Effect of curvature squared corrections in AdS on the viscosity of the dual gauge theory

TL;DR

The paper investigates curvature-squared corrections to AdS gravity and their impact on the viscosity-to-entropy ratio in the dual gauge theory. Using real-time AdS/CFT and a black brane background, it derives how $R^2$ terms modify the metric, perturbations, and thermodynamics, and computes the resulting $\eta/s$. The key finding is that while $c_1$ and $c_2$ shift $\eta$ and $s$ separately, the ratio is affected only by $c_3$, leading to possible violations of the universal bound in certain large-N, finite-N CFTs via the relation to central charges $a$ and $c$. This provides a framework to explore swampland-like constraints and the behavior of strongly coupled plasmas beyond the classic AdS/CFT setup. Higher-derivative corrections from stringy effects (e.g., $R^4$) are subleading in the relevant regimes they analyze.

Abstract

We use the real-time finite-temperature AdS/CFT correspondence to compute the effect of general R^2 corrections to the gravitational action in AdS space on the shear viscosity of the dual gauge theory. The R^2 terms in AdS_5 are determined by the central charges of the CFT. We present an example of a four-dimensional gauge theory in which the conjectured lower bound of 1/(4π) on the viscosity-to-entropy ratio is violated for finite N.