Measurements of Ω and Λ from 42 High‐Redshift Supernovae

We report measurements of the mass density, Omega_M, and\ncosmological-constant energy density, Omega_Lambda, of the universe based on\nthe analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology\nProject. The magnitude-redshift data for these SNe, at redshifts between 0.18\nand 0.83, are fit jointly with a set of SNe from the Calan/Tololo Supernova\nSurvey, at redshifts below 0.1, to yield values for the cosmological\nparameters. All SN peak magnitudes are standardized using a SN Ia lightcurve\nwidth-luminosity relation. The measurement yields a joint probability\ndistribution of the cosmological parameters that is approximated by the\nrelation 0.8 Omega_M - 0.6 Omega_Lambda ~= -0.2 +/- 0.1 in the region of\ninterest (Omega_M <~ 1.5). For a flat (Omega_M + Omega_Lambda = 1) cosmology we\nfind Omega_M = 0.28{+0.09,-0.08} (1 sigma statistical) {+0.05,-0.04}\n(identified systematics). The data are strongly inconsistent with a Lambda = 0\nflat cosmology, the simplest inflationary universe model. An open, Lambda = 0\ncosmology also does not fit the data well: the data indicate that the\ncosmological constant is non-zero and positive, with a confidence of P(Lambda >\n0) = 99%, including the identified systematic uncertainties. The best-fit age\nof the universe relative to the Hubble time is t_0 = 14.9{+1.4,-1.1} (0.63/h)\nGyr for a flat cosmology. The size of our sample allows us to perform a variety\nof statistical tests to check for possible systematic errors and biases. We\nfind no significant differences in either the host reddening distribution or\nMalmquist bias between the low-redshift Calan/Tololo sample and our\nhigh-redshift sample. The conclusions are robust whether or not a\nwidth-luminosity relation is used to standardize the SN peak magnitudes.