Testing statistics of the CMB B-mode polarization toward unambiguously establishing quantum fluctuation of vacuum

TL;DR

This paper investigates how analyzing non-Gaussian features in the CMB B-mode polarization can confirm the quantum origin of primordial gravitational waves, distinguishing them from source-field generated signals.

Contribution

It demonstrates that the B-mode bispectrum, especially the 'BBB' component, can detect source-field non-Gaussianity with high significance, aiding in confirming quantum vacuum fluctuations.

Findings

B-mode bispectrum 'BBB' detectable at >3σ significance with LiteBIRD.

Minkowski functional is less sensitive but useful for cross-checks.

Analysis accounts for foregrounds and lensing effects.

Abstract

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.