The case for dynamical dark energy revisited

We investigate the behaviour of dark energy using the recently released supernova data of Riess et al ~(2004) and a model independent parameterization for dark energy (DE). We find that, if no priors are imposed on $\Omega_{0m}$ and $h$, DE which evolves with time provides a better fit to the SNe data than $\Lambda$CDM. This is also true if we include results from the WMAP CMB data. From a joint analysis of SNe+CMB, the best-fit DE model has $w_0 < -1$ at the present epoch and the transition from deceleration to acceleration occurs at $z_T = 0.39 \pm 0.03$. However, DE evolution becomes weaker if the $\Lambda$CDM based CMB results $\Omega_{0m} = 0.27 \pm 0.04$, $h = 0.71 \pm 0.06$ are incorporated in the analysis. In this case, $z_T = 0.57 \pm 0.07$. Our results also show that the extent of DE evolution is sensitive to the manner in which the supernova data is sampled.