Dipole Cosmology: The Copernican Paradigm Beyond FLRW

TL;DR

This paper proposes a new cosmological model called dipole cosmology, which generalizes the FLRW paradigm by incorporating cosmic flow and anisotropy, leading to modified Einstein equations with four functions.

Contribution

It introduces the dipole cosmological principle and develops the corresponding axially isotropic, tilted Bianchi V/VII_h cosmology, expanding the framework beyond traditional FLRW models.

Findings

Cosmic flow can grow while anisotropy diminishes.

The model accommodates late-time acceleration with tilt.

Modified Einstein equations involve four functions.

Abstract

We introduce the $dipole$ $cosmological$ $principle$, the idea that the Universe is a maximally Copernican cosmology, compatible with a cosmic flow. It serves as the most symmetric paradigm that generalizes the FLRW ansatz, in light of the increasingly numerous (but still tentative) hints that have emerged in the last two decades for a non-kinematic component in the CMB dipole. Einstein equations in our "dipole cosmology" are still ordinary differential equations -- but instead of the two Friedmann equations, now we have four. The two new functions can be viewed as an anisotropic scale factor that breaks the isotropy group from $SO(3)$ to $U(1)$, and a "tilt" that captures the cosmic flow velocity. The result is an axially isotropic, tilted Bianchi V/VII$_h$ cosmology. We assess the possibility of model building within the dipole cosmology paradigm, and discuss the dynamics of expansion rate, anisotropic shear and tilt, in various examples. A key observation is that the cosmic flow (tilt) can grow even while the anisotropy (shear) dies down. Remarkably, this can happen even in an era of late time acceleration.