The kinematic contribution to the cosmic number count dipole

¹ Max-Planck Institut fur Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, UK
³ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany

^⋆ Corresponding author.

Received: 11 December 2024
Accepted: 1 March 2025

Abstract

Measurements of the number-count dipole with large surveys have shown amplitudes in tension with kinematic predictions based on the observed Doppler dipole of the cosmic microwave background (CMB). These observations seem to be in direct conflict with a homogeneous and isotropic universe as asserted by the cosmological principle, demanding further investigation into the origin of the tension. Here, we investigated whether the observed number-count dipoles are consistent with being fully kinematic, regardless of boost, or if there is any residual anisotropy contributing to the total observed dipole, independent of the kinematic part. To disentangle these contributions, we aim to leverage the fact that the kinematic matter dipole expected in a given galaxy catalogue scales with observed properties of the sample, and different catalogues used in the literature therefore have different kinematic dipole expectations. We performed joint dipole fits using the NRAO VLA Sky Survey (NVSS), the Rapid ASKAP Continuum Survey (RACS), and the active galactic nuclei (AGN) catalogue derived from the Wide-field Infrared Survey Explorer (CatWISE). The direction of the common dipole between these catalogues is offset from the CMB dipole direction by 23 ± 5 degrees. Assuming a common kinematic and non-kinematic dipole component between all catalogues, we find that a large residual, non-kinematic dipole anisotropy is detected, though a common direction between the two components is disfavoured by model selection. Freeing up both amplitude and direction for this residual dipole while fixing the kinematic dipole to the CMB dipole expectation, we recover a significant residual dipole with 𝒟_resid = (0.81 ± 0.14)×10⁻², which is offset from the CMB dipole direction by 39 ± 8 degrees. While these results cannot explain the origin of the unexpectedly large number-count dipoles, they offer a rephrasing of the anomaly in terms of kinematic and non-kinematic contributions, providing evidence for the existence of the latter within the models explored here. The present work provides a valuable first test of this concept, although its scrutinising power is limited by the currently employed catalogues. Larger catalogues, especially in radio, will be needed to further lift the degeneracy between the kinematic and residual dipole components.