Quasinormal modes and holography

Quasinormal frequencies of electromagnetic and gravitational perturbations in asymptotically anti-de Sitter spacetime can be identified with poles of the corresponding real-time Green's functions in a holographically dual finite temperature field theory. The quasinormal modes are defined for gauge-invariant quantities which obey an incoming-wave boundary condition at the horizon and a Dirichlet condition at the boundary. As an application, we explicitly find poles of retarded correlation functions of $R$-symmetry currents and the energy-momentum tensor in strongly coupled finite temperature $\mathcal{N}=4$ supersymmetric $SU({N}_{c})$ Yang-Mills theory in the limit of large ${N}_{c}$.