Such systems are likely to be found in the centers of galaxies, where stellar mass compact objects, such as stellar black holes and neutron stars, may be found orbiting a supermassive black hole.^[1][2][3] In the case of a black hole in orbit around another black hole this is an extreme mass ratio binary black hole. The term EMRI is sometimes used as a shorthand to denote the emitted gravitational waveform as well as the orbit itself.

The main reason for scientific interest in EMRIs is that they are one of the most promising sources for gravitational wave astronomy using future space-based detectors such as the Laser Interferometer Space Antenna (LISA).^[4] If such signals are successfully detected, they will allow accurate measurements of the mass and angular momentum of the central object, which in turn gives crucial input for models for the formation and evolution of supermassive black holes.^[5] Moreover, the gravitational wave signal provides a detailed map of the spacetime geometry surrounding the central object, allowing unprecedented tests of the predictions of general relativity in the strong gravity regime.^[6]