A test of the CPL parameterization for rapid dark energy equation of   state transitions
  We test the robustness and flexibility of the Chevallier-Polarski-Linder (CPL) parameterization of the Dark Energy equation of state $w(z)=w_0+w_a \frac{z}{1+z}$ in recovering a four-parameter step-like fiducial model. We constrain the parameter space region of the underlying fiducial model where the CPL parameterization offers a reliable reconstruction. It turns out that non negligible biases leak into the results for recent ($z<2.5$) rapid transitions, but that CPL yields a good reconstruction in all other cases. The presented analysis is performed with supernova Ia data as forecasted for a space mission like SNAP/JDEM, combined with future expectations for the CMB shift parameter $R$ and the BAO parameter $A$. 
Sebastian Linden and Jean-Marc Virey