Fundamental Aspects of the Early Ringdown

The ringdown of a perturbed black hole is a well-established prediction of the theory of general relativity (GR). Black hole ringdown has been the subject of a great deal of research in the past five decades, with substantial progress made in understanding the structure of quasinormal modes (QNMs) and their characteristic frequencies as well as in identifying them in gravitational wave data from compact object mergers. In recent years, other fundamental aspects of ringdown have received greater attention, such as the imprint of the perturbing source on the QNM amplitudes, nonlinear aspects of the black hole response, and the role of the horizon properties on the gravitational waveform. In this work, we examine some of these aspects of ringdown and present new results which are most relevant in the merger and early ringdown of a compact object merger. Given that gravity reaches its peak nonlinearity in this stage, we then consider how an improved understanding of these phenomena will enable more precise parameter estimation and tests of GR.