The Cosmological Dipole in Tilted Anisotropic Universes

TL;DR

This work addresses whether a cosmological dipole, compatible with the quasar dipole anomaly, can arise in tilted anisotropic universes. It develops a formalism based on a tilted Bianchi V framework and analyzes four sourcing mechanisms—curvature, heat flow, large-scale electromagnetic fields, and a Khronon field—to express the dipole amplitude $\beta$ in terms of observable quantities and shear. Across the four scenarios, ancillary observational constraints from curvature bounds, CMB shear, spectral distortions, and isocurvature limits severely restrict $\beta$, with the Khronon case remaining the most plausible but requiring more detailed perturbative modelling. The study highlights the predictive tension between a large-scale dipole and established cosmological observations, suggesting that simple homogeneous models struggle to accommodate the dipole unless more complex or dark-sector physics is invoked.

Abstract

There is tentative evidence for a mismatch between the rest frames of matter and the cosmic microwave background, the "quasar dipole anomaly". We consider such a dipole in tilted anisotropic models, for a range of scenarios and sources: spatial curvature, cosmic heat flux, large scale electromagnetic fields and a Khronon field. Crucially, we determine the ancillary effects on other cosmological observables in each of these models and we show that, apart from the case of the Khronon field, it is unlikely that one can obtain a dipole with the amplitude that is being observed unless one considers additional exotica.