Neutron stars and the dense matter equation of state: from microscopic theory to macroscopic observations

TL;DR

This paper reviews how microscopic nuclear physics models and astrophysical observations, including gravitational waves, jointly constrain the dense matter equation of state in neutron stars, highlighting recent progress and future challenges.

Contribution

It provides a comprehensive overview of theoretical models and observational data used to understand the neutron-star equation of state across different densities.

Findings

Constraints from gravitational waves and electromagnetic observations inform the dense matter properties.

Nuclear interactions constrained by experiments help model the equation of state.

Future observations will further refine the understanding of neutron-star interiors.

Abstract

The past years have witnessed tremendous progress in understanding the properties of neutron stars and of the dense matter in their cores, made possible by electromagnetic observations of neutron stars and the detection of gravitational waves from their mergers. These observations provided novel constraints on neutron-star structure, that is intimately related to the properties of dense neutron-rich matter described by the nuclear equation of state. Nevertheless, constraining the equation of state over the wide range of densities probed by astrophysical observations is still challenging, as the physics involved is very broad and the system spans many orders of magnitude in densities. Here, we review theoretical approaches to calculate and model the neutron-star equation of state in various regimes of densities, and discuss the related consequent properties of neutron stars. We describe how the equation of state can be calculated from nuclear interactions that are constrained and benchmarked by nuclear experiments. We review neutron-star observations, with particular emphasis on information provided by gravitational-wave signals and electromagnetic observations. Finally, we discuss future challenges and opportunities in the field.