Scrutinizing coupled vector dark energy in light of data

Abstract

Since current challenges faced by $\Lambda$CDM might be hinting at new unravelled physics, here we investigate a plausible cosmological model where a vector field acts as source of dark energy. In particular, we examine whether an energy-momentum exchange between dark energy and dark matter could provide an explanation for current discrepancies in cosmological parameters. We carefully work out equations governing both background and linear order perturbations and implement them in a Boltzmann code. We found that a negative coupling makes the dark energy equation of state less negative and closer to a cosmological constant during the matter dominated epoch than an uncoupled vector dark energy model. While the effect of the coupling is hardly noticeable on the growth of matter density perturbations, matter velocity perturbations are enhanced at late-times when dark energy dominates. Therefore, data of redshift space distortions help to narrow down these kinds of couplings in the dark sector. We computed cosmological constraints and found common parameters also present in $\Lambda$CDM are in good agreement with the Planck Collaboration baseline result. However, our best fit predicts a much higher growth rate of matter perturbations at low redshift, thus exacerbating the disagreement with redshift space distortions data. We conclude that our coupled vector dark energy model does not solve current tensions (i.e., $H_0$ and $\sigma_8$). Moreover, having three additional parameters with respect to $\Lambda$CDM, the coupled vector dark energy model is heavily disfavoured by Bayesian evidence.Comment: 30 pages, 7 figures, 2 tables. A few references were adde