Anisotropic time evolution of sound modes in Bjorken expanding holographic plasma

TL;DR

This study investigates how sound waves evolve in an anisotropically expanding quark-gluon plasma using holographic methods, revealing distinct sound speeds and providing insights for heavy-ion collision data analysis.

Contribution

It introduces a numerical analysis of sound mode evolution in anisotropic plasma during Bjorken expansion within holography, highlighting anisotropic effects on sound speeds and attenuation.

Findings

Two distinct sound speeds due to anisotropy

Sound speeds vary from below conformal value to light speed

Anisotropic hydrodynamics effectively describes the system

Abstract

The speed of sound is a key parameter for characterizing equilibrium states. However, sound waves change their properties when propagating through rapidly evolving anisotropic media, such as the quark-gluon plasma created in heavy-ion collisions. This paper uses $\mathcal{N}=4$ Super-Yang-Mills theory to numerically study the time evolution of the speed and attenuation of sound modes along with the relaxation time in a plasma undergoing Bjorken expansion from various initial states in a quasi-static approximation. The longitudinal Bjorken expansion breaks the isotropy, resulting in two distinct sound speeds that range from just below the conformal value to the speed of light. An anisotropic hydrodynamic description is constructed and its applicability is discussed. Implications for the analysis of heavy ion data are considered.