Resolving disagreement for [formula omitted] in a CFT plasma at finite coupling

Abstract

The ratio of shear viscosity to entropy density in a strongly coupled CFT plasma can be computed using the AdS/CFT correspondence either from equilibrium correlation functions or from the Janik–Peschanski dual of the boost invariant plasma expansion. We point out that the previously found disagreement for η / s at finite 't Hooft coupling is resolved once the incoming-wave boundary condition for metric fluctuations at the horizon of the dual geometry is properly imposed.