  We discuss the possibility of a scalar field being the fundamental
description of dark energy without the need of self-tuning any parameter.
Thereby, we focus on shift-symmetric scalar-tensor theories satisfying also
that the propagation speed of gravitational waves is equal to the speed of
light. Analysing the stability of scalar linear perturbations, we discuss the
conditions that seems to be necessary to describe (super) accelerated cosmic
expansion without introducing instabilities. It has been established, however,
that this stability can be ruined when taking into account the interaction with
tensor perturbations (essentially gravitational waves). Indeed, although we
shall point out that the standard proof of absence of dark energy stable
braiding models due to this interaction has a possible way-out, we find general
arguments suggesting that there are no dark energy stable solutions that can
exploit this loophole. Thus, we discuss future research directions for finding
viable fundamental descriptions of dark energy. We also provide a dictionary
between the covariant version of the scalar field theory and that of the
Effective Field Theory approach, explicitly computing the parameters in the
latter formalism in terms of the functions appearing in the covariant version,
and its derivatives. To the best of our knowledge, this is the first time these
expressions are explicitly obtained up-to arbitrary order in perturbation
theory.
