We construct quasimodes for the Klein–Gordon equation on the black hole exterior of
Kerr–AdS (anti- de Sitter) 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 semiclassical nonlinear eigenvalue problem arising
after separation of variables, with the (inverse of the) angular momentum
playing the role of the semiclassical parameter. Our construction results in
exponentially small errors in the semiclassical 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.
Keywords
wave equation, black holes, decay estimates, Kerr – anti-de
Sitter