Chasing super-horizon correlations. The promise of LiteBIRD polarization measurements

It is well known that, since Cosmic Microwave Background (CMB) polarization originates at the surface of last scattering, scenarios in which perturbations are generated causally within the horizon cannot produce correlations at angular scales larger than about 2 degrees. In contrast, inflationary models predict a distinctive peak in the correlation functions at these large angular scales. By comparing the null hypothesis (no TE signal at large scales) with the ΛCDM prediction, we show that while Planck already provides strong statistical evidence for super-horizon perturbations, a future CMB satellite experiment as LiteBIRD will further reinforce this conclusion by improving current constraints by more than a factor of 3. We then consider an alternative explanation for the observed suppression of large-scale polarization: a low-k cutoff in the primordial power spectrum. We forecast the sensitivity of LiteBIRD to this cutoff scale, kc. In particular, we find that LiteBIRD will improve the constraint on kc by a factor of 1.4, outperforming Planck. These results highlight the critical role of large-scale polarization in establishing the presence of super-horizon correlations and offer new avenues to probe deviations from a scale-invariant primordial spectrum.