Beyond Linear Ringdown

After the merger of two black holes (BHs), there is a distorted remnant black hole that rings down to its final stationary state by emitting gravitational waves (GWs). This ringdown post-merger signal is typically modelled analytically using linear perturbation theory, which assumes that the spacetime distortions associated to the GWs are small. Surprisingly, research has shown that linear theory can model the GW signal extremely well even at the moment of the merger, when the spacetime is expected to be largely distorted and the nonlinear nature of gravity is expected to become important.

In this talk, I will first discuss the surprising success of linear ringdown models and then argue why nonlinearities have a seemingly small effect. Nonetheless, I will show that high-accuracy numerical simulations of black hole mergers do show the presence of nonlinearities, which could be observable with future gravitational wave detectors and could allow to better probe the behavior of gravity.