Compact Objects in Alternative Gravities

TL;DR

This paper explores the properties of neutron stars and black holes within scalar-tensor theories of alternative gravity, highlighting phenomena like spontaneous scalarization, quasi-normal modes, and black hole shadows, with an emphasis on universal relations.

Contribution

It provides a comprehensive analysis of compact objects in scalar-tensor theories, including phenomena like scalarization and quasi-normal modes, and introduces universal relations to simplify their characterization.

Findings

Spontaneous scalarization occurs in neutron stars and black holes.

Universal relations help reduce dependence on the equation of state.

Black hole shadows are briefly analyzed in the context of alternative gravities.

Abstract

We address neutron stars and black holes in alternative gravities, after recalling their basic properties in General Relativity. Among the plethora of interesting alternative gravities we here focus on an interesting set of scalar-tensor theories. We discuss the phenomenon of spontaneous scalarization, that is matter induced for neutron stars and curvature induced for black holes. Along with other relevant physical properties, we address the quasi-normal modes of these compact objects. In particular, we consider \textit{universal relations} of neutron stars to largely reduce the dependence on the equation of state, and we briefly address the shadow of black holes.