Cosmology with moving bimetric fluids

TL;DR

This paper explores how bigravity and massive gravity theories with non-simultaneously diagonal metrics can mimic dark fluids and influence cosmic microwave background anisotropies, providing new insights into dark sector modeling.

Contribution

It introduces a framework using generalized Gordon and Kerr-Schild ansatzes to model effective dark fluids within ghost-free bimetric theories, linking them to observable cosmological effects.

Findings

Effective fluids can reproduce various dark energy and dark matter behaviors.

Different ansatzes describe slow-moving and null dark energy components.

CMB dipole data constrains the properties of the effective bimetric dark sector.

Abstract

We study cosmological implications of bigravity and massive gravity solutions with non-simultaneously diagonal metrics by considering the generalized Gordon and Kerr-Schild ansatzes. The scenario that we obtain is equivalent to that of General Relativity with additional non-comoving perfect fluids. We show that the most general ghost-free bimetric theory generates three kinds of effective fluids whose equations of state are fixed by a function of the ansatz. Different choices of such function allow to reproduce the behaviour of different dark fluids. In particular, the Gordon ansatz is suitable for the description of various kinds of slowly-moving fluids, whereas the Kerr-Schild one is shown to describe a null dark energy component. The motion of those dark fluids with respect to the CMB is shown to generate, in turn, a relative motion of baryonic matter with respect to radition which contributes to the CMB anisotropies. CMB dipole observations are able to set stringent limits on the dark sector described by the effective bimetric fluid.