The Evens and Odds of CMB Anomalies

TL;DR

This paper analyzes large-scale anomalies in the CMB, revealing a robust suppression of even multipoles linked to a pre-inflationary scale, with implications for future polarization surveys.

Contribution

It identifies the dominance of even multipole suppression at large scales and demonstrates how polarization data can enhance detection of the pre-inflationary scale 4444.

Findings

Suppression of even 4444; robust against Galactic masking.

Detection of pre-inflationary scale 4444 with about 344 confidence.

Future polarization surveys could increase detection significance beyond 544.

Abstract

The lack of power of large--angle CMB anisotropies is known to increase its statistical significance at higher Galactic latitudes, where a string--inspired pre--inflationary scale $\Delta$ can also be detected. Considering the Planck 2015 data, and relying largely on a Bayesian approach, we show that the effect is mostly driven by the \emph{even}--$\ell$ harmonic multipoles with $\ell \lesssim 20$, which appear sizably suppressed in a way that is robust with respect to Galactic masking, along with the corresponding detections of $\Delta$. On the other hand, the first \emph{odd}--$\ell$ multipoles are only suppressed at high Galactic latitudes. We investigate this behavior in different sky masks, constraining $\Delta$ through even and odd multipoles, and we elaborate on possible implications. We include low--$\ell$ polarization data which, despite being noise--limited, help in attaining confidence levels of about 3 $\sigma$ in the detection of $\Delta$. We also show by direct forecasts that a future all--sky $E$--mode cosmic--variance--limited polarization survey may push the constraining power for $\Delta$ beyond 5 $\sigma$.