CMB-S4 and the Hemispherical Variance Anomaly

TL;DR

This paper investigates the hemispherical variance anomaly in the CMB, predicting polarization map variances for CMB-S4 and discussing how these measurements can test the anomaly's nature.

Contribution

It introduces polarization variance predictions for CMB-S4 under different sky coverage scenarios, providing a method to test if temperature anomalies are statistical flukes.

Findings

Polarization variance in the northern hemisphere is unlikely to be low if the temperature anomaly is a fluke.

Full northern sky coverage yields more reliable variance measurements than partial coverage.

A low polarization variance would challenge the hypothesis that the temperature anomaly is a statistical fluctuation.

Abstract

Cosmic Microwave Background (CMB) full-sky temperature data show a hemispherical asymmetry in power nearly aligned with the Ecliptic. In real space, this anomaly can be quantified by the temperature variance in the northern and southern Ecliptic hemispheres. In this context, the northern hemisphere displays an anomalously low variance while the southern hemisphere appears unremarkable (consistent with expectations from the best-fitting theory, $\Lambda$CDM). While this is a well established result in temperature, the low signal-to-noise ratio in current polarization data prevents a similar comparison. This will change with a proposed ground-based CMB experiment, CMB-S4. With that in mind, we generate realizations of polarization maps constrained by the temperature data and predict the distribution of the hemispherical variance in polarization considering two different sky coverage scenarios possible in CMB-S4: full Ecliptic north coverage and just the portion of the North that can be observed from a ground based telescope at the high Chilean Atacama plateau. We find that even in the set of realizations constrained by the temperature data, the low northern hemisphere variance observed in temperature is not expected in polarization. Therefore, an anomalously low variance detection in polarization would provide strong evidence against the hypothesis that the temperature anomaly is simply a statistical fluke. We show, within $\Lambda$CDM, how variance measurements in both sky coverage scenarios are related. We find that the variance makes for a good statistic in cases where the sky coverage is limited, however a full northern coverage is still preferable.