  In certain models of inflation, the postinflationary reheating of the
Universe is not primarily due to perturbative decay of the inflaton field into
particles, but proceeds through a tachyonic instability. In the process,
long-wavelength modes of an unstable field, which is often distinct from the
inflaton itself, acquire very large occupation numbers, which are subsequently
redistributed into a thermal equilibrium state. We investigate this process
numerically through quantum real-time lattice simulations of the Kadanoff-Baym
equation, using a 1/N-NLO truncation of the 2PI-effective action. We identify
the early-time maximum occupation number, the "classical" momentum range, the
validity of the classical approximation and the effective IR temperature, and
study the kinetic equilibration of the system and the equation of state.
