Bulk Viscosity in Dense Nuclear Matter

TL;DR

This paper reviews bulk viscosity in dense nuclear matter, focusing on its calculation in neutron star conditions, its historical development, and its role in neutron star mergers, emphasizing weak interactions and dissipation mechanisms.

Contribution

It provides a comprehensive overview of bulk viscosity in dense matter, including calculations for Urca processes and insights into its impact on neutron star mergers.

Findings

Bulk viscosity affects damping of oscillations in neutron stars.

Calculations for Urca processes in npe and npeμ matter are presented.

Bulk viscosity plays a significant role in neutron star merger dynamics.

Abstract

In this chapter, I describe bulk viscosity as a general concept, and then focus on bulk viscosity in the dense matter present in compact objects. While this review is focused on bulk viscosity in the conditions present in neutron star mergers, I present a history of bulk viscosity research in dense matter, from its role in damping radial oscillations in neutron stars through its current applications in neutron star mergers. The majority of the chapter consists of calculations of the bulk viscosity from Urca processes in generic neutron-proton-electron ($npe$) matter, and then in dense matter containing muons ($npe\mu$ matter) as well. I make several approximations in these calculations to keep the focus on the concepts. More precise calculations exist in the literature, to which I refer the reader. One concept I attempt to elucidate is the thermodynamic behavior of a fluid element throughout an oscillation and how that leads to bulk-viscous dissipation. I conclude with a discussion of the recent research into the role of weak interactions and bulk viscosity in neutron star mergers.