A Novel Formula for Bulk Viscosity from the Null Horizon Focusing Equation

TL;DR

This paper derives a new, simple formula for the bulk viscosity of fluids using the null horizon focusing equation, linking horizon scalar fields to thermodynamic variables, with applications to various gauge theory plasmas.

Contribution

It introduces a novel formula for bulk viscosity based on the null horizon focusing equation, connecting horizon scalar fields to thermodynamic properties.

Findings

The formula applies to non-conformal branes, N=4 SYM perturbations, and holographic QCD models.

It provides a unified approach to compute bulk viscosity in holographic fluids.

The applicability range of the formula is discussed.

Abstract

The null horizon focusing equation is equivalent via the fluid/gravity correspondence to the entropy balance law of the fluid. Using this equation we derive a simple novel formula for the bulk viscosity of the fluid. The formula is expressed in terms of the dependence of scalar fields at the horizon on thermodynamic variables such as the entropy and charge densities. We apply the formula to three classes of gauge theory plasmas: non-conformal branes, perturbations of the N=4 supersymmetric Yang-Mills theory and holographic models of QCD, and discuss its range of applicability.