Neutron Stars

TL;DR

This paper reviews neutron stars as probes of gravity theories, focusing on universal relations between their properties and gravitational radiation, and explores how alternative gravity theories could produce observable differences.

Contribution

It introduces neutron star properties and universal relations, and presents two examples of alternative gravity theories with distinctive effects on neutron stars.

Findings

Universal relations help constrain neutron star equations of state.

Alternative gravity theories can alter neutron star gravitational wave signatures.

Pulsar and gravitational wave observations can test these theories.

Abstract

Neutron stars are highly compact astrophysical objects and therefore of utmost relevance to learn about theories of gravity. Whereas the proper equation of state of the nuclear matter inside neutron stars is not yet known, and a wide range of equations of state is still compatible with observations, this uncertainty can be overcome to a large extent, when dimensionless neutron star properties are considered. In this case universal relations between neutron star properties and for the gravitational radiation in the form of quasi-normal modes arise. These universal relations can be rather distinct for alternative theories of gravity as compared to General Relativity. Moreover, the presence of new degrees of freedom in alternative theories of gravity leads to further types of gravitational radiation, that may be revealed by pulsar observations and gravitational wave detectors. Here an introduction to neutron stars, their properties and universal relations is presented, followed by two examples of alternative theories of gravity featuring interesting effects for neutrons stars.