  The semiclassical approximation has frequently been used to describe the
initial stage of particle production, often called preheating, which occurs
after the inflationary epoch in chaotic models of inflation. During this phase
backreaction effects from the produced particles on the inflaton field are
significant, and one might be concerned about the validity of the semiclassical
approximation, even though large backreaction effects are allowed if the
inflaton field is coupled to a large number of quantum fields. A criterion is
presented for the validity of the semiclassical approximation in this case and
the question of whether this criterion is satisfied during preheating is
addressed.
