Scattering of Scalar Fields in Charged Black Holes Surrounded by an Anisotropic Fluid

Black holes in realistic, non-vacuum settings need models that capture environmental effects. In this study, we explore black hole dynamics using a generalization of Einstein's equations in general relativity, with the black hole surrounded by an anisotropic fluid described by the Kiselev spacetime. How does the presence of this anisotropic fluid alter the black hole's structure and influence the behavior of perturbations in its surroundings? We addressed this question by focusing on a specific example that can mimic a cosmic-string–type fluid. To link to potential observables, we analyze the perturbations of a charged, massless scalar field around a charged black hole and characterize the frequency spectrum of the field by studying its characteristic vibrations, known as quasinormal modes. For this, we developed a novel extension of Leaver's method coupled to automatic differentiation to be able to find the frequencies even in the most challenging parameter spaces. The results show interesting features in the spectrum, such as long-lived modes, breaking of symmetry in the spectrum, and regions not allowed for the frequencies, known as avoided-crossing regions.Scattering of Scalar Fields in Charged Black Holes Surrounded by an Anisotropic Fluid