Critical slowing down and bulk viscosity in binary neutron star mergers

TL;DR

This paper explores how proximity to a QCD critical point during neutron star mergers can cause critical slowing down, significantly increasing bulk viscosity and potentially affecting observable merger dynamics.

Contribution

It demonstrates that QCD critical phenomena can enhance bulk viscosity in neutron star mergers, challenging standard assumptions about dissipation mechanisms.

Findings

Bulk viscosity can rival electroweak contributions near a QCD critical point.

Critical slowing down affects the hydrodynamic evolution of neutron star mergers.

Observable imprints of critical dynamics are possible in merger signals.

Abstract

Hydrodynamic simulations of neutron star mergers rely on the clear separation between the strong-interaction, weak-interaction, and hydrodynamic timescales. In this effective framework, weak Urca interactions are typically the slowest microscopic processes, and therefore the Urca rate determines the bulk-viscous dissipation. This assumed hierarchy of dissipative mechanisms can be decisively altered, without invalidating hydrodynamics, if the trajectory of the matter in a neutron star merger passes through the vicinity of a possible low temperature QCD critical point. The enhanced density fluctuations lead to critical slowing down and rapid growth of transport coefficients including bulk viscosity. While this growth is regulated by finite-time effects, finite-size effects, and the breakdown of hydrodynamic scale separation, which bound the correlation length, we demonstrate that the QCD contribution to bulk viscosity can rival the electroweak contribution in realistic conditions. Thus, critical dynamics could leave observable imprints on the hydrodynamic evolution of neutron star mergers.