  In two previous works [arXiv:1009.4363,arXiv:1107.0668], we studied the time
evolution of a system of real scalar fields with quartic coupling which shares
important features with the Color Glass Condensate description of heavy ion
collisions. Our primary objective was to understand how such a system, when
initialized with a non-perturbatively large classical field configuration,
reaches thermal equilibrium. An essential goal of these works was to highlight
the role played by the quantum fluctuations. However, these studies considered
only a system confined within a box of fixed volume. In the present paper, we
extend this work to a system that expands in the longitudinal direction thereby
more closely mimicking a heavy ion collision. We conclude that the microscopic
processes that drive the system towards equilibrium are able to keep up with
the expansion of the system; the pressure tensor becomes isotropic despite the
anisotropic expansion.
