Testing statistics of the CMB B-mode polarization toward unambiguously
  establishing quantum fluctuation of vacuum
  The B-mode polarization in the cosmic microwave background (CMB) anisotropies
at large angular scales provides a smoking-gun evidence for the primordial
gravitational waves (GWs). It is often stated that a discovery of the GWs
establishes the quantum fluctuation of vacuum during the cosmic inflation.
Since the GWs could also be generated by source fields, however, we need to
check if a sizable signal exists due to such source fields before reaching a
firm conclusion when the B-mode is discovered. Source fields of particular
types can generate non-Gaussianity (NG) in the GWs. Testing statistics of the
B-mode is a powerful way of detecting such NG. As a concrete example, we show a
model in which a gauge field sources chiral GWs via a pseudoscalar coupling,
and forecast the detection significance at the future CMB satellite LiteBIRD.
Effects of residual foregrounds and lensing B-mode are both taken into account.
We find the B-mode bispectrum "BBB" is in particular sensitive to the
source-field NG, which is detectable at LiteBIRD with a $> 3 \sigma$
significance. Therefore the search for the "BBB" will be indispensable toward
unambiguously establishing quantum fluctuation of vacuum when the B-mode is
discovered. We also introduced the Minkowski functional to detect the NGs.
While we find that the Minkowski functional is less efficient than the
harmonic-space bispectrum estimator, it still serves as a useful cross check.
Finally, we also discuss the possibility of extracting clean information on
parity violation of GWs, and new types of parity-violating observables induced
by lensing.
