Hints of tensions in the cosmic microwave background temperature and polarization quadrupoles

Abstract

The large-angular-scale falloff in the autocorrelation function for the cosmic microwave background (CMB) temperature has long intrigued cosmologists and fueled speculation about suppressed superhorizon power. Here we highlight an inconsistency between the temperature quadrupole and the more recently obtained E-mode polarization quadrupole from Planck PR3. The temperature quadrupole arises primarily at the CMB surface of last scatter, while the polarization primarily from the epoch of reionization, but the two still probe comparable distance scales. Although the temperature quadrupole is intriguingly low (much greater than a $1\sigma$ fluctuation) compared with that expected in the standard $\Lambda$CDM cosmological model, the polarization quadrupole turns out to be somewhat high, at the $1\sigma$ level. We calculate the joint probability distribution function for both and find a slight tension: the observed pair of quadrupoles is inconsistent at a $2.3\sigma$ confidence level. The problem is robust to simple changes to the cosmological model. If the high polarization quadrupole survives further scrutiny, then this result disfavors, at comparable significance, new superhorizon physics. The full-sky coverage and pristine foreground subtraction of the LiteBIRD satellite will be ideal to help resolve this question.