WMAP anomaly : Weak lensing in disguise

TL;DR

This paper shows that gravitational lensing effects can mimic violations of statistical isotropy in the CMB, explaining anomalies detected by WMAP and emphasizing the need for combined data sets to accurately test cosmological principles.

Contribution

It demonstrates that lensing-induced modifications can account for observed SI violations in WMAP data, challenging previous interpretations of these anomalies.

Findings

Lensing effects can mimic SI violation signals in CMB data.

Differences in WMAP band signals are explained by instrumental beam sensitivities.

Lensing modifications may account for WMAP anomalies without new physics.

Abstract

Statistical isotropy (SI) has been one of the simplifying assumptions in cosmological model building. Experiments like WMAP and PLANCK are attempting to test this assumption by searching for specific signals in the Cosmic Microwave Background (CMB) two point correlation function. Modifications to this correlation function due to gravitational lensing by the large scale structure (LSS) surrounding us have been ignored in this context. Gravitational lensing will induce signals which mimic isotropy violation even in an isotropic universe. The signal detected in the Bipolar Spherical Harmonic (BipoSH) coefficients $A^{20}_{ll}$ by the WMAP team may be explained by accounting for the lensing modifications to these coefficients. Further the difference in the amplitude of the signal detected in the V-band and W-band maps can be explained by accounting for the differences in the designed angular sensitivity of the instrumental beams. The arguments presented in this article have crucial implications for SI violation studies. Constraining SI violation will only be possible by complementing CMB data sets with all sky measurements of the large scale dark matter distribution. Till that time, the signal detected in the BipoSH coefficients from WMAP-7 could also be yet another suggested evidence of strong deviations from the standard $\Lambda$CDM cosmology based on homogeneous and isotropic FRW models.