Relic Gravitational Waves: Gravitons, Vacuum And Regularization

The spectrum of relic gravitational wave (RGW) contains high-frequency diver-gences, which should be removed. We present a systematic study of the issue, based on the exact RGW solution that covers the five stages, from inflation to the acceler-ation, each being a power law expansion. We show that the present RGW consists of vacuum dominating at f> 1011Hz and graviton dominating at f<1011Hz, re-spectively. The gravitons are produced by the four cosmic transitions, mostly by the inflation-reheating one. We perform adiabatic regularization to remove vacuum diver-gences in three schemes:at present, at the end of inflation, and at horizon-exit, to the 2-nd adiabatic order for the spectrum, and the 4-th order for energy density and pres-sure. In the first scheme a cutoff is needed to remove graviton divergences. We find that all three schemes yield the spectra of a similar profile, and the primordial spectrum defined far outside horizon during inflation is practically unaffected. We also regularize the gauge-invariant perturbed inflaton and the scalar curvature perturbation by the last two schemes, and find that the scalar spectra, the tensor-scalar ratio, and the consistency relation remain unchanged.