The Ellis and Baldwin test of the Cosmic Dipole: Exploring the impact of multiple flux density cuts

Abstract

The cosmic dipole tension - the discrepancy between the Cosmic Microwave Background kinematic dipole and the matter dipole inferred from all-sky surveys poses a significant challenge to the Cosmological Principle, which dictates that the universe is homogeneous and isotropic at the largest scales. Traditional measurement of the matter dipole requires selecting an appropriate limiting flux and calculating the dipolar modulation using sources brighter than the flux. This approach, however, ignores the shape of the source luminosity function (LF) and deprives the analysis of this crucial information. In this study, we present a new approach to calculate the matter dipole by integrating the source flux distribution into the analysis. We achieve this by dividing the catalogue into disjoint flux bins and simultaneously fitting the matter dipole across them. For non-power-law LFs, this method gives a higher Bayes factor - and hence a better description of the matter dipole - as compared to the traditional approach. The method works best when the flux cuts are selected in regions where the LF's shape changes significantly. We discuss the feasibility of this method for upcoming cosmological surveys and show that it has the potential to yield decisive results at both radio and infrared wavelengths.

arXiv comment26 pages, 14 figures, accepted for publication in Journal of Cosmology and Astroparticle Physics (JCAP)

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