*both* baryons and dark matter, between which we will not differentiate here. Any other terms are ascribed to the DE Lagrangian $\mathcal{L}_{x}$, which represents some consistent theory potentially depending on extra degrees of freedom or $g_{\mu\nu}$ (i.e. modifications of gravity).[^1] We will neglect the radiation component because all the observations are assumed to be performed well after decoupling. In non-minimally coupled models, the Lagrangian $\mathcal{L_{\text{m}}}$ depends on a different metric, related to $g_{\mu\nu}$ through some transformation. Here we assume, however, that we have already reformulated the action so that matter moves on the geodesics of $g_{\mu\nu}$.

We employ the above minimal framework to address question (1) by considering the background observables in section \[sec:Bckg\] and those arising from linear perturbations in section \[sec:LinPerts\]. To answer question (2) we need another crucial assumption, concerning the degrees of freedom in the dark energy Lagrangian:

1.  The Lagrangian $\mathcal{L}_{x}$, which describes dark energy, is any one of the Lagrangians describing a single scalar field governed by second-order equations of motion. We call this scalar field dark energy, but we do not necessarily require it to be driving the current acceleration. For example, it could be that the late time acceleration is effectively driven 