Narrowing constraints with type Ia supernovae: converging on a cosmological constant

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Abstract
We apply a parameterization-independent approach to fitting the dark energy equation-of-state (EOS). Utilizing the latest type Ia supernova data, combined with results from the cosmic microwave background and baryon acoustic oscillations, we find that the dark energy is consistent with a cosmological constant. We establish independent estimates of the evolution of the dark energy EOS by diagonalizing 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 range 0<z1.2 and rolls to a 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 at z<0.2. Finally, we propose a parameterization-independent figure of merit (FOM), to help assess the ability of future surveys to constrain dark energy. While the more common FOM presumes a specific dark energy parameterization with two parameters, we suggest a binning approach to evaluate dark energy constraints with a minimum number of assumptions.
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