2007ApJ...666..694W

We present constraints on the dark energy equation-of-state parameter, w=P/(ρc²), using 60 SNe Ia from the ESSENCE supernova survey. We derive a set of constraints on the nature of the dark energy assuming a flat universe. By including constraints on (Ω_M, w) from baryon acoustic oscillations, we obtain a value for a static equation-of-state parameter w=-1.05^+0.13_–0.12(stat 1 σ)±0.13 (sys) and Ω_M=0.274^+0.033_–0.020 (stat 1 σ) with a best-fit χ²/dof of 0.96. These results are consistent with those reported by the Supernova Legacy Survey from the first year of a similar program measuring supernova distances and redshifts. We evaluate sources of systematic error that afflict supernova observations and present Monte Carlo simulations that explore these effects. Currently, the largest systematic with the potential to affect our measurements is the treatment of extinction due to dust in the supernova host galaxies. Combining our set of ESSENCE SNe Ia with the first-results Supernova Legacy Survey SNe Ia, we obtain a joint constraint of w=-1.07^+0.09_–0.09(stat 1 σ)±0.13 (sys), Ω_M=0.267^+0.028_–0.018(stat 1 σ) with a best-fit χ²/dof of 0.91. The current global SN Ia data alone rule out empty (Ω_M=0), matter-only Ω_M=0.3, and Ω_M=1 universes at >4.5 σ. The current SN Ia data are fully consistent with a cosmological constant.