Toward the AdS/CFT Gravity Dual for High Energy Collisions. 3.Gravitationally Collapsing Shell and Quasiequilibrium

TL;DR

This paper models gravitational collapse in AdS space using a falling shell to understand high-energy collision equilibration, revealing a peculiar quasiequilibrium with oscillating spectral densities due to an echo phenomenon.

Contribution

It introduces a simple falling shell model in AdS to study nonequilibrium dynamics and finds a novel quasiequilibrium state with oscillating spectral densities.

Findings

Boundary stress tensor shows equilibrium energy and pressure.

Spectral densities exhibit oscillations due to echo effects.

Model provides insights into holographic thermalization processes.

Abstract

The equilibration of matter and onset of hydrodynamics can be understood in the AdS/CFT context as a gravitational collapse process, in which "collision debris" create a horizon. In this paper we consider the simplest geometry possible, a flat shell (or membrane) falling in the holographic direction toward the horizon. The metric is a combination of two well known solutions: thermal AdS above the shell and pure AdS below, while motion of the shell is given by the Israel junction condition. Furthermore, when the shell motion can be considered slow, we were able to solve for two-point functions of all boundary stress tensor and found that an observer on the boundary sees a very peculiar $quasiequilibrium$: while the average stress tensor $<T_{\mu\nu>}$ contains the equilibrium plasma energy and pressure at all times, the spectral densities of the correlators(related with occupation probabilities of the modes) reveal additional oscillating terms absent in equilibrium. This is explained by the "echo" phenomenon, a partial return of the field coherence at certain "echo" times.