Constraining alternatives to a cosmological constant: generalized couplings and scale invariance

TL;DR

This paper compares three models explaining low-redshift cosmic acceleration using observational data, finding that the traditional cosmological constant remains necessary in some models, while others require alternative explanations.

Contribution

It provides a comparative analysis of generalized coupling and scale invariant models against standard parametrizations using observational constraints.

Findings

The generalized coupling model needs a cosmological constant to fit data.

The scale invariant model favors a low-density fluid with a positive equation of state.

Standard matter density of 0.3 is a poor fit for the scale invariant model.

Abstract

We present a comparative analysis of observational low-redshift background constraints on three candidate models for explaining the low-redshift acceleration of the universe. The generalized coupling model by Feng and Carloni and the scale invariant model by Maeder (both of which can be interpreted as bimetric theories) are compared to the traditional parametrization of Chevallier, Polarski and Linder. In principle the generalized coupling model, which in vacuum is equivalent to General Relativity, contains two types of vacuum energy: the usual cosmological constant plus a second contribution due to the matter fields. We show that the former is necessary for the model to agree with low-redshift observations, while there is no statistically significant evidence for the presence of the second. On the other hand the scale invariant model effectively has a time-dependent cosmological constant. In this case we show that a matter density $\Omega_m\sim0.3$ is a relatively poor fit to the data, and the best-fit model would require a fluid with a much smaller density and a significantly positive equation of state parameter.