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A. V. Filippenko (U. California, Berkeley)
Type Ia (hydrogen-deficient) supernovae have enormous potential for the determination of fundamental cosmological quantities. Relatively low-redshift Type Ia supernovae demonstrate that the Hubble expansion is linear, that the Hubble constant has a value of 65 +/- 2 km/s/Mpc (statistical uncertainty), that the bulk motion of the Local Group is consistent with the COBE result, and that the properties of dust in other galaxies are similar to those of dust in the Milky Way. The light curves of high-redshift (z = 0.3-1) supernovae are stretched in a manner consistent with the expansion of space; similarly, their spectra exhibit slower temporal evolution (by a factor of 1+z) than those of nearby supernovae. Our most important conclusion is that the expansion of the Universe is accelerating, perhaps due to the presence of vacuum energy, also referred to as Lambda or Einstein's ``cosmological constant." Thus, Einstein seems to have been right after all, but for the wrong reason (and with the incorrect numerical value for Lambda). We derive a current dynamical age of 14 +/- 2 billion years for the Universe, consistent with the ages of globular star clusters. Moreover, combining our results with existing measurements of the cosmic microwave background radiation, we find a best fit for the normalized matter density and vacuum energy density in the Universe of about 0.3 and 0.7, respectively. With a sum close to unity, this agrees with the value predicted by most inflationary models for the evolution of the Universe, and indicates that the Universe is flat (Euclidean geometry) on large scales. A number of systematic effects (dust, evolution) might be affecting our results and will be discussed, but so far they don't seem to eliminate the need for nonzero Lambda. We are continuing to test for systematic effects by obtaining accurate color curves and spectra of supernovae at z ~ 0.5, and by measuring the Hubble diagram of supernovae near redshift 1.