Instability induced pressure isotropization in a longitudinally expanding system

TL;DR

This paper investigates how a longitudinally expanding scalar field system evolves towards isotropic pressure, demonstrating that microscopic processes can maintain isotropy despite anisotropic expansion, akin to conditions in heavy ion collisions.

Contribution

It extends previous fixed-volume studies to an expanding system, showing that pressure isotropization occurs despite the system's longitudinal expansion.

Findings

Pressure tensor becomes isotropic despite expansion

Microscopic processes sustain equilibrium during expansion

System mimics conditions of heavy ion collisions

Abstract

In two previous works [arXiv:1009.4363,arXiv:1107.0668], we studied the time evolution of a system of real scalar fields with quartic coupling which shares important features with the Color Glass Condensate description of heavy ion collisions. Our primary objective was to understand how such a system, when initialized with a non-perturbatively large classical field configuration, reaches thermal equilibrium. An essential goal of these works was to highlight the role played by the quantum fluctuations. However, these studies considered only a system confined within a box of fixed volume. In the present paper, we extend this work to a system that expands in the longitudinal direction thereby more closely mimicking a heavy ion collision. We conclude that the microscopic processes that drive the system towards equilibrium are able to keep up with the expansion of the system; the pressure tensor becomes isotropic despite the anisotropic expansion.