Dark Energy in vector-tensor theories of gravity

TL;DR

This paper explores vector-tensor theories of gravity, identifying models that produce accelerated cosmic expansion, analyzing their stability, and proposing candidates for dark energy including a cosmic vector and electromagnetic field effects.

Contribution

It classifies vector-tensor theories with small-scale behavior similar to General Relativity and identifies those free from instabilities, proposing two models for dark energy.

Findings

Solutions with accelerated expansion and future singularities are common in these models.

Only six theories match small-scale behavior of General Relativity, with two being fully stable.

Electromagnetic fields can generate an effective cosmological constant explainable by inflation.

Abstract

We consider a general class of vector-tensor theories of gravity and show that solutions with accelerated expansion and a future type III singularity are a common feature in these models. We also show that there are only six vector-tensor theories with the same small scales behavior as General Relativity and, in addition, only two of them can be made completely free from instabilities. Finally, two particular models as candidates for dark energy are proposed: on one hand, a cosmic vector that allows to alleviate the usual naturalness and coincidence problems and, on the other hand, the electromagnetic field is shown to give rise to an effective cosmological constant on large scales whose value can be explained in terms of inflation at the electroweak scale.