Technically natural dark energy from Lorentz breaking

TL;DR

This paper introduces Theta-CDM, a Lorentz-violating dark energy model with a naturally small contribution to cosmic acceleration, matching Lambda-CDM expansion but differing in matter power spectrum predictions at subhorizon scales.

Contribution

It presents a technically natural dark energy model derived from Lorentz-violating gravity theories with a derivatively coupled scalar field, compatible with high-energy completion and observational data.

Findings

Expansion history similar to Lambda-CDM without a cosmological constant

Enhanced matter power spectrum at subhorizon scales compared to Lambda-CDM

Model can be distinguished from Lambda-CDM using current cosmological data

Abstract

We construct a model of dark energy with a technically natural small contribution to cosmic acceleration, i.e. this contribution does not receive corrections from other scales in the theory. The proposed acceleration mechanism appears generically in the low-energy limit of gravity theories with violation of Lorentz invariance that contain a derivatively coupled scalar field Theta. The latter may be the Goldstone field of a broken global symmetry. The model, that we call Theta-CDM, is a valid effective field theory up to a high cutoff just a few orders of magnitude below the Planck scale. Furthermore, it can be ultraviolet-completed in the context of Horava gravity. We discuss the observational predictions of the model. Even in the absence of a cosmological constant term, the expansion history of the Universe is essentially indistinguishable from that of Lambda-CDM. The difference between the two theories appears at the level of cosmological perturbations. We find that in Theta-CDM the matter power spectrum is enhanced at subhorizon scales compared to Lambda-CDM. This property can be used to discriminate the model from Lambda-CDM with current cosmological data.