Non-universal lower bound for the shear viscosity to entropy density ratio

TL;DR

This paper demonstrates that the lower bound of the shear viscosity to entropy density ratio is not universal but depends on the entropy density, challenging the previously assumed universal value.

Contribution

It provides an exact representation of the ratio via the density of states and shows that the bound varies across different physical systems.

Findings

The lower bound is not universal and depends on entropy density.

Examples show possible violations of the conformal 1/4pi bound.

The ratio's lower bound is system-dependent.

Abstract

The lower bound of the shear viscosity to entropy density ratio is examined using an exact representation of the ratio through the density of states. It is shown that the lower bound in a generic physical system is not universal, its value is determined by the entropy density. Some examples of physical systems are discussed in the paper where one can expect violation of the conformal 1/4pi value.