The cosmological fit to 313 SNe Ia (112 PS1 SNe Ia + 201 low-z SNe Ia), using only SNe and assuming a constant dark energy equation of state and flatness, yields w = -1.015^{+0.319}_{-0.201}(Stat)+{0.164}_{-0.122}(Sys). When combined with BAO+CMB(Planck)+H0, the analysis yields Omega_M = 0.277^{+0.010}_{-0.012} and w = -1.186^{+0.076}_{-0.065} including all identified systematics, as spelled out in the companion paper by Scolnic et al. (2013a).The full Pan-STARRS1 supernova sample will be 3 times as large as this initial sample, which should provide more conclusive results.
The value of w is inconsistent with the cosmological constant value of -1 at the 2.4 sigma level. This tension has been seen in other high-z SN surveys and endures after removing either the BAO or the H0 constraint. If we include WMAP9 CMB constraints instead of those from Planck, we find w = -1.142^{+0.076}_{-0.087}, which diminishes the discord to <2 data-blogger-escaped-b="" data-blogger-escaped-sigma.="">We cannot conclude whether the tension with flat CDM is a feature of dark energy, new physics, or a combination of chance and systematic errors.
But, as Motl explains, there are sound theoretical reasons to doubt that the deviation observed is more than a statistical fluke because if the universe had the properties claimed (a w constant more negative than -1), this would lead to a variety of implications that appear non-physical, particularly in light of the modest statistical significance of the result. For instance, it would imply a speed of sound greater than the speed of light in some instances. Sean Carroll is co-author of a paper that at least admits the possibility as one not to be dismissed out of hand
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