Can We Detect Deviations from Einstein's Gravity in Black Hole Ringdowns?

The quasinormal mode spectrum of gravitational waves emitted during the black hole ringdown relaxation phase, following the merger of a black hole binary, is a crucial target of gravitational wave astronomy. By considering causality constraints on the on-shell graviton three-point couplings within a weakly coupled gravity theory, we present arguments indicating that the contributions to the physics of linear and quadratic quasinormal modes from higher derivative gravity theories are either negligible or vastly suppressed for Schwarzschild and Kerr black holes. Their spectrum and interactions are dictated solely by Einstein's gravity.