Power Asymmetry in Cosmic Microwave Background Fluctuations from Full Sky to Sub-degree Scales: Is the Universe Isotropic?

TL;DR

This study confirms a persistent hemispherical power asymmetry in the Cosmic Microwave Background across a broad range of scales, challenging the assumption of isotropy in the universe.

Contribution

It extends previous analyses of CMB anisotropy to smaller scales and introduces a model selection approach to quantify the asymmetry's significance.

Findings

Asymmetry extends from large to smaller scales (l=2-600).

A preferred asymmetry direction is consistently found across multiple multipole ranges.

The asymmetry is statistically significant at the 0.4% level, with no known systematic explanation.

Abstract

We repeat and extend the analysis of Eriksen et al 2004 and Hansen et al 2004 testing the isotropy of the Cosmic Microwave Background (CMB) fluctuations. We find that the hemispherical power asymmetry previously reported for the largest scales l=2-40 extend to much smaller scales. In fact, for the full multipole range l=2-600, significantly more power is found in the hemisphere centered at (theta=107 deg., phi=226 deg.) in galactic co-latitude and longitude than in the opposite hemisphere consistent with the previously detected direction of asymmetry for l=2-40. We adopt a model selection test where the direction and amplitude of asymmetry as well as the multipole range are free parameters. A model with an asymmetric distribution of power for l=2-600 is found to be preferred over the isotropic model at the 0.4% significance level taking into account the additional parameters required to describe it. A similar direction of asymmetry is found independently in all six subranges of 100 multipoles between l=2-600 and none of our 9800 isotropic simulated maps show a similarly consistent direction of asymmetry over such a large multipole range. No known systematic effects or foregrounds are found to be able to explain the asymmetry.