Cosmological backreaction of a quantized massless scalar field
  We consider the backreaction problem of a quantized minimally coupled
massless scalar field in cosmology. The adiabatically regularized stress-energy
tensor in a general Friedmann-Robertson-Walker background is approximately
evaluated by using the fact that subhorizon modes evolve adiabatically and
superhorizon modes are frozen. The vacuum energy density is verified to obey a
new first order differential equation depending on a dimensionless parameter of
order unity, which calibrates subhorizon/superhorizon division. We check the
validity of the approximation by calculating the corresponding vacuum energy
densities in fixed backgrounds, which are shown to agree with the known results
in de Sitter space and space-times undergoing power law expansions. We then
apply our findings to slow-roll inflationary models. Although backreaction
effects are found to be negligible during the near exponential expansion, the
vacuum energy density generated during this period might be important at later
stages since it decreases slower than radiation or dust.
