Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibit Hall 1

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*G. Aldering, S. Perlmutter, R.A. Knop, P. Nugent, G. Goldhaber, D.E. Groom, M.Y. Kim, C.R. Pennypacker, S. Deustua, R. Quimby (LBNL/CfPA, Berkeley), A. Goobar (U. Stockholm), R. Pain, S. Fabbro (CRNS-IN2P3), I.M. Hook, C. Lidman (ESO), A. Kim (College de France), B. E. Schaefer (Yale), R. Ellis, M. Irwin, N. Walton (IoA), P. Ruiz-Lapuente (U. Barcelona), A. S. Fruchter, N. Panagia (STScI), Supernova Cosmology Project Collaboration*

We report measurements of the mass density, \Omega_{\rm
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 Calán/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_{\rm M}- 0.6\,\Omega_{\}Lambda \approx -0.2 ±
0.1 in the region of interest (\Omega_{\rm M} \lesssim
1.5). For a flat (zero curvature) cosmology we find
\Omega_{\rm 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 Calán/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.

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The author(s) of this abstract have provided an email address for comments about the abstract: galdering@lbl.gov