Resolving disagreement for eta/s in a CFT plasma at finite coupling

TL;DR

This paper resolves a discrepancy in calculating the shear viscosity to entropy density ratio in a strongly coupled CFT plasma at finite coupling by emphasizing the importance of boundary conditions in the dual gravity description.

Contribution

It demonstrates that correctly imposing the incoming-wave boundary condition at the horizon resolves previous disagreements in eta/s calculations.

Findings

Disagreement in eta/s at finite coupling is resolved.

Proper boundary conditions are crucial for accurate holographic computations.

The corrected approach aligns different methods of calculation.

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 eta/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.