Observational constraints on vector-like dark energy

The canonical cosmological model to explain the recent acceleration of the universe relies on a cosmological constant, and most dynamical dark energy and modified gravity model alternatives are based on scalar fields. Still, further alternatives are possible. One of these involves vector fields: under certain conditions, they can lead to accelerating universes while preserving large-scale homogeneity and isotropy. We report quantitative observational constraints on a model previously proposed by Armend\'ariz-Pic\'on and known as the cosmic triad. We consider several subclasses of the model, which generically is a parametric extension of the canonical $\Lambda$CDM model, as well as two possible choices of the triad's potential. Our analysis shows that any deviations from this limit are constrained to be small. In particular the preferred present-day values of the matter density and the dark energy equation of state are fully consistent with those obtained, for the same datasets, in flat $\Lambda$CDM and $w_0$CDM. The constraints mildly depend on the priors on the dark energy equation of state, specifically on whether phantom values thereof are allowed, while the choice of potential does not play a significant role since any such potential is constrained to be relatively flat.