Clustering properties of the CatWISE2020 quasar catalogue and their impact on the cosmic dipole anomaly

TL;DR

This study investigates the clustering properties of the CatWISE2020 quasar catalogue to understand their impact on the cosmic dipole anomaly, reaffirming the high dipole signal and its implications for cosmology.

Contribution

It provides the first detailed analysis of large-scale clustering in the CatWISE2020 quasar sample, assessing its influence on the cosmic dipole anomaly.

Findings

Large-scale power is consistent with noise after accounting for known trends.

No evidence found for an octupole ($rac{3}{})$ in the data.

The clustering dipole remains marginal, supporting the anomaly.

Abstract

The cosmic dipole anomaly -- the observation of a significant mismatch between the dipole observed in the matter distribution and that expected given the kinematic interpretation of the cosmic microwave background dipole -- poses a serious challenge to the Cosmological Principle upon which the standard model of cosmology rests. Measurements of the dipole ($\ell=1$) in a given sample crucially depend on having control over other large-scale power ($\ell > 1$) so as to avoid biases, in particular those potentially caused by correlations among multipoles during fitting, and those by local source clustering. Currently, the most powerful catalogue that exhibits the cosmic dipole anomaly is the sample of 1.6~million mid-infrared quasars derived from CatWISE2020. We therefore analyse clustering properties of this catalogue by performing an inference analysis of large-scale multipoles in real space, and by computing its angular power spectrum on small scales to test for convergence with $\Lambda$CDM. After accounting for the known trend of the quasar number counts with ecliptic latitude, we find that any other large-scale power is consistent with noise, find no evidence for the presence of an octupole ($\ell=3$) in the data, and quantify the clustering dipole's proportion to be marginal. Our results therefore reaffirm the anomalously high dipole in the distribution of quasars.