OBSERVATIONAL EVIDENCE FROM SUPERNOVAE FOR AN ACCELERATING UNIVERSE AND A COSMOLOGICAL CONSTANT

We present spectral and photometric observations of 10 Type Ia superno vae (SNe Ia) in the redshift range 0.16 less than or equal to z less t han or equal to 0.62. The luminosity distances of these objects are de termined by methods that employ relations between SN Ia luminosity and light curve shape. Combined with previous data from our High-z Supern ova Search Team and recent results by Riess et al., this expanded set of 16 high-redshift supernovae and a set of 34 nearby supernovae are u sed to place constraints on the following cosmological parameters: the Hubble constant (H-o), the mass density (Omega(M)), the cosmological constant (i.e., the vacuum energy density, Omega(Lambda)), the deceler ation parameter (q(o)), and the dynamical age of the universe (t(o)). The distances of the high-redshift SNe Ia are, on average, 10%-15% far ther than expected in a low mass density (Omega(M) = 0.2) universe wit hout a cosmological constant. Different light curve fitting methods, S N Ia subsamples, and prior constraints unanimously favor eternally exp anding models with positive cosmological constant (i.e., Omega(Lambda) > 0) and a current acceleration of the expansion (i.e., q(o) < 0). Wi th no prior constraint on mass density other than Omega(M) greater tha n or equal to 0, the spectroscopically confirmed SNe Ia are statistica lly consistent with q(o) < 0 at the 2.8 sigma and 3.9 sigma confidence levels, and with Omega(Lambda) > 0 at the 3.0 sigma and 4.0 sigma con fidence levels, for two different fitting methods, respectively. Fixin g a ''minimal'' mass density, Omega(M) = 0.2, results in the weakest d etection, Omega(Lambda) > 0 at the 3.0 sigma confidence level from one of the two methods. For a flat universe prior (Omega(M) + Omega(Lambd a) = 1), the spectroscopically confirmed SNe Ia require Omega(Lambda) > 0 at 7 sigma and 9 sigma formal statistical significance for the two different fitting methods. A universe closed by ordinary matter (i.e. , Omega(M) = 1) is formally ruled out at the 7 sigma to 8 sigma confid ence level for the two different fitting methods. We estimate the dyna mical age of the universe to be 14.2 +/- 1.7 Gyr including systematic uncertainties in the current Cepheid distance scale. We estimate the l ikely effect of several sources of systematic error, including progeni tor and metallicity evolution, extinction, sample selection bias, loca l perturbations in the expansion rate, gravitational lensing, and samp le contamination. Presently, none of these effects appear to reconcile the data with Omega(Lambda) = 0 and q(o) greater than or equal to 0.