Observational constraints on dark energy with a fast varying equation of   state
  We place observational constraints on models with the late-time cosmic acceleration based on a number of parametrizations allowing fast transitions for the equation of state of dark energy. In addition to the model of Linder and Huterer where the dark energy equation of state $w$ monotonically grows or decreases in time, we propose two new parametrizations in which $w$ has an extremum. We carry out the likelihood analysis with the three parametrizations by using the observational data of supernovae type Ia, cosmic microwave background, and baryon acoustic oscillations. Although the transient cosmic acceleration models with fast transitions can give rise to the total chi square smaller than that in the $\Lambda$-Cold-Dark-Matter ($\Lambda$CDM) model, these models are not favored over $\Lambda$CDM when one uses the Akaike information criterion which penalizes the extra degrees of freedom present in the parametrizations. 
Antonio De Felice, Savvas Nesseris, Shinji Tsujikawa
astro-ph.CO gr-qc hep-ph hep-th