Cross-correlations as a Diagnostic Tool for Primordial Gravitational Waves

Abstract: We explore and corroborate, by working out explicit examples, the effectiveness of cross-correlating stochastic gravitational wave background anisotropies with CMB temperature fluctuations as a way to establish the primordial nature of a given gravitational wave signal. We consider the case of gravitational wave anisotropies induced by scalar-tensor-tensor primordial non-Gaussianity. Our analysis spans anisotropies exhibiting different angular behaviours, including a quadrupolar dependence. We calculate the expected uncertainty on the non-linearity parameter $F_{\rm NL}$ obtained as a result of cross-correlation measurements for several proposed experiments such as the ground-based Einstein Telescope, Cosmic Explorer, and the space-based Big-Bang Observer. As a benchmark for future survey planning, we also calculate the theoretical, cosmic-variance-limited, error on the non-linearity parameter.