The ringing of a different bell: quasinormal modes and their excitation beyond general relativity

The inference of quasinormal mode frequencies from the ringdown part of gravitational-wave signals produced by the coalescence of black-hole binaries can be used to infer the mass and spin of the remnant black hole and, possibly, test general relativity and extensions thereof in the strong field regime. In this presentation, I will share some findings of a long-term project aimed at studying the quasinormal-mode spectrum of black holes beyond general relativity, and how these modes are excited given an initial perturbation. Working with an effective-field-theory extension of general relativity with cubic-in-curvature terms, I discuss the properties of quasinormal mode overtones and excitation factors of nonrotating black holes in this theory. I will interpret these properties from an effective-field-theory perspective and discuss some surprising consequences.