Cosmic dipole tensions: confronting the cosmic microwave background with infrared and radio populations of cosmological sources

Abstract

The cosmic dipole measured in surveys of cosmologically distant sources is generally found to be in disagreement with the kinematic expectation of the Cosmic Microwave Background (CMB). This discrepancy represents severe tension with the Cosmological Principle and challenges the standard model of cosmology. Here, we present a Bayesian analysis of the tension between datasets used to measure the cosmic dipole. We examine the NRAO VLA Sky Survey (NVSS), the Rapid ASKAP Continuum Survey (RACS) and the Wide-field Infrared Survey Explorer catalogue (CatWISE), and jointly analyse them with the Planck observations of the CMB. Under the kinematic interpretation, we find that Planck is in severe tension with CatWISE above 5$\sigma$, strong tension with RACS, and moderate tension with NVSS. Moreover, the strong concordance between CatWISE and NVSS suggests that their dipoles arise from a common astrophysical signal. Conversely, the high discordance between RACS and both CatWISE and NVSS indicates a possible systematic difference in the RACS catalogue itself. Whilst the tension between Planck and infrared-selected quasars is already significant, the question of whether or not the dipole in individual radio surveys adds to the challenge against the standard model is yet to be seen. We estimate that $\mathcal{O}(10^6)$ radio sources are required to measure the tension to a significance of 5$\sigma$. Therefore, in light of the upcoming SKA radio surveys, we are on the cusp of disentangling the anomaly of the cosmic dipole.