Early-Matter-Like Dark Energy and the Cosmic Microwave Background
  Early-matter-like dark energy is defined as a dark energy component whose
equation of state approaches that of cold dark matter (CDM) at early times.
Such a component is an ingredient of unified dark matter (UDM) models, which
unify the cold dark matter and the cosmological constant of the LambdaCDM
concordance model into a single dark fluid. Power series expansions in
conformal time of the perturbations of the various components for a model with
early-matter-like dark energy are provided. They allow the calculation of the
cosmic microwave background (CMB) anisotropy from the primordial initial values
of the perturbations. For a phenomenological UDM model, which agrees with the
observations of the local Universe, the CMB anisotropy is computed and compared
with the CMB data. It is found that a match to the CMB observations is possible
if the so-called effective velocity of sound c_eff of the early-matter-like
dark energy component is very close to zero. The modifications on the CMB
temperature and polarization power spectra caused by varying the effective
velocity of sound are studied.
