Quasimodes and a Lower Bound on the Uniform Energy Decay Rate for Kerr-AdS Spacetimes

Abstract

We construct quasimodes for the Klein-Gordon equation on the black hole exterior of Kerr-Anti-de Sitter (Kerr-AdS) spacetimes.Such quasimodes are associated with time-periodic approximate solutions of the Klein Gordon equation and provide natural candidates to probe the decay of solutions on these backgrounds.They are constructed as the solutions of a semi-classical non-linear eigenvalue problem arising after separation of variables, with the (inverse of the) angular momentum playing the role of the semi-classical parameter.Our construction results in exponentially small errors in the semi-classical parameter.This implies that general solutions to the Klein Gordon equation on Kerr-AdS cannot decay faster than logarithmically.The latter result completes previous work by the authors, where a logarithmic decay rate was established as an upper bound.