Towards A Realistic Dipole Cosmology: The Dipole $\Lambda$CDM Model

TL;DR

This paper develops a dipole $ ext{Lambda}$CDM cosmological model incorporating multiple fluid components with independent flows, potentially explaining observed cosmic dipoles and addressing late-time cosmic tensions.

Contribution

It introduces a realistic dipole cosmology framework with fluid mixtures and independent flows, extending the standard model to include bulk flows and their late-time effects.

Findings

Radiation and matter can have independent flows in the model.

Relative flows between radiation and matter can increase at late times.

This behavior may contribute to observed CMB dipole and cosmic tensions.

Abstract

Dipole cosmology is the maximally Copernican generalization of the FLRW paradigm that can incorporate bulk flows in the cosmic fluid. In this paper, we first discuss how multiple fluid components with independent flows can be realized in this set up. This is the necessary step to promote ``tilted" Bianchi cosmologies to a viable framework for cosmological model building involving fluid mixtures (as in FLRW). We present a dipole $\Lambda$CDM model which has radiation and matter with independent flows, with (or without) a positive cosmological constant. A remarkable feature of models containing radiation (including dipole $\Lambda$CDM) is that the $relative$ flow between radiation and matter can increase at late times, which can contribute to eg., the CMB dipole. This can happen generically in the space of initial conditions. We discuss the significance of this observation for late time cosmic tensions.