Shear viscosity of CFT plasma at finite coupling

TL;DR

The paper investigates whether the shear viscosity to entropy density ratio $\eta/s$ remains universal for conformal gauge theory plasmas at finite 't Hooft coupling. By comparing Klebanov–Witten and N=4 SYM plasmas with the same holographic string tension, it shows that their full shear quasinormal-mode spectra coincide at order $\lambda^{-3/2}$, implying identical finite-coupling corrections to $\eta/s$. It also demonstrates that naive lower-dimensional effective actions can misrepresent the spectrum, underscoring the importance of proper Kaluza-Klein reductions. The results support a conjectured universality of finite-coupling transport in holographic CFTs, with caveats and directions for rigorous verification via KK reductions and boost-invariant analyses.

Abstract

We present evidence for the universality of the shear viscosity of conformal gauge theory plasmas beyond infinite coupling. We comment of subtleties of computing the shear viscosity in effective models of gauge/gravity correspondence rather than in string theory.