  Time-dependent scalar fields provide a candidate explanation for the dark
energy. For these to vary on cosmological time scales, the derivative of the
scalar potential in Planck units should have roughly the same magnitude as the
potential itself. We emphasize that scalars with this property are present in
any four-dimensional gravitational effective theory with a known UV completion
via holography, provided that the dual CFT has scalar operators with dimensions
of order one. Cosmological solutions without time-dependent scalars are
analogous to solutions dual to the vacuum states of such CFTs while solutions
with time-dependent scalars are analogous to solutions dual to the vacuum state
of quantum field theories with RG flow where these CFTs provide the UV or IR
fixed point. If time-dependent scalars do explain the dark energy, the
gravitational effective theory describing our universe could be a $\Lambda < 0$
model associated to a holographic CFT.
