Viscosity Bound Violation in Higher Derivative Gravity

TL;DR

This paper demonstrates that in certain higher derivative gravity theories, specifically Gauss-Bonnet gravity, the shear viscosity to entropy density ratio can be tuned to violate the previously conjectured universal bound, with consistent calculations across multiple methods.

Contribution

It shows that the viscosity bound can be violated in conformal field theories dual to Gauss-Bonnet gravity, providing a nonperturbative computation and verifying multiple calculation methods.

Findings

Viscosity ratio can be adjusted from infinity to zero.

All four calculation methods agree at linear order.

No pathologies found in theories violating the bound.

Abstract

Motivated by the vast string landscape, we consider the shear viscosity to entropy density ratio in conformal field theories dual to Einstein gravity with curvature square corrections. After field redefinitions these theories reduce to Gauss-Bonnet gravity, which has special properties that allow us to compute the shear viscosity nonperturbatively in the Gauss-Bonnet coupling. By tuning of the coupling, the value of the shear viscosity to entropy density ratio can be adjusted to any positive value from infinity down to zero, thus violating the conjectured viscosity bound. At linear order in the coupling, we also check consistency of four different methods to calculate the shear viscosity, and we find that all of them agree. We search for possible pathologies associated with this class of theories violating the viscosity bound.