Cosmic microwave background and parametric resonance in reheating

The variation of the perturbative 3-curvature parameter zeta is investigated in the period of reheating after inflation. The two-field model used has the inflaton, with an extra scalar field coupled to it, and the nonlinear effects of both fields are included as well as a slow decay mechanism into the hydrodynamic fluid of the radiation era. Changes in zeta occur and persist into the succeeding cosmic eras to influence the generation of the cosmic microwave background fluctuations. With appropriate coupling, the extra field rises from almost zero at the end of inflation to parametric resonance, which is important in giving ultimate changes to zeta. Inflaton particles are always produced in important quantities and their interaction with the extra field may limit its parametric resonance. When the field coupling is such that parametric resonance is small or absent, then the nonlinear effects of the produced inflaton particles by themselves also give ultimate changes in zeta, though these are small.