Quasinormal modes and their anomalous behavior for black holes in $f(R)$ gravity

TLDR: In this article, the authors studied the scalar field propagation in the background of an asymptotically de-Sitter black hole solution in $f(R)$ gravity, with the aim of analyzing the existence of an anomalous behavior in the quasinormal modes (QNMs) spectrum in alternatives theories of gravity and to study the stability of scalar fields propagation.

Abstract: We study the propagation of scalar fields in the background of an asymptotically de-Sitter black hole solution in $f(R)$ gravity, with the aim of analyzing the existence of an anomalous behavior in the quasinormal modes (QNMs) spectrum in alternatives theories of gravity and to study the stability of the scalar field propagation. We study the QNMs for various overtone numbers of different branches that they depend on a parameter $\beta$ which appears in the metric and characterizes the $f(R)$ gravity. For small deviations from the Schwarzschild-dS black hole the anomalous behavior in the QNMs is present, and the critical value of the mass of the scalar field depends on the parameter $\beta$ while for large deviations the anomalous behavior does not appear. Also, the critical mass of the scalar field increases when the overtone number increases until the $f(R)$ gravity parameter $\beta$ approaches the near extremal limit at which the critical mass of the scalar field does not depend anymore on the overtone number. Also, we find that the imaginary part of the quasinormal frequencies is always negative for all branches leading to a stable propagation of the scalar fields in this background.