Parametrizing the transition to the phantom epoch with Supernovae Ia and
  Standard Rulers
  The reconstruction of a (non)canonical scalar field Lagrangian from the dark
energy Equation of State (EoS) parameter is studied, where it is shown that any
EoS parametrization can be well reconstructed in terms of scalar fields.
Several examples of EoS parameters are studied and the particular scalar field
Lagrangian is reconstructed. Then, we propose some new parametrizations that
may present a (fast) transition to a phantom dark energy EoS (where
$w_{DE}<-1$) and the scalar field Lagrangian is also reconstructed numerically.
Furthermore, the properties of these parametrizations of the dark energy EoS
are studied by using supernovae Ia data (HST Cluster Supernova Survey) combined
with Standard Ruler datasets [Cosmic Microwave Background (CMB) and Baryon
Acoustic Oscillations (BAO)] and its comparison with the $\Lambda$CDM model is
analyzed. Then, the best fit of the models is obtained, which provides some
information about whether a phantom transition may be supported by the
observations. In this regard, the crossing of the phantom barrier is allowed
statistically but the occurrence of a future singularity seems unlikely.
