ÃÛÌÒTV

The historic first detection of a gravitational waves from merging black holes came with an unexpected bonus: the first observation of a black hole ringdown. The ringdown of a black hole through "quasinormal modes" provides us with a unique measurement of the properties of a black hole. Deviations from General Relativity are imprinted on the spectrum of modes, and even in the case of Kerr, there is an unexpected richness in ringdown phenomena. In this talk, I will discuss past work on the spectrum of nearly extremal black holes and their unique ringdown signature. I will also discuss a method for computing the spectrum of black hole spacetimes which are perturbed away from Kerr. The application of this method reveals the possibility of a parametric instability in rapidly rotating black holes, and allows for the calculation of quasinormal mode frequencies in weakly charged Kerr-Newman black holes.