QNMs of charged black holes in AdS spacetime: a geometrical optics perspective

Abstract: We investigate the quasinormal modes of the Reissner$-$Nordstr\"om anti$-$de Sitter black hole using the Penrose limit, motivated by the geometrical optics approximation. This approach offers a novel framework for approximating quasinormal modes with large real frequencies by associating a plane wave to spacetime regions near null geodesics, providing a geometric interpretation of the geometrical optics approximation. Applying this limit to bound null orbits around black holes allows us to explore the black hole response to perturbations. We analyze the effects of black hole charge and negative cosmological constant on the quasinormal spectrum, finding that increasing charge enhances both the real and imaginary parts of the frequencies, while a decreasing cosmological constant leads to higher real frequencies and longer lived perturbations, with the spectrum stabilizing at larger values of the cosmological constant.