Tensor Non-Gaussianity Search: Current Status and Future Prospects
Primordial gravitational waves (GWs) are said to be a smoking gun of cosmic inflation, while, even if they are detected, the specification of their origins will still be required for establishing the true inflationary model. Testing non-Gaussianity in the tensor-mode anisotropies of the cosmic microwave background (CMB) is one of the most powerful ways to identify sources of GW signals. In this paper, we review studies of the search for tensor non-Gaussianities employing the CMB bispectrum and forecast future developments. No significant signal has so far been found from temperature and $E$-mode polarization data, while orders-of-magnitude improvement in detection limit can be achieved by adding the information of $B$-mode polarization. There is already an established methodology for bispectrum estimation, which encourages a follow-up investigation with next-decadal CMB $B$-mode surveys.