Cosmic Dipoles from Large-Scale Structure Surveys

TL;DR

This paper investigates the origins and expected alignments of cosmic dipoles from different large-scale structure surveys within the standard cosmological model, addressing observed inconsistencies and clarifying misconceptions.

Contribution

It provides a detailed theoretical analysis of cosmic dipoles from CMB, supernovae, and galaxy surveys, explaining their expected alignments and amplitudes within the ΛCDM model.

Findings

Observer motion dominates the dipole signal.

Intrinsic velocities decrease with source redshift.

Dipoles should be aligned but vary in amplitude.

Abstract

Large-scale structure surveys can be used to measure the dipole in the cosmic microwave background (CMB), in the luminosity distances inferred from type-Ia supernova observations, and in the spatial distribution of galaxies and quasars. The measurements of these cosmic dipoles appear to be mutually inconsistent, even though they are expected to indicate the common observer velocity. This observational tension may represent a significant challenge to the standard model of cosmology. Here we study in detail what contributes to the cosmic dipoles from CMB, supernova, and galaxy survey in the standard $\Lambda$CDM model, though our theoretical model can be applied beyond the standard model. While measurements of the cosmic dipoles yield the relative velocities between the source samples and the observer velocity, the motion of the observer is the dominant contribution in the conformal Newtonian gauge, and the intrinsic velocities of the samples fall steeply with increasing redshift of the sources. Hence the cosmic dipoles of CMB, type-Ia supernovae, and galaxies should be aligned but can have different amplitudes. We also clarify several misconceptions that are commonly found in the literature.