A sticky business: the status of the conjectured viscosity/entropy density bound

TL;DR

This paper critically examines various forms of the conjectured universal lower bound on the shear viscosity to entropy density ratio, highlighting counterexamples and limitations of existing evidence for the bound.

Contribution

The paper analyzes different variants of the viscosity/entropy density bound, identifying which are viable and which are invalidated by counterexamples.

Findings

Some variants of the conjecture are invalidated by counterexamples

Much of the supporting evidence does not apply to unruled-out variants

The conjecture's universality is more limited than previously thought

Abstract

There have been a number of forms of a conjecture that there is a universal lower bound on the ratio, eta/s, of the shear viscosity, eta, to entropy density, s, with several different domains of validity. We examine the various forms of the conjecture. We argue that a number of variants of the conjecture are not viable due to the existence of theoretically consistent counterexamples. We also note that much of the evidence in favor of a bound does not apply to the variants which have not yet been ruled out.