# Hadron matter in neutron stars in view of gravitational wave observations

## TL;DR

This review discusses the physical properties of neutron stars, their equation of state, and how gravitational wave observations like GW170817 provide insights into hadron matter at extreme densities, linking astrophysics with nuclear physics.

## Contribution

It synthesizes current knowledge on neutron star matter, emphasizing the role of gravitational wave data in testing hadron physics and the equation of state at high densities.

## Key findings

- Gravitational wave data constrains the neutron star equation of state.
- Transport in neutron stars is dominated by turbulence, not diffusion.
- Phase transitions in hadron matter can be probed through gravitational wave observations.

## Abstract

In this review we highlight a few physical properties of neutron stars and their theoretical treatment inasmuch as they can be useful for nuclear and particle physicists concerned with matter at finite density (and newly, temperature). Conversely, we lay out some of the hadron physics necessary to test General Relativity with binary mergers including at least one neutron star, in view of the event GW170817: neutron stars and their mergers reach the highest matter densities known, offering access to the matter side of Einstein's equations. In addition to minimum introductory material for those interested in starting research in the field of neutron stars, we dedicate quite some effort to a discussion of the Equation of State of hadron matter in view of gravitational wave developments; we address phase transitions and how the new data may help; we show why transport is expected to be dominated by turbulence instead of diffusion through most if not all of the star, in view of the transport coefficients that have been calculated from microscopic hadron physics; and we relate many of the interesting physics topics in neutron stars to the radius and tidal deformability.