Probing general relativity with mergers of supermassive and intermediate-mass black holes

Recent observations and stellar dynamics simulations suggest that similar to10(3) M. black holes can form in compact massive young star clusters. Any such cluster in the bulge of its host galaxy will, within a few hundred million years, spiral to the center, where its intermediate-mass black hole is likely to eventually merge with the galaxy's supermassive black hole. If such mergers are common, then future space-based gravitational wave detectors such as the Laser Interferometer Space Antenna will detect them with such a high signal-to-noise ratio that toward the end of the inspiral the orbits will be visible in a simple power density spectrum, without the need for matched filtering. We discuss the astrophysics of the inspiral of clusters in the nuclear region of a galaxy and the subsequent merger of intermediate-mass with supermassive black holes. We also examine the prospects for understanding the spacetime geometry of rotating black holes, based on phase connection of the strong signals visible near the end of these extreme mass ratio inspirals.