On SUGRA description of boost-invariant conformal plasma at strong coupling

TL;DR

The paper constructs a holographic description of boost-invariant conformal plasmas at strong coupling via a late-time, non-singular supergravity expansion. By enforcing bulk nonsingularity, it reproduces the equilibrium equation of state and the shear-viscosity to entropy-density ratio η/s = 1/(4π), along with a Müller-Israel-Stewart–type relaxation structure for the expanding plasma. However, a logarithmic curvature singularity at third subleading order cannot be canceled within a finite supergravity truncation, implying that an infinite tower of bulk fields (i.e., an infinite set of gauge-invariant operator VEVs) would be required to fully capture the non-equilibrium evolution, suggesting that full stringy dynamics may be necessary. The work discusses interpretations of these singularities, including classifications of truncations (A,B,C) and the potential universality of relaxation times, and points toward non-conformal generalizations and future non-equilibrium holography research.

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. We comment on the possible interpretations of this singularity.