Cosmological Tests of Coupled Galileons

TL;DR

This paper examines cosmological Galileon models with matter couplings, finding that observational constraints severely limit their parameter space, especially for couplings to baryons and photons, leaving viable models mainly coupled to dark matter.

Contribution

It provides a detailed analysis of the constraints on conformal and disformal couplings in Galileon models from cosmological observations, highlighting the necessity of suppressing baryonic and photon couplings.

Findings

Disformal coupling to baryons is highly constrained by astrophysics.

Variation of the speed of light due to photon coupling is tightly limited.

Galileon models can be viable dark energy theories if couplings to baryons and photons are suppressed.

Abstract

We investigate the cosmological properties of Galileon models with positive kinetic terms. We include both conformal and disformal couplings to matter and focus on constraints on the theory that arise because of these couplings. The disformal coupling to baryonic matter is extremely constrained by astrophysical and particle physics effects. The disformal coupling to photons induces a cosmological variation of the speed of light and therefore distortions of the Cosmic Microwave Background spectrum which are known to be very small. The conformal coupling to baryons leads to a variation of particle masses since Big Bang Nucleosynthesis which is also tightly constrained. We consider the background cosmology of Galileon models coupled to Cold Dark Matter (CDM), photons and baryons and impose that the speed of light and particle masses respect the observational bounds on cosmological time scales. We find that requiring that the equation of state for the Galileon models must be close to -1 now restricts severely their parameter space and can only be achieved with a combination of the conformal and disformal couplings. This leads to large variations of particle masses and the speed of light which are not compatible with observations. As a result, we find that cosmological Galileon models are viable dark energy theories coupled to dark matter but their couplings, both disformal and conformal, to baryons and photons must be heavily suppressed making them only sensitive to CDM.