Anisotropic Scalar Field Dark Energy with a Disformally Coupled Yang-Mills Field

TL;DR

This paper explores how a disformally coupled Yang-Mills field in scalar-tensor theories can produce anisotropic scalar field dark energy, revealing new scaling solutions and potential contributions to dark matter and dark radiation.

Contribution

It introduces a novel model combining scalar fields with a disformally coupled Yang-Mills field, analyzing its cosmological dynamics and implications for dark energy, dark matter, and dark radiation.

Findings

Disformal coupling alters the gauge field's equation of state from radiation to matter.

New scaling solutions with non-zero gauge fields are found, some exhibiting anisotropy.

The gauge field can contribute to dark matter and dark radiation depending on initial conditions.

Abstract

In the context of scalar-tensor theories, the inclusion of new degrees of freedom coupled non-minimally to the gravitational sector might produce some appealing effects on the cosmic expansion history. We investigate this premise by including a canonical SU(2) Yang- Mills field to the total energy budget of the universe coupled to the standard quintessential field by a disformal transformation. From the dynamical system analysis, we study three cases of cosmological interest that span most of the physical phase space of the model: the uncoupled limit, the isotropic, and the Abelian cases. New scaling solutions with a non-vanishing gauge field are found in all cases which can be interesting for early cosmological scenarios. Some of these scaling solutions even exhibit anisotropic features. Also, the background evolution of the universe is studied by means of numerical analysis. As an interesting result, the disformal coupling changes the equation of state of the gauge field from radiation to matter at some stages of the evolution of the universe, thereby the gauge field can contribute to some fraction of the total dark matter. We have also quantified the redshift-dependent contribution of the gauge field in the form of dark radiation during the radiation era to the effective number of relativistic species. This depends essentially on the initial conditions and, more importantly, on the disformal coupling function.