The acceleration of the Universe: Measurements of cosmological parameters from type Ia supernovae

The fate of the Universe, infinite expansion or a big crunch': can be determined by measuring the redshifts and brightness of very distant supernovae. These provide a record of changes in the expansion rate of the Universe over the past several billion years. The mass density, Omega(M), and cosmological-constant energy density, Omega(Lambda), are measured from a data-set consisting of 42 high-redshift Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these supernovae, at redshifts between 0.18 and 0.83, 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. we find Omega(M)(flat) = 0.28(-0.08)(+0.09) (1 sigma statistical)(-0.04)(+0.05) (identified systematics). The data are strongly inconsistent with a Lambda = 0 flat 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 identified systematic uncertainties. Thus, the Universe is found to be accelerating, i.e., q(0) = Omega(M)/2 - Omega(Lambda) < 0. The best-fit age of the universe relative to the Hubble time is t(0)(flat) = 14.9(-1.1)(+1.4) (0.63/h) Gyr for a Rat cosmology.