Hydrodynamic dispersion relations at finite coupling

TL;DR

This paper investigates how the convergence radii of hydrodynamic dispersion relations in strongly coupled gauge theories vary with finite coupling, revealing non-perturbative effects and piecewise continuous behavior through holographic methods.

Contribution

It extends previous infinite coupling analyses to finite coupling regimes, using non-perturbative resummation and Einstein-Gauss-Bonnet gravity models to study convergence radii behavior.

Findings

Radii grow with inverse coupling initially

Piecewise continuous dependence on coupling

Decreases in radii observed with non-perturbative methods

Abstract

By using holographic methods, the radii of convergence of the hydrodynamic shear and sound dispersion relations were previously computed in the ${\cal N} = 4$ supersymmetric Yang-Mills theory at infinite 't Hooft coupling and infinite number of colours. Here, we extend this analysis to the domain of large but finite 't Hooft coupling. To leading order in the perturbative expansion, we find that the radii grow with increasing inverse coupling, contrary to naive expectations. However, when the equations of motion are solved using a qualitative non-perturbative resummation, the dependence on the coupling becomes piecewise continuous and the initial growth is followed by a decrease. The piecewise nature of the dependence is related to the dynamics of branch point singularities of the energy-momentum tensor finite-temperature two-point functions in the complex plane of spatial momentum squared. We repeat the study using the Einstein-Gauss-Bonnet gravity as a model where the equations can be solved fully non-perturbatively, and find the expected decrease of the radii of convergence with the effective inverse coupling which is also piecewise continuous. Finally, we provide arguments in favour of the non-perturbative approach and show that the presence of non-perturbative modes in the quasinormal spectrum can be indirectly inferred from the analysis of perturbative critical points.