Modelling the WMAP large-angle anomalies as an effect of a local density inhomogeneity

TL;DR

This paper models large-angle CMB anomalies as effects of local density inhomogeneities, showing that accounting for these can explain some observed low multipole anomalies in WMAP data.

Contribution

It introduces a model attributing WMAP large-angle anomalies to local density fluctuations, providing a potential explanation for the low quadrupole and alignment issues.

Findings

Local density inhomogeneity model increases quadrupole power from ~200-260 to ~1000 μK².

Model reduces quadrupole-octopole alignment statistic S from ~0.7-0.74 to ~0.31-0.37.

Predicted quadrupole power and alignment are consistent with observed anomalies after correction.

Abstract

We investigate large-angle scale temperature anisotropy in the Cosmic Microwave Background (CMB) with the Wilkinson Microwave Anisotropy Probe (WMAP) data and model the large-angle anomalies as the effect of the CMB quadrupole anisotropies caused by the local density inhomogeneities. The quadrupole caused by the local density inhomogeneities is different from the special relativity kinematic quadrupole. If the observer inhabits a strong inhomogeneous region, the local quadrupole should not be neglected. We calculate such local quadrupole under the assumption that there is a huge density fluctuation field in direction $(284^{\circ},74^{\circ})$, where the density fluctuation is $10^{-3}$, and its center is $\sim 112h^{-1} \rm {Mpc}$ away from us. After removing such mock signals from WMAP data, the power in quadrupole, $C_2$, increases from the range $(200\sim260\mu \rm{K^2})$ to $\sim1000\mu \rm{K^2}$. The quantity S, which is used to estimate the alignment between the quadrupole and the octopole, decreases from $(0.7\sim0.74)$ to $(0.31\sim0.37)$, while the model predict that $C_2=1071.5\mu \rm{K^2}$, $S=0.412$. So our local density inhomogeneity model can, in part, explain the WMAP low-$\ell$ anomalies.