  We consider the nonlinear dynamics of inflaton fragmentation during and after
preheating in the simplest model of chaotic inflation. While the earlier regime
of parametric resonant particle production and the later turbulent regime of
interacting fields evolving towards equilibrium are well identified and
understood, the short intermediate stage of violent nonlinear dynamics remains
less explored. Lattice simulations of fully nonlinear preheating dynamics show
specific features of this intermediate stage: occupation numbers of the scalar
particles are peaked, scalar fields become significantly non-gaussian and the
field dynamics become chaotic and irreversible. Visualization of the field
dynamics in configuration space reveals that nonlinear interactions generate
non-gaussian inflaton inhomogeneities with very fast growing amplitudes. The
peaks of the inflaton inhomogeneities coincide with the peaks of the scalar
field(s) produced by parametric resonance. When the inflaton peaks reach their
maxima, they stop growing and begin to expand. The subsequent dynamics is
determined by expansion and superposition of the scalar waves originating from
the peaks. Multiple wave superposition results in phase mixing and turbulent
wave dynamics. Thus, the short intermediate stage is defined by the formation,
expansion and collision of bubble-like field inhomogeneities associated with
the peaks of the original gaussian field. This process is qualitatively similar
to the bubble-like inflaton fragmentation that occurs during tachyonic
preheating after hybrid or new inflation.
