Measurements of Omega and Lambda from High-Redshift Supernovae

We report measurements of the mass density, Omega_M , and cosmological-constant energy density, Omega_ Lambda , of the universe based on the analysis of more than forty Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these high redshift supernovae are fit jointly with a set of supernovae from the Calan/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. All supernova peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation. The measurement yields a joint probability distribution of the cosmological parameters that is approximated by the relation 0.8 Omega_M - 0.6 Omega_ Lambda ~ -0.2 +/- 0.1 in the region of interest (Omega_M ≲ 1.5). For a flat (zero curvature) cosmology we find Omega_M = 0.28(+0.09}_{-0.08) (statistical) (+0.05}_{-0.04) (systematic). The data are strongly inconsistent with a Lambda = 0 zero-curvature cosmology, the simplest inflationary universe model. An open, Lambda = 0 cosmology also does not fit the data well: the data indicate that the cosmological constant is non-zero and positive, with a confidence of P(Lambda > 0) = 99%, including the systematic uncertainty. The best-fit age of the universe relative to the Hubble time is t_0=14.9(+1.4}_{-1.1) (0.63/h) Gyr for a zero-curvature universe. The size of our sample allows us to perform a variety of statistical tests to check for possible systematic errors and biases. We find no significant differences in either the host reddening distribution or Malmquist bias between the low-redshift Calan/Tololo sample and our high-redshift sample. Excluding those few supernovae which are outliers in color excess or fit residual does not significantly change the results. The conclusions are also robust whether or not a width-luminosity relation is used to standardize the supernova peak magnitudes.