  The dissipative dynamics of an expanding massless gas with constant cross
section in a spatially flat Friedmann-Lema\^itre-Robertson-Walker (FLRW)
universe is studied. The mathematical problem of solving the full nonlinear
relativistic Boltzmann equation is recast into an infinite set of nonlinear
ordinary differential equations for the moments of the one-particle
distribution function. Momentum-space resolution is determined by the number of
non-hydrodynamic modes included in the moment hierarchy, i.e., by the
truncation order. We show that in the FLRW spacetime the non-hydrodynamic modes
decouple completely from the hydrodynamic degrees of freedom. This results in
the system flowing as an ideal fluid while at the same time producing entropy.
The solutions to the nonlinear Boltzmann equation exhibit transient tails of
the distribution function with nontrivial momentum dependence. The evolution of
this tail is not correctly captured by the relaxation time approximation nor by
the linearized Boltzmann equation. However, the latter probes additional
high-momentum details unresolved by the relaxation time approximation. While
the expansion of the FLRW spacetime is slow enough for the system to move
towards (and not away from) local thermal equilibrium, it is not sufficiently
slow for the system to actually ever reach complete local equilibrium.
Equilibration is fastest in the relaxation time approximation, followed, in
turn, by kinetic evolution with a linearized and a fully nonlinear Boltzmann
collision term.
