Approximate Universal Relations for Neutron Stars and Quark Stars

TL;DR

This paper reviews the universal relations among neutron star observables, such as the I-Love-Q relations, which are independent of the star's internal structure and have diverse astrophysical applications.

Contribution

It provides a comprehensive review of the origin, properties, and applications of approximate universal relations for neutron and quark stars.

Findings

Universal I-Love-Q relations are approximately independent of the equation of state.

These relations help break degeneracies in astrophysical measurements.

Applications include constraining nuclear matter and testing gravity theories.

Abstract

Neutron stars and quark stars are ideal laboratories to study fundamental physics at supra nuclear densities and strong gravitational fields. Astrophysical observables, however, depend strongly on the star's internal structure, which is currently unknown due to uncertainties in the equation of state. Universal relations, however, exist among certain stellar observables that do not depend sensitively on the star's internal structure. One such set of relations is between the star's moment of inertia ($I$), its tidal Love number (Love) and its quadrupole moment ($Q$), the so-called I-Love-Q relations. Similar relations hold among the star's multipole moments, which resemble the well-known black hole no-hair theorems. Universal relations break degeneracies among astrophysical observables, leading to a variety of applications: (i) X-ray measurements of the nuclear matter equation of state, (ii) gravitational wave measurements of the intrinsic spin of inspiraling compact objects, and (iii) gravitational and astrophysical tests of General Relativity that are independent of the equation of state. We here review how the universal relations come about and all the applications that have been devised to date.