  We investigate the far-from-equilibrium behavior of the Boltzmann equation
for a gas of massless scalar field particles with quartic (tree level)
self-interactions ($\lambda \phi^4$) in Friedmann-Lemaitre-Robertson-Walker
spacetime. Using a new covariant generating function for the moments of the
Boltzmann distribution function, we analytically determine a subset of the
spectrum and the corresponding eigenfunctions of the linearized Boltzmann
collision operator. We show how the covariant generating function can be also
used to find the exact equations for the moments in the full nonlinear regime.
Different than the case of a ultrarelativistic gas of hard spheres (where the
total cross section is constant), for $\lambda \phi^4$ the fact that the cross
section decreases with energy implies that moments of arbitrarily high order
directly couple to low order moments. Numerical solutions for the scalar field
case are presented and compared to those found for a gas of hard spheres.
