Gravitational wave tomography: the ringdown regime

As a black hole remnant settles down to its final state, its dynamical horizon asymptotically approaches equilibrium. However, the horizon dynamics cannot be directly observed from null infinity because it is causally disconnected. Nonetheless, using Einstein's equations and the linearized approximation in the ringdown regime, we show that we can infer the strong field dynamics of the black hole horizon from the weak field gravitational waves. In particular, this can be used to test the starting time of ringdown regime and in numerical simulations and identify overfitting of the ringdown waveform.