Bulk Viscosity and Cavitation in Boost-Invariant Hydrodynamic Expansion

TL;DR

This paper models the evolution of a quark-gluon plasma using second order relativistic hydrodynamics, highlighting how bulk viscosity can induce cavitation, potentially explaining the breakdown of the fluid during hadronization.

Contribution

It provides a detailed analysis of how bulk viscosity influences hydrodynamic expansion and predicts cavitation at temperatures relevant to heavy ion collisions.

Findings

Bulk viscosity peaks can cause cavitation in the fluid.

Shear viscosity significantly affects the energy density evolution.

Cavitation occurs near the QCD crossover temperature.

Abstract

We solve second order relativistic hydrodynamics equations for a boost-invariant 1+1-dimensional expanding fluid with an equation of state taken from lattice calculations of the thermodynamics of strongly coupled quark-gluon plasma. We investigate the dependence of the energy density as a function of proper time on the values of the shear viscosity, the bulk viscosity, and second order coefficients, confirming that large changes in the values of the latter have negligible effects. Varying the shear viscosity between zero and a few times s/(4 pi), with s the entropy density, has significant effects, as expected based on other studies. Introducing a nonzero bulk viscosity also has significant effects. In fact, if the bulk viscosity peaks near the crossover temperature Tc to the degree indicated by recent lattice calculations in QCD without quarks, it can make the fluid cavitate -- falling apart into droplets. It is interesting to see a hydrodynamic calculation predicting its own breakdown, via cavitation, at the temperatures where hadronization is thought to occur in ultrarelativistic heavy ion collisions.