  The current Universe is composed by a mixture of relativistic species,
baryonic matter, dark matter and dark energy which evolve in a non-trivial way
at perturbative level. An advanced description of the cosmological dynamics
should include non-standard features beyond the simplistic approach idealized
by the standard cosmology in which cosmic components do not interact, are
adiabatic and dissipationless. We promote a full perturbative analysis of
linear scalar perturbations of a non-interacting cosmological model containing
baryons, dark matter (both pressureless) and a scalar field allowing for the
presence of relative entropic perturbations between the three fluids. Assuming
an effective scalar-field sound speed equal to one and neglecting anisotropic
stresses we establish a new set of equations for the scalar cosmological
perturbations. As a consequence of this new approach, we show that tiny
departures from a constant scalar field equation of state $w_S=-1$ damage
structure formation in a non-acceptable manner. Hence, by strongly constraining
$w_S$ our results provide compelling evidence in favor of the standard
cosmological model and rule out a large class of dynamical dark energy models
