An exact analytic spectrum of relic gravitational waves in an accelerating universe

An exact analytic calculation is presented for the spectrum of relic gravitational waves in the scenario of accelerating Universe $\Omega_{\Lambda}+\Omega_m = 1$. The spectrum formula contains explicitly the parameters of acceleration, inflation, reheating, and the (tensor/scalar) ratio, so that it can be employed for a variety of cosmological models. We find that the spectrum depends on the behavior of the present accelerating expansion. The amplitude of gravitational waves for the model $\Omega_{\Lambda}=0.65$ is about $\sim 50 %$ greater than that of the model $\Omega_{\Lambda}=0.7$, an effect accessible to the designed sensitivities of LIGO and LISA. The spectrum sensitively depends on the inflationary models with $a(\tau) \propto |\tau|^{1+\beta}$, and a larger $\beta$ yields a flatter spectrum, producing more power. The current LIGO results rule out the inflationary models of $\beta \geq -1.8$. The LIGO with its design sensitivity and the LISA will also be able to test the model of $\beta=-1.9$. We also examine the constraints on the spectral energy density of relic gravitational waves. Both the LIGO bound and the nucleosynthesis bound point out that the model $\beta=-1.8$ is ruled out, but the model $\beta=-2.0$ is still alive. The exact analytic results also confirm the approximate spectrum and the numerical one in our previous work.