extreme mass ratio inspiral

The abbreviation EMRI for extreme mass ratio inspiral is used for orbits expected to inspiral yet have a mass ratio of 10,000 or greater, specifically, a supermassive black hole (SMBH) with a massive star or compact object orbiting it. They are proposed as a source of detectable gravitational waves (GWs) and are some of the targets of planned space-based gravitational-wave detectors such as the LISA:

• This type of signal is calculated to be strong enough to be detectable.
• They should be sufficiently frequent that detection is plausible, because of the high density of stars in the center of may galaxies, in the vicinity of the galaxy's central black hole.

The orbital period and wave period would be in the range of hours. This circumstance was considered promising enough to contribute to the motivation for such space-based GW detectors, but there are also factors which could make detectable spirals rare:

• The need for an extreme eccentricity (for a sufficiently close pass between the objects to make the waves strong enough to stand out) and theories that suggest the strong-field gravity orbits tend to lose their eccentricity.
• Tidal forces destroying the orbiting object, particularly suggesting early main sequence stars wouldn't last long.

The term intermediate mass ratio inspiral (IMRI) is used for a similar circumstance surrounding a (theorized) intermediate-mass black hole (IMBH), a possible target for ground detectors such as LIGO.