  Preheating after inflation may lead to nonthermal phase transitions with
symmetry restoration. These phase transitions may occur even if the total
energy density of fluctuations produced during reheating is relatively small as
compared with the vacuum energy in the state with restored symmetry. As a
result, in some inflationary models one encounters a secondary, nonthermal
stage of inflation due to symmetry restoration after preheating. We review the
theory of nonthermal phase transitions and make a prediction about the
expansion factor during the secondary inflationary stage. We then present the
results of lattice simulations which verify these predictions, and discuss
possible implications of our results for the theory of formation of topological
defects during nonthermal phase transitions.
