Universality of second order transport coefficients from the gauge-string duality

TL;DR

The paper explores second-order hydrodynamic transport in strongly coupled, conformal gauge theories with gravity duals and conserved U(1) charges. Using the fluid-gravity framework, it confirms a universal relation among second-order coefficients, 4λ1+λ2=2ητπ, and provides an explicit bulk integral for λ2 in terms of the dual geometry. It also reproduces η via holographic methods, yielding η/s=1/(4π), and discusses the dependence on conformal symmetry, holographic renormalization, and potential extensions beyond conformality. The work highlights a deeper universality in dissipative transport beyond η/s and sets the stage for exploring non-conformal generalizations and comparisons with weak-coupling results.

Abstract

We consider the strongly coupled limit of conformal gauge theory plasmas with conserved U(1) charges which have a gravity dual. We show that, under mild restrictions, the second order transport coefficients of such theories satisfy a universal relation among themselves, similar to the shear viscosity to entropy ratio.