Oscillatory amplitude of stochastic gravitational wave spectrum

Primordial gravitational waves generated from early universe are placed in the squeezed vacuum state and the resulting stochastic background is studied for various models of the expanding universe. The quantum effect on the stochastic gravitational waves leads to overall enhancement of the amplitude and spectral energy density when compared to those in the absence of squeezing effect with continued increase in the amplitude in the accelerating stage and oscillatory behavior at higher frequency range of the spectrum in the accelerating universe. Through the quantum effect, it is also found that the reheating phenomenon affects the entire spectrum. The results of the present study may be useful to test the possibility of detection of the stochastic gravitational waves by current and future gravitational wave detectors and whether these waves exist in the squeezed vacuum state.