Nonlinear evolution of density and flow perturbations on a Bjorken background

TL;DR

This paper investigates the nonlinear evolution of density and flow perturbations in a longitudinally expanding medium post-heavy-ion collision, highlighting the generation of bispectrum and the impact of initial conditions on nonlinear effects.

Contribution

It provides a detailed analysis of the spectrum and bispectrum evolution, including nonlinear effects, in a Bjorken background relevant to heavy-ion collisions.

Findings

Nonlinear terms generate a non-trivial bispectrum over time.

Smoother initial conditions lead to smaller nonlinear effects.

Linear evolution dominates when initial conditions are sufficiently smooth.

Abstract

Density perturbations and their dynamic evolution from early to late times can be used for an improved understanding of interesting physical phenomena both in cosmology and in the context of heavy-ion collisions. We discuss the spectrum and bispectrum of these perturbations around a longitudinally expanding fireball after a heavy-ion collision. The time-evolution equations couple the spectrum and bispectrum to each other, as well as to higher-order correlation functions through nonlinear terms. A non-trivial bispectrum is thus always generated, even if absent initially. For initial conditions corresponding to a model of independent sources, we discuss the linear and nonlinear evolution is detail. We show that, if the initial conditions are sufficiently smooth for fluid dynamics to be applicable, the nonlinear effects are relatively small.