Universality of the shear viscosity in supergravity

TL;DR

The paper proves that the Kovtun–Son–Starinets bound on the shear viscosity to entropy density, ${\eta}/{s} \ge {1}/{4\pi}$, is saturated in the supergravity limit for gauge theories with holographic duals, i.e. ${\eta}/{s} = {1}/{4\pi}$. By computing the shear diffusion constant ${\cal D}$ for the PW, KT, and MN backgrounds, the authors show ${\cal D} = {1}/{(4\pi T)}$, implying ${\eta}/{s} = {1}/{4\pi}$ in each case, and then derive a general argument that this saturation holds universally for nonextremal supergravity backgrounds. They also discuss $\alpha'$ corrections, proposing a coupling-dependent factor $f(\lambda) \ge 1$ with $f(\lambda) \to 1$ as $\lambda \to \infty$, supported by preliminary corrections that may alter the dispersion relation and require a full fluctuation analysis. The results reinforce the view that strong coupling holographic plasmas attain a universal minimal viscosity, with potential finite-$\lambda$ departures identifiable via higher-derivative corrections.

Abstract

Kovtun, Son and Starinets proposed a bound on the shear viscosity of any fluid in terms of its entropy density. We argue that this bound is always saturated for gauge theories at large 't Hooft coupling, which admit holographically dual supergravity description.