Narrowing constraints with type Ia supernovae: converging on a cosmologicalconstant

Abstract

We apply a parameterization-independent approach to fitting the dark energyequation-of-state (EOS). Utilizing the latest type Ia supernova data, combined withresults from the cosmic microwave background and baryon acoustic oscillations,we find that the dark energy is consistent with a cosmological constant. Weestablish independent estimates of the evolution of the dark energy EOS bydiagonalizing the covariance matrix. We find three independent constraints,which taken together imply that the equation of state is more negative than−0.2 at the 68% confidence level in the redshift range0<z<1.8, independent of the flat universe assumption. Our estimates argue against previous claimsof dark energy ‘metamorphosis’, where the EOS was found to be strongly varying at lowredshifts. Our results are inconsistent with extreme models of dynamical dark energy, both inthe form of ‘freezing’ models where the dark energy EOS begins with a value greater than−0.2 atz>1.2 and rolls toa value of −1 today, and ‘thawing’ models where the EOS is near−1 at high redshifts, but rapidly evolves to values greater than−0.85 atz<0.2. Finally, we propose a parameterization-independent figure of merit (FOM), to help assessthe ability of future surveys to constrain dark energy. While the more common FOMpresumes a specific dark energy parameterization with two parameters, we suggest abinning approach to evaluate dark energy constraints with a minimum number ofassumptions.