Quantum Fluctuations and Particle Production of Coherently Oscillating Inflaton

We study a massive inflaton minimally coupled to the FRW Universe using the semiclassical gravity derived from canonical quantum gravity. It is found that the semiclassical quantum gravity leads to the power-law expansion $t^{2/3}$ in an oscillatory phase of the inflaton. The particle production of the inflaton in the expanding Universe restricts the duration of stable coherent oscillations and parametric resonance. The semiclassical gravity shows a significant difference that the Hubble constant does not oscillate in contrast with the oscillatory behavior in classical gravity.