Effects of a preinflation radiation-dominated epoch to CMB anisotropy

We consider that the preinflation era is radiation dominated, transiting smoothly to the inflationary era. We work out in detail the dynamics of inflaton fluctuations across the phase transition and the proper choices of initial vacuum states. It is found that this phase transition can suppress long-wavelength quantum fluctuations of inflaton. This may attribute to the large-scale CMB anisotropy a lower power than predicted in the standard Lambda CDM model. In constraining this transitional effect by WMAP anisotropy data, we use the WMAP best-fit scale-invariant Lambda CDM model with the density power spectrum replaced by the one found in this work. We find that the transition occurs at least about 10 e-folds before the comoving scales comparable to the size of our present horizon cross the Hubble radius during inflation.