Derivation of relativistic hydrodynamic equations consistent with relativistic Boltzmann equation by renormalization-group method

TL;DR

This paper applies the renormalization-group method to derive relativistic hydrodynamic equations from the Boltzmann equation, establishing the energy frame as fundamental and providing new insights into transport coefficients and relaxation parameters.

Contribution

It presents a novel derivation of relativistic hydrodynamics using the renormalization-group method, clarifying the frame choice and extending the invariant manifold approach.

Findings

Derivation of first-order relativistic hydrodynamics consistent with the Boltzmann equation.

Explicit form of the distribution function as the invariant manifold.

New expressions for relaxation times and lengths in relativistic hydrodynamics.

Abstract

We review our work on the application of the renormalization-group method to obtain first- and second-order relativistic hydrodynamics of the relativistic Boltzmann equation (RBE) as a dynamical system, with some corrections and new unpublished results. For the first-order equation, we explicitly obtain the distribution function in the asymptotic regime as the invariant manifold of the dynamical system, which turns out to be nothing but the matching condition defining the energy frame, i.e., the Landau-Lifshitz one. It is argued that the frame on which the flow of the relativistic hydrodynamic equation is defined must be the energy frame, if the dynamics should be consistent with the underlying RBE. A sketch is also given for derivation of the second-order hydrodynamic equation, i.e., extended thermodynamics, which is accomplished by extending the invariant manifold so that it is spanned by excited modes as well as the zero modes (hydrodynamic modes) of the linearized collision operator. On the basis of thus constructed resummed distribution function, we propose a novel ansatz for the functional form to be used in Grad moment method; it is shown that our theory gives the same expressions for the transport coefficients as those given in the Chapman-Enskog theory as well as the novel expressions for the relaxation times and lengths allowing natural interpretation.