Relativistic perturbation theory for black-hole boson clouds

Linear perturbations around black hole spacetimes can be quasinormal, quasibound, or even superradiantly unstable, the latter leading to the formation of macroscopic boson clouds. Here, we develop a relativistic perturbation theory for boson clouds around rotating black holes that supersedes the non-relativistic "gravitational atom" approximation and brings close agreement with numerical relativity. We first introduce a relativistic product and corresponding orthogonality relation between (massive) scalar modes, which extends a recent result for gravitational perturbations, and forms the basis for our framework. We then derive the analog of time-dependent perturbation theory in quantum mechanics. As a demonstration, we apply these techniques to calculate the self-gravitational frequency shift of a Kerr superradiant mode, improving the error by a factor of four (from 28% to 7%) for the largest masses considered. We thereby provide a conceptually new approach to calculate black-hole mode dynamics, with practical application for precision gravitational-wave astronomy.