Validity of the semiclassical approximation during the preheating phase   of chaotic inflation
  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. 
Paul R. Anderson, Carmen Molina-Paris, Dillon H. Sanders
gr-qc hep-th