New Dark Energy Constraints from Supernovae, Microwave Background, and Galaxy Clustering

Using supernova, cosmic microwave background, 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 simplest 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 Gyr away for a broader class of models allowing such cataclysmic events. We discuss popular pitfalls and hidden priors.

Figure 1

$1\sigma$ constraints on the density of matter and dark energy from SN Ia (Riess sample, flux averaged with $\Delta z=0.05$), CMB, and LSS data, all in units of the current dark energy density. From inside out, the four nested dark energy constraints are for models making increasingly strong assumptions, corresponding, respectively, to the four-parameter spline, the three-parameter spline, the two-parameter $(f_{\infty },w_i)$ case, and the one-parameter constant $w$ case (hatched area). The Universe starts accelerating when the total density slope $d\ln \rho /d\ln (1+z)>-2$, which roughly corresponds to when dark energy begins to dominate, i.e., to where the matter and dark energy bands cross. In the distant future, the Universe recollapses if the dark energy density ${\rho }_X$ goes negative and ends in a “big rip” if it keeps growing [$d\ln {\rho }_X/d\ln (1+z)<0$].

Figure 2

How constraints on $w_0$ and $w_0^{\prime }$ depend on assumptions and data used. Darker shaded regions are ruled out at 95% confidence by SNe Ia alone; lighter shaded regions are ruled out when adding other information as indicated. 68% contours are dotted. Models above the dotted line end in a big rip. The 157 SNe Ia (Riess sample) have been flux averaged with $\Delta z=0.05$.