  Systems that exhibit pattern formation are typically driven and dissipative.
In the early universe, parametric resonance can drive explosive particle
production called preheating. The fields that are populated then decay quantum
mechanically if their particles are unstable. Thus, during preheating, a
driven-dissipative system exists. We have shown previously that pattern
formation can occur in two dimensions in a self-coupled inflaton system
undergoing parametric resonance. In this paper, we provide evidence of pattern
formation for more realistic initial conditions in both two and three
dimensions. In the one-field case, we have the novel interpretation that these
patterns can be thought of as a network of domain walls. We also show that the
patterns are spatio-temporal, leading to a distinctive, but probably
low-amplitude peak in the gravitational wave spectrum. In the context of a
two-field model, we discuss putting power from resonance into patterns on
cosmological scales, in particular to explain the observed excess power at 100
h^{-1}Mpc, but why this seems unlikely in the absence of a period of
post-preheating inflation. Finally we note our model is similar to that of the
decay of DCCs and therefore pattern formation may also occur at RHIC and LHC.
