Horizon flux-balance laws in the multiscale perturbation

Abstract

Black-hole event horizons obey a set of evolution equations governing their intrinsic and extrinsic geometry. We study these equations in a \textit{two-timescale} perturbative expansion about a Kerr background and derive strong constraints on the coarse-grained horizon dynamics. At leading order, the coarse-grained linear shear vanishes, while the horizon exhibits an \textit{adiabatic rigidity}: the leading corrections to its angular velocity and inaffinity remain uniform on each horizon cut, while evolving on the slow timescale. We then provide a systematic procedure for transforming a perturbative bulk solution, given for example in Lorenz gauge, to an ingoing Newman--Unti gauge adapted to the horizon, allowing the perturbed horizon geometry to be extracted directly from the bulk metric. Finally, we formulate black-hole conservation laws associated with horizon symmetries, including energy, dynamical entropy, and angular momentum. We expand the charges through second order and their fluxes through third order in perturbation theory, providing a framework for future applications to horizon absorption and backreaction in extreme mass-ratio inspirals.