Statistical isotropy violation in WMAP CMB maps resulting from non-circular beams

Abstract

Statistical isotropy (SI) of Cosmic Microwave Background (CMB) fluctuations is a key observational test to validate the cosmological principle underlying the standard model of cosmology. While a detection of SI violation would have immense cosmological ramification, it is important to recognise their possible origin in systematic effects of observations. WMAP seven year (WMAP-7) release claimed significant deviation from SI in the bipolar spherical harmonic (BipoSH) coefficients $A_{ll}^{20}$ and $A_{l-2l}^{20}$. Here we present the first explicit reproduction of the measurements reported in WMAP-7, confirming that beam systematics alone can completely account for the measured SI violation. The possibility of such a systematic origin was alluded to in WMAP-7 paper itself and other authors but not as explicitly so as to account for it accurately. We simulate CMB maps using the actual WMAP non-circular beams and scanning strategy. Our estimated BipoSH spectra from these maps match the WMAP-7 results very well. It is also evident that only a very careful and adequately detailed modelling, as carried out here, can conclusively establish that the entire signal arises from non-circular beam effect. This is important since cosmic SI violation signals are expected to be subtle and dismissing a large SI violation signal as observational artefact based on simplistic plausibility arguments run the serious risk of "throwing the baby out with the bathwater".

4 pages, 3 figures. Published version. Includes major revision in the text and one important figure. No change in the results

References in corpus (11)

• Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Temperature Analysis
• Search for TeV Gamma-ray Emission from GRB 100621A, an extremely bright GRB in X-rays, with H.E.S.S
• Cosmic Microwave Background Statistics for a Direction-Dependent Primordial Power Spectrum
• Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Beam Profiles, Data Processing, Radiometer Characterization and Systematic Error Limits
• Asymmetric Beams and CMB Statistical Anisotropy
• Making sky maps from Planck data
• Constraining anisotropic models of the early Universe with WMAP9 data
• CMB power spectrum estimation using noncircular beams
• CMB power spectrum estimation with non-circular beam and incomplete sky coverage
• Orthogonal BipoSH measures : Scrutinizing sources of isotropy violation
• Estimating SI violation in CMB due to non-circular beam and complex scan in minutes

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• Reconstruction of a direction-dependent primordial power spectrum from Planck CMB data
• Isotropy-Violation Diagnostics for $B$-mode Polarization Foregrounds to the Cosmic Microwave Background
• SIToolBox : A package for Bayesian estimation of the isotropy violation in the CMB sky