Cosmology seminar: Quasinormal Modes and Black Hole Ringdowns

Binary black holes are the most numerous gravitational wave (GW) sources. Their GW signals are usually short and, in the loudest signals, the merger and *ringdown* portion of the signal is clearly visible. Modeling the final stages of binary black hole mergers is a cornerstone of gravitational-wave astronomy. The *ringdown* is associated with the system settling down into its final, stationary state which (within general relativity) assumed to be a Kerr black hole, fully described by only a mass and spin (the "no hair" theorem). The ringdown signal contains a superposition of exponentially damped quasinormal modes the frequencies of which depend only on the properties of the remnant Kerr black hole. In this talk I will review recent developments aimed at extracting multiple quasinormal mode frequencies from ringdown signals allowing the remnant black holes to be studied in unprecedented detail and for tests of general relativity and the "no hair" theorem to be performed.