On the supergravity description of boost invariant conformal plasma at strong coupling

TL;DR

<3-5 sentence high-level summary>This paper investigates holographic descriptions of strongly coupled, boost-invariant conformal plasmas using late-time expansions of gravity duals. By performing complete (N=4) and Klebanov-Witten KK reductions, the authors show that non-singular backgrounds up to third order fix the equilibrium equation of state, shear viscosity, and a relaxation time that matches MIS-type hydrodynamics, but a residual logarithmic curvature singularity persists at third order that cannot be canceled within a finite supergravity field content. They demonstrate that while certain higher-curvature invariants can be made finite by introducing additional massive modes, an infinite tower of fields would be required to cancel all logs, implying an intrinsic limitation of the supergravity approximation for these dynamical flows. The results hint at a possible universality of the relaxation time and raise questions about the feasibility of a fully non-singular string-theory dual for time-dependent, boost-invariant conformal plasmas, with implications for modeling QGP dynamics.

Abstract

We study string theory duals of the expanding boost invariant conformal gauge theory plasmas at strong coupling. The dual supergravity background is constructed as an asymptotic late-time expansion, corresponding to equilibration of the gauge theory plasma. The absence of curvature singularities in the first few orders of the late-time expansion of the dual gravitational background unambiguously determines the equilibrium equation of the state, and the shear viscosity of the gauge theory plasma. While the absence of the leading pole singularities in the gravitational curvature invariants at the third order in late-time expansion determines the relaxation time of the plasma, the subleading logarithmic singularity can not be canceled within a supergravity approximation. Thus, a supergravity approximation to a dual description of the strongly coupled boost invariant expanding plasma is inconsistent. Nevertheless we find that the relaxation time determined from cancellation of pole singularities is quite robust.