Resolution of the incongruency of dipole asymmetries within various large radio surveys -- implications for the Cosmological Principle

TL;DR

This study analyzes dipole asymmetries in four large radio surveys, identifies systematic biases, and finds that while dipole directions align with the CMB, their amplitudes challenge the Cosmological Principle and standard cosmological models.

Contribution

It provides a detailed investigation of radio survey dipoles, identifies declination-dependent systematics, and offers a method to mitigate these effects, questioning the origin of cosmic dipoles.

Findings

Radio survey dipoles are affected by declination-dependent systematics.

The combined dipole direction aligns with the CMB within 1.2 sigma.

The dipole amplitude is significantly larger than expected from the CMB dipole.

Abstract

We investigate dipole asymmetries in four large radio surveys, each spanning more than 80\% of the sky. Two of them, the Very Large Array Sky Survey (VLASS) and the Rapid ASKAP Continuum Survey (RACS), have recently yielded dipoles that appear incongruent with each other as well as seem inconsistent with previous radio survey dipoles and the Cosmic Microwave Background (CMB) dipole. Because these radio surveys have large overlaps in sky coverage, comprising hence large majority of common radio sources, one would not expect significant differences between their radio dipoles, irrespective of their underlying source of origin. We examine here in detail these radio dipoles, to ascertain the source of incongruency amongst them. We find the VLASS and RACS data to be containing some declination-dependent systematics, seemingly in the vicinity of the declination limit of each survey. We show that the effects of such systematics can be mitigated by restricting the declination limits of the respective survey during the dipole determination. A weighted mean of the sky coordinates of thus derived dipoles from the four radio surveys lies within $1.2\sigma$ of the CMB dipole direction. However, the amplitude appears significantly larger, $3.7\pm 0.6$ times or more than the CMB dipole. This puts in doubt not only the conventional wisdom that the genesis of all these dipoles, including that of the CMB dipole, is due to the Solar peculiar motion, it also raises uncomfortable questions about the Cosmological Principle (CP), the basis of the standard $\Lambda$CDM cosmological model.