ESO Expanding Horizon White Paper: Revealing the properties of matter at supranuclear densities with gravitational waves

Abstract

Understanding dense matter under extreme conditions is one of the most fundamental puzzles in modern physics. Complex interactions give rise to emergent, collective phenomena. While nuclear experiments and Earth - based colliders provide valuable insights, much of the quantum chromodynamics phase diagram at high density and low temperature remains accessible only through astrophysical observations of neutron stars, neutron star mergers, and stellar collapse. Astronomical observations thus offer a direct window to the physics on subatomic scales with gravitational waves presenting an especially clean channel. Next-generation gravitational - wave observatories, such as the Einstein Telescope, would serve as unparalleled instruments to transform our understanding of neutron star matter. They will enable the detection of up to tens of thousands of binary neutron star and neutron star - black hole mergers per year, a dramatic increase over the few events accessible with current detectors. They will provide an unprecedented precision in probing cold, dense matter during the binary inspiral, exceeding by at least an order of magnitude what current facilities can achieve. Moreover, these observatories will allow us to explore uncharted regimes of dense matter at finite temperatures produced in a subset of neutron star mergers, areas that remain entirely inaccessible to current instruments. Together with multimessenger observations, these measurements will significantly deepen our knowledge of dense nuclear matter.

Figures (2)

• Figure 1: Phase diagram of QCD. Orange shading indicates the range of thermodynamic conditions spanned by NSs, NS mergers, core-collapse supernovae (CCSN), and proto-NSs, all of which are accessible with GWs. Other parts of the diagram are explored in various terrestrial facilities.
• Figure 2: Binary NS coalescence seen by GW detectors. The blue curve is an averaged GW strain amplitude, and snapshots of the density from numerical-relativity simulations indicate interesting regimes: (i) early inspiral, where resonant tidal excitations of NS oscillation modes with low frequency could reveal detailed information about NS interiors, (ii) tidal deformations that encode information on the EOS, and (iii) the postmerger phase in which the finite-temperature equation of state can be probed. Black curves indicate the most recent sensitivity of LIGO (dotted) and of ET (dashed).