Superradiance and instabilities in black holes surrounded by anisotropic fluids

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In this paper we analyze the propagation of a charged scalar field in a Reissner-Nordstr\"om black hole endowed with one anisotropic fluid that can play the role of a cosmological term for certain set of parameters.

• The evolution of a scalar wave scattering is examined giving rise to the same superradiant scattering condition as in the de Sitter case.
• In addition, an analysis of the modes coming from the application of quasinormal boundary conditions is presented.
• Some special cases displaying analytical solutions for the quasinormal frequencies are discussed.