Prospects for direct detection of inflationary gravitational waves by next generation interferometric detectors

We study the potential impact of detecting the inflationary gravitational wave background by the future space-based gravitational wave detectors, such as DECIGO and BBO. The signal-to-noise ratio of each experiment is calculated for chaotic/natural/hybrid inflation models by using the precise predictions of the gravitational wave spectrum based on numerical calculations. We investigate the dependence of each inflation model on the reheating temperature which influences the amplitude and shape of the spectrum, and find that the gravitational waves could be detected for chaotic/natural inflation models with high reheating temperature. From the detection of the gravitational waves, a lower bound on the reheating temperature could be obtained. The implications of this lower bound on the reheating temperature for particle physics are also discussed.

Figure 1

Spectra of the gravitational wave background for different inflation models, shown with the sensitivity curves of DECIGO (dotted) and BBO (solid). The spectra are calculated assuming $T_{\mathrm{RH}}={10}^7\mathrm{GeV}$. The cases of $T_{\mathrm{RH}}={10}^6\mathrm{GeV}$ and ${10}^8\mathrm{GeV}$ are also plotted assuming the quadratic potential model. Note that the spectrum lines mean the time-averaged value of ${\Omega }_{\mathrm{GW}}$.

Figure 2

Signal-to-noise ratio vs reheating temperature calculated for the four different inflation models. The upper panel shows the SNR for DECIGO, and the lower panel is for BBO. The gray region shows the $1\sigma$ uncertainty in the normalization ${\Delta }_{\mathcal{R}}^2=({2.441}_{-0.092}^{+0.088})\times {10}^{-9}$ from WMAP 7 yr, which is used to determine the energy scale of inflation.

Figure 3

Parameter dependence of the signal-to-noise ratio for the natural inflation model.

Figure 4

Parameter dependencies of the signal-to-noise ratio for the hybrid inflation model. The stars show the fiducial point, at which parameters are set as $\lambda =1$, $g=8\times {10}^{-4}$, and $m=2.5\times {10}^{-7}{m}_{\mathrm{Pl}}$. The dotted line in the top panel shows $y=1$. The shaded region means $\sigma \ne 0$ when the observable scale (the present Hubble horizon) exits the horizon.