Since current challenges faced by ΛCDM 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 ΛCDM 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., H0 and
σ8). Moreover, having three additional parameters with respect to
ΛCDM, the coupled vector dark energy model is heavily disfavoured by
Bayesian evidence.Comment: 30 pages, 7 figures, 2 tables. A few references were adde