Quasi-normal mode expansions of black hole perturbations: a hyperboloidal Keldysh's approach

We study quasinormal mode expansions by adopting a Keldysh scheme for the spectral construction of asymptotic resonant expansions. Quasinormal modes are first cast in terms of a non-selfadjoint problem by adopting, in a black hole perturbation setting, a spacetime hyperboloidal approach. Then the Keldysh expansion of the resolvent, built on bi-orthogonal systems, provides a spectral version of Lax-Phillips expansions on scattering resonances. We clarify the role of scalar product structures in the Keldysh setting, that prove non-necessary to construct the resonant expansions (in particular the quasinormal mode time-series at null infinity), but are required to define the (constant) excitation coefficients in the bulk resonant expansion. We demonstrate the efficiency and accuracy of the Keldysh spectral approach to (non-selfadjoint) dynamics, even beyond its limits of validity, in particular recovering Schwarzschild black hole late power-law tails. We also study early dynamics by exploring i) the existence of an earliest time of validity of the resonant expansion and ii) the interplay between overtones extracted with the Keldysh scheme and regularity. Specifically, we address convergence aspects of the series and, on the other hand, we implement non-modal analysis tools, namely assessing $H^p$-Sobolev dynamical transient growths and constructing $H^p$-pseudospectra. Finally, we apply the Keldysh scheme to calculate ''second-order'' quasinormal modes and complement the qualitative study of overtone distribution by presenting the Weyl law for the counting of quasinormal modes in black holes with different (flat, De Sitter, anti-De Sitter) spacetime asymptotics.