Revisiting the CMB homogeneity scale: low multipoles removal effect and extragalactic foreground masking

TL;DR

This study investigates the apparent inconsistency in the CMB homogeneity scale by analyzing the effects of low multipole removal and extragalactic foreground masking, highlighting the quadrupole anomaly's role in observed discrepancies.

Contribution

It demonstrates that removing low multipoles, especially the quadrupole, improves model-data consistency and explores the impact of extragalactic foreground masking on this discrepancy.

Findings

Removing low multipoles improves model-data consistency.

Masking extragalactic foregrounds has limited effect on the discrepancy.

The quadrupole anomaly is central to the observed homogeneity scale inconsistency.

Abstract

The Cosmic Microwave Background (CMB) reaches homogeneity at relatively modest angular scales compared to the expectation of the standard $\Lambda$CDM model revealing an important challenge to the theoretical predictions. We analyze this inconsistency through the homogeneity scale $H$ and the slope of the homogeneity index at $\theta = 90^\circ$. We find that the removal of low multipoles, in particular the quadrupole, from both the data and the $\Lambda$CDM synthetic CMB maps, significantly improve the consistency between models and observations. This adds to indications of the relevant contribution of the low value of the CMB quadrupole to the observed anomalies in the homogeneity scale. Due to the presence of a new extragalactic foreground in the CMB maps, we have performed statistical analyses with different masking taking into account the regions mostly affected. In particular we consider galaxies in the local neighborhood which are expected to affect more significantly the large angular scales. We find that by masking these regions, the analysis cannot solve the discrepancy between the observations and the $\Lambda$CDM model in spite of a small improvement of their mutual consistency. The studies with both foreground masking and low-$\ell$ removed CMB maps show similar results than those of the full CMB map indicating that the main discrepancy between theory and observations is associated to the quadrupole anomaly and may require more exhaustive analysis.