Quasinormal modes of compact objects in alternative theories of gravity

TL;DR

This paper investigates how quasinormal modes of black holes and neutron stars are affected by alternative theories of gravity, revealing richer spectra and stability features due to scalar fields and hair, with implications for gravitational wave observations.

Contribution

It provides a comprehensive analysis of quasinormal modes in various alternative gravity theories, including scalarized black holes and neutron stars, highlighting new spectral features and stability properties.

Findings

Scalar hair modifies quasinormal mode spectra.

Presence of a minimum mass in certain models.

Universal relations for neutron star modes independent of matter content.

Abstract

We address quasinormal modes of compact objects in several alternative theories of gravity. In particular, we focus on black holes and neutron stars with scalar hair. We consider black holes in dilaton-Einstein-Gau\ss -Bonnet theory, and in a generalized scalar-Einstein-Gau\ss -Bonnet theory. In the latter case scalarized black holes arise, and we study the stability of the different branches of solutions. In particular, we discuss how the spectrum of quasinormal modes is changed by the presence of a non-trivial scalar field outside the black hole horizon. We discuss the existence of an (effective) minimum mass in these models, and how the spectrum of modes becomes richer as compared to general relativity, when a scalar field is present. Subsequently we discuss the effect of scalar hair for realistic neutron star models. Here we consider $R^2$ gravity, scalar-tensor theory, a particular subsector of Horndeski theory with a non-minimal derivative coupling, and again dilatonic-Einstein-Gau\ss -Bonnet theory. Because of the current lack of knowledge on the internal composition of the neutron stars, we focus on universal relations for the quasinormal modes, that are largely independent of the equations of state and thus the matter content of the stars.