Discordance of dipole asymmetries seen in recent large radio surveys with the Cosmological Principle

Abstract

In recent years, large radio surveys of Active Galactic Nuclei (AGNs), comprising millions of sources, have become available where one could investigate dipole asymmetries, assumedly arising due to a peculiar motion of the Solar system. Investigations of such dipoles have yielded in past much larger amplitudes than the cosmic microwave background (CMB) dipole, though their directions seem to lie close to the CMB dipole. Here we investigate dipole asymmetries in two recent large radio surveys, Very Large Array Sky Survey (VLASS) containing 1.9 million sources, covering the sky north of $-40^\circ$ declination, and the Rapid ASKAP Continuum Survey (RACS) containing 2.1 million sources, covering the sky south of $+30^\circ$ declination. We find dipoles determined from the VLASS and RACS surveys to be significantly larger than the CMB dipole. Dipole directions from the VLASS and RACS data differ significantly from each other. Nevertheless, along with a number of other previously determined dipoles, including the CMB, they all appear to lie in a narrow sky region, which argues for the various dipoles to be related somehow. However, significant differences in their derived peculiar velocities, including that of the CMB, cannot be explained by a peculiar motion of the Solar system, which should necessarily be a single value. Instead, their discordant peculiar velocities may be indicating that different cosmic reference frames are moving relative to each other or that the matter distribution on cosmic scales is not homogeneous and isotropic, either scenario being in contravention of what expected from the Cosmological Principle (CP).