The Shear Viscosity to Entropy Ratio: A Status Report

TL;DR

This review discusses the universal shear viscosity to entropy density ratio in strongly coupled gauge theories with gravity duals, its modifications by higher derivative corrections, and implications for the conjectured lower bound.

Contribution

It provides a comprehensive overview of how higher derivative corrections affect the viscosity to entropy ratio and challenges the notion of a universal lower bound in holographic theories.

Findings

Higher derivative corrections can lower the viscosity to entropy ratio below the universal value.

The universal ratio is not a strict lower bound for all fluids.

Theories with certain curvature corrections violate the previously conjectured bound.

Abstract

This review highlights some of the lessons that the holographic gauge/gravity duality has taught us regarding the behavior of the shear viscosity to entropy density in strongly coupled field theories. The viscosity to entropy ratio has been shown to take on a very simple universal value in all gauge theories with an Einstein gravity dual. Here we describe the origin of this universal ratio, and focus on how it is modified by generic higher derivative corrections corresponding to curvature corrections on the gravity side of the duality. In particular, certain curvature corrections are known to push the viscosity to entropy ratio below its universal value. This disproves a longstanding conjecture that such a universal value represents a strict lower bound for any fluid in nature. We discuss the main developments that have led to insight into the violation of this bound, and consider whether the consistency of the theory is responsible for setting a fundamental lower bound on the viscosity to entropy ratio.