Dark Energy Scaling from Dark Matter to Acceleration
  The dark sector of the Universe need not be completely separable into
distinct dark matter and dark energy components. We consider a model of early
dark energy in which the dark energy mimics a dark matter component in both
evolution and perturbations at early times. Barotropic aether dark energy
scales as a fixed fraction, possibly greater than one, of the dark matter
density and has vanishing sound speed at early times before undergoing a
transition. This gives signatures not only in cosmic expansion but in sound
speed and inhomogeneities, and in number of effective neutrino species. Model
parameters describe the timing, sharpness of the transition, and the relative
abundance at early times. Upon comparison with current data, we find viable
regimes in which the dark energy behaves like dark matter at early times: for
transitions well before recombination the dark energy to dark matter fraction
can equal or exceed unity, while for transitions near recombination the ratio
can only be a few percent. After the transition, dark energy goes its separate
way, ultimately driving cosmic acceleration and approaching a cosmological
constant in this scenario.
