Technically natural dark energy from Lorentz breaking

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 Θ. The latter may be the Goldstone field of a broken global symmetry. The model, that we call ΘCDM, 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 Hořava 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 ΛCDM. The difference between the two theories appears at the level of cosmological perturbations. We find that in ΘCDM the matter power spectrum is enhanced at subhorizon scales compared to ΛCDM. This property can be used to discriminate the model from ΛCDM with current cosmological data.