The prospects for direct measurements of inflationary gravitational waves by next generation interferometric detectors inferred from the possible detection of B-mode polarization of the cosmic microwave background are studied .
We compute the spectra of the gravitational wave background and the signal-to-noise ratios by various types of interferometric detectors ( DECIGO , BBO , and Ultimate DECIGO ) for large-field inflationary models in which the tensor-to-scalar ratio is greater than the order of 0.01 .
If the reheating temperature T _ { RH } of chaotic inflation with the quadratic potential is high ( T _ { RH } > 7.9 \times 10 ^ { 6 } GeV for upgraded DECIGO , T _ { RH } > 1.8 \times 10 ^ { 6 } GeV for BBO , and T _ { RH } > 2.8 \times 10 ^ { 3 } GeV for Ultimate DECIGO ) , it will be possible to reach the sensitivity of the gravitational background in future experiments at 3 \sigma confidence level .
The direct detection is also possible for natural inflation with the potential V ( \phi ) = \Lambda ^ { 4 } [ 1 - \cos ( \phi / f ) ] , provided that f > 4.2 M _ { pl } ( upgraded DECIGO ) and f > 3.6 M _ { pl } ( BBO ) with T _ { RH } higher than 10 ^ { 8 } GeV .
The quartic potential V ( \phi ) = \lambda \phi ^ { 4 } / 4 with a non-minimal coupling \xi between the inflaton field \phi and the Ricci scalar R gives rise to a detectable level of gravitational waves for | \xi| smaller than the order of 0.01 , irrespective of the reheating temperature .