Shear viscosity of boost invariant plasma at finite coupling

TL;DR

This work studies string-theory $\alpha'$ corrections to the holographic dual of the expanding ${\cal N}=4$ SYM plasma in Bjorken flow and reports a finite-$\lambda$ correction to the shear viscosity to entropy density ratio $\\eta/s$. Using an $\alpha'$-corrected supergravity background and a late-time, scaling-variable expansion, the authors derive the corrected equations of motion, perform consistency checks via curvature invariants, and map the results to Muller-Israel-Stewart hydrodynamics. They find $\\eta/s=\\frac{1}{4\\pi}(1+120\\gamma)$ with $\\gamma$ encoding the finite coupling correction, but this result disagrees with corresponding equilibrium two-point function computations, prompting discussion of possible sources such as missing RR flux corrections or an incomplete effective action. The paper highlights the subtleties in extending holographic non-equilibrium frameworks beyond the supergravity limit and suggests avenues for resolving the mismatch between non-equilibrium and equilibrium transport predictions.

Abstract

We discuss string theory alpha' corrections in the dual description of the expanding boost invariant N=4 supersymmetric Yang-Mills plasma at strong coupling. We compute finite 't Hooft coupling corrections to the shear viscosity and find that it disagrees with the equilibrium correlation function computations. We comment on the possible source of the discrepancy.