  The two most popular candidates for dark energy, i.e. a cosmological constant
and quintessence, are very difficult to distinguish observationally, mostly
because the quintessence field does not have sizable fluctuations. We study a
scalar field model for dark energy in which the scalar field is invariant under
reflection symmetry, phi -> -phi. Under general assumptions, there is a phase
transition at late times (z < 0.5). Before the phase transition, the field
behaves as a cosmological constant. After the phase transition, a
time-dependent scalar condensate forms, the field couples with dark matter and
develops sizable perturbations tracking those of dark matter. The background
cosmological evolution is in agreement with existing observations, but might be
distinguished from that of a cosmological constant by future Supernovae
surveys. The growth of cosmological perturbations carries the imprint of the
phase transition, however a non-linear approach has to be developed in order to
study it quantitatively.
