Local Voids as the Origin of Large‐Angle Cosmic Microwave Background Anomalies. I.

We explore the large angular scale temperature anisotropies in the cosmic microwave background due to expanding homogeneous local voids at redshift z ≲ 1. A compensated spherically symmetric homogeneous dust-filled void with radius ~3 × 10 2 h -1 Mpc and density contrast δ ~ -0.3 can be observed as a cold spot with a temperature anisotropy Δ T / T ~ -1 × 10 -5 surrounded by a slightly hotter ring. We find that a pair of these circular cold spots separated by ~50° can account both for the planarity of the octopole and for the alignment between the quadrupole and the octopole in the cosmic microwave background (CMB) anisotropy. The cold spot in the Galactic southern hemisphere, which is anomalous at the ~3 σ level, can be explained by such a large void at z ~ 1. The observed north-south asymmetry in the large-angle CMB power can be attributed to the asymmetric distribution of these local voids between the two hemispheres. The statistical significance of the low quadrupole is further reduced in this interpretation of the large angular scale CMB anomalies.