Universality of energy-momentum response in kinetic theories

TL;DR

This paper demonstrates a universal behavior in the energy-momentum response functions across various kinetic theories, including QCD, highlighting the dominance of specific modes even outside hydrodynamic regimes.

Contribution

It reveals a universal pattern in energy-momentum responses in kinetic theories, characterized by a limited set of modes, regardless of microscopic differences.

Findings

Response functions are universal across theories.

Scalar perturbations are described by one hydrodynamic and one non-hydrodynamic mode.

Single-mode description remains effective beyond hydrodynamic validity.

Abstract

We study the response of the energy-momentum tensor in several kinetic theories, from the simple relaxation time approximation (RTA) to Quantum Chromodynamics (QCD). Irrespective of the differences in microscopic properties, we find a remarkable degree of universality in the response functions from conformal theories. We find that the response to scalar perturbations in kinetic theory can be effectively described by a pair of one hydrodynamic sound mode and one non-hydrodynamic mode. We find that even beyond the range of validity of hydrodynamics, the energy-momentum response in position space can be effectively described by one single mode with non-trivial dispersion relation and residue.