Hemispherical Variance Anomaly and Reionization Optical Depth

TL;DR

This paper investigates the hemispherical variance anomaly in CMB temperature data and explores how polarization variance, sensitive to reionization optical depth, can serve as a future test for this anomaly with upcoming high-precision measurements.

Contribution

It introduces a forecast of polarization variance dependence on cosmological parameters, especially reionization optical depth, and discusses how improved tau measurements can test the temperature anomaly.

Findings

Polarization variance is sensitive to current uncertainties in cosmological parameters.

Improved measurements of tau can tighten polarization variance expectations.

Polarization variance can serve as a future test for the hemispherical anomaly.

Abstract

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, with the north displaying an anomalously low variance while the south appears consistent with expectations from the best-fitting theory, LCDM. While this is a well-established result in temperature, the low signal-to-noise ratio in current polarization data prevents a similar comparison. Even though temperature and polarization are correlated, polarization realizations constrained by temperature data show that the lack of variance is not expected to be present in polarization data. Therefore, a natural way of testing whether the temperature result is a fluke is to measure the variance of CMB polarization components. In anticipation of future CMB experiments that will allow for high-precision large-scale polarization measurements, we study how variance of polarization depends on LCDM parameters' uncertainties by forecasting polarization maps with Planck's MCMC chains. We find that, unlike temperature variance, polarization variance is noticeably sensitive to present uncertainties in cosmological parameters. This comes mainly from the current poor constraints on the reionization optical depth, tau, and the fact that tau drives variance at low multipoles. In this work we show how the variance of polarization maps generically depends on the cosmological parameters. We demonstrate how the improvement in the tau measurement seen between Planck's two latest data releases results in a tighter constraint on polarization variance expectations. Finally, we consider even smaller uncertainties on tau and how more precise measurements of tau can drive the expectation for polarization variance in a hemisphere close to that of the cosmic-variance-limited distribution.