CMB-S4 and the Hemispherical Variance Anomaly

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.

arXiv comment7 pages, 3 figures

References in corpus (17)

• HEALPix -- a Framework for High Resolution Discretization, and Fast Analysis of Data Distributed on the Sphere
• Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Temperature Analysis
• Asymmetries in the CMB anisotropy field
• Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies?
• Planck 2013 results. XXIII. Isotropy and statistics of the CMB
• The Microwave Anisotropy Probe (MAP) Mission
• Planck 2018 results. VII. Isotropy and Statistics of the CMB
• Testing the cosmological principle of isotropy: local power spectrum estimates of the WMAP data
• Planck 2015 results. IX. Diffuse component separation: CMB maps
• Large-angle anomalies in the CMB
• Power asymmetry in WMAP and Planck temperature sky maps as measured by a local variance estimator
• Mapping the large-scale anisotropy in the WMAP data
• A low CMB variance in the WMAP data
• On the significance of power asymmetries in Planck CMB data at all scales
• Microwave Background Correlations from Dipole Anisotropy Modulation
• Anomalous variance in the WMAP data and Galactic Foreground residuals
• Large-Angle CMB Suppression and Polarization Predictions

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• Testing cosmological models with large-scale power modulation using microwave background polarization observations
• The low level of correlation observed in the CMB sky at large angular scales and the low quadrupole variance