Disformal Scalar Fields and the Dark Sector of the Universe
  Disformal transformations have proven to be very useful to devise models of
the dark sector. In the present paper we apply such transformation to a single
scalar field theory as a way to drive the field into a slow roll phase. The
canonical scalar field Lagrangian, when coupled to a disformal metric, turns
out to have relations to bimetric dark matter theories and to describe many
specific dark energy models at various limits, thus providing a surprisingly
simple parametrisation of a wide variety of models including tachyon, Chaplygin
gas, K-essence and dilatonic ghost condensate. We investigate the evolution of
the background and linear perturbations in disformal quintessence in order to
perform a full comparison of the predictions with the cosmological data. The
dynamics of the expansion, in particular the mechanism of the transition to
accelerating phase, is described in detail. We then study the effects of
disformal quintessence on cosmic microwave background (CMB) anisotropies and
large scale structures (LSS). A likelihood analysis using the latest data on
wide-ranging SNIa, CMB and LSS observations is performed allowing variations in
six cosmological parameters and the two parameters specifying the model. We
find that while a large region of parameter space remains compatible with
observations, models featuring either too much early dark energy or too slow
transition to acceleration are ruled out.
