Fluctuations in horizon-fluid lead to negative bulk viscosity

TL;DR

This paper derives the bulk viscosity of horizon-fluids using fluctuation theory, showing it matches previous values and suggesting a model-independent statistical mechanical description for such fluids.

Contribution

It provides an explicit, mostly model-independent derivation of horizon-fluid bulk viscosity using Kubo's fluctuation theory, linking it to horizon dynamics.

Findings

Bulk viscosity coefficient matches previous theoretical values.

Analysis is largely independent of microscopic details.

Supports the existence of a statistical mechanical framework for horizon-fluids.

Abstract

Einstein equations projected on to a black hole horizon gives rise to Navier-Stokes equations. Horizon-fluids typically possess unusual features like negative bulk viscosity and it is not clear whether a statistical mechanical description exists for such fluids. In this work, we provide an explicit derivation of the Bulk viscosity of the horizon-fluid based on the theory of fluctuations a la Kubo. The main advantage of our approach is that our analysis remains for the most part independent of the details of the underlying microscopic theory and hence the conclusions reached here are model independent. We show that the coefficient of bulk viscosity for the horizon-fluid matches exactly with the value found from the equations of motion for the horizon-fluid.