New dark energy constraints from supernovae, microwave background and   galaxy clustering
  Using the spectacular new high redshift supernova observations from the HST/GOODS program and previous supernova, CMB and galaxy clustering data, we make the most accurate measurements to date of the dark energy density rho_X as a function of cosmic time, constraining it in a rather model-independent way, assuming a flat universe. We find that Einstein's vanilla scenario where rho_X(z) is constant remains consistent with these new tight constraints, and that a Big Crunch or Big Rip is more than 50 gigayears away for a broader class of models allowing such cataclysmic events. We discuss popular pitfalls and hidden priors: parametrizing the equation-of-state w_X(z) assumes positive dark energy density and no Big Crunch, and the popular parametrization w_X(z)=w_0 +w_0' z has nominally strong constraints from CMB merely because w_0' > 0 implies an unphysical exponential blow-up rho_X ~ e^{3 w_0' z}. 
Yun Wang, Max Tegmark