Universality of sound modes in kinetic theory

TL;DR

This paper demonstrates that sound modes exhibit a universal behavior across various kinetic theories, extending beyond traditional hydrodynamic regimes, by analyzing response functions of the energy-momentum tensor.

Contribution

It introduces a simple method to extract and compare sound and non-hydrodynamic modes in different kinetic theories, revealing their universal characteristics.

Findings

Sound modes are remarkably universal across different kinetic theories.

The approach effectively distinguishes hydrodynamic and non-hydrodynamic modes.

Universality persists even beyond the hydrodynamic regime.

Abstract

We present a simple approach to extract hydrodynamic sound modes and non-hydrodynamic modes in kinetic theories from response functions of the energy-momentum tensor. By comparing the response functions in four types of kinetic theories, namely the Relaxation-Time Approximation, scalar $\phi^4$ theory, SU(3) Yang-Mills theory and QCD kinetic theory, we find a remarkable degree of universality for the sound mode, even beyond the hydrodynamic regime.