Phases of dense matter in compact stars

TL;DR

This paper reviews the various phases of dense matter inside neutron stars, highlighting their properties, observational implications, and unresolved puzzles in understanding matter under extreme conditions.

Contribution

It provides a comprehensive review of the phases of dense matter in neutron stars, including recent perspectives and observational prospects.

Findings

Different phases of dense matter influence neutron star mass-radius relations.

Outer layers include crust, neutron ocean, and possibly nuclear pasta.

Inner core remains a subject of ongoing research and puzzles.

Abstract

Formed in the aftermath of gravitational core-collapse supernova explosions, neutron stars are unique cosmic laboratories for probing the properties of matter under extreme conditions that cannot be reproduced in terrestrial laboratories. The interior of a neutron star, endowed with the highest magnetic fields known and with densities spanning about ten orders of magnitude from the surface to the centre, is predicted to exhibit various phases of dense strongly interacting matter, whose physics is reviewed in this chapter. The outer layers of a neutron star consist of a solid nuclear crust, permeated by a neutron ocean in its densest region, possibly on top of a nuclear "pasta" mantle. The properties of these layers and of the homogeneous isospin asymmetric nuclear matter beneath constituting the outer core may still be constrained by terrestrial experiments. The inner core of highly degenerate, strongly interacting matter poses a few puzzles and questions which are reviewed here together with perspectives for their resolution. Consequences of the dense-matter phases for observables such the neutron-star mass-radius relationship and the prospects to uncover their structure with modern observational programmes are touched upon.