Probing the Cosmic Dipole in the Radio Sky using ASKAP for Cosmology

Abstract

The dipole of the Cosmic Microwave Background (“CMB”) is conventionally understood to be due to our heliocentric motion through the Universe at a speed of about 370 km/s. It is therefore called the kinematic dipole. If this interpretation is correct, this motion should impact other observables, for example large-scale radio galaxy surveys surveyed up to some limiting flux density. Our motion is anticipated to induce a variation in galaxy source density described by a dipole — the “cosmic dipole”. Analysing the consistency between this dipole and the kinematic dipole functions as a test of the cosmological principle, which is the assumption that our Universe is homogeneous and isotropic on large scales. We present a Bayesian analysis of two radio galaxy surveys — RACS-low and NVSS — and find that, even after accounting for local sources, the dipole points roughly in the same direction as the kinematic dipole but has an amplitude 2 to 3 times larger than anticipated. This accords with other studies in the literature and represents an ongoing challenge to the kinematic interpretation of the CMB, and therefore the cosmological principle.