On higher order and anisotropic hydrodynamics for Bjorken and Gubser flows

TL;DR

This paper compares anisotropic and third-order Chapman-Enskog hydrodynamics in modeling the evolution of moments in systems undergoing Bjorken and Gubser flows, highlighting anisotropic hydrodynamics' superior accuracy especially for entropy evolution.

Contribution

It demonstrates that anisotropic hydrodynamics more accurately describes the evolution of highly anisotropic systems compared to third-order Chapman-Enskog hydrodynamics, especially for entropy.

Findings

Hydrodynamic moments are well described by both schemes.

Entropy evolution is less accurately captured by hydrodynamics.

Anisotropic hydrodynamics outperforms Chapman-Enskog in accuracy.

Abstract

We study the evolution of hydrodynamic and non-hydrodynamic moments of the distribution function using anisotropic and third-order Chapman-Enskog hydrodynamics for systems undergoing Bjorken and Gubser flows. The hydrodynamic results are compared with the exact solution of the Boltzmann equation with a collision term in relaxation time approximation. While the evolution of the hydrodynamic moments of the distribution function (i.e. of the energy momentum tensor) can be described with high accuracy by both hydrodynamic approximation schemes, their description of the evolution of the entropy of the system is much less precise. We attribute this to large contributions from non-hydrodynamic modes coupling into the entropy evolution which are not well captured by the hydrodynamic approximations. The differences between the exact solution and the hydrodynamic approximations are larger for the third-order Chapman-Enskog hydrodynamics than for anisotropic hydrodynamics, which effectively resums some of the dissipative effects from anisotropic expansion to all orders in the anisotropy, and are larger for Gubser flow than for Bjorken flow. Overall, anisotropic hydrodynamics provides the most precise macroscopic description for these highly anisotropically expanding systems.