Abstract

Recent analyses have shown that the gravitational-wave signal emitted by merging black holes is surprisingly simple in its final stages. It has long been known that the very end stage of a black hole merger—the late ringdown—is well-described by black hole perturbation theory. The latter, however, has been found to be in excellent agreement with numerical simulations even at much earlier times, immediately following the merger. In this talk, I will describe efforts to understand this phenomenon by characterizing the size and nature of backreaction effects in black-hole ringdowns. Our analysis, which we validate against numerical relativity simulations, is based on a novel perturbation theory incorporating a bilinear form between quasinormal modes. From a toy-model system (an asymptotically anti-de Sitter black hole with a ringing scalar field), to gravitational perturbations of Schwarzschild, the lessons we learn can help explain the simplicity of ringdown signals. I will also discuss ongoing efforts to extend these studies to more realistic (Kerr) black holes.