Invited Review given at the Nobel Symposium, "Particle Physics and the Universe", Haga Slott, Sweden, August, 1998. Published in Physica Scripta, Vol. T85, 37-46, 2000

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Wendy L. Freedman

Carnegie Observatories, 813 Santa Barbara St., Pasadena, CA 91101

ABSTRACT. Rapid progress has been made recently toward the measurement of cosmological parameters. Still, there are areas remaining where future progress will be relatively slow and difficult, and where further attention is needed. In this review, the status of measurements of the matter density (Ωm), the vacuum energy density or cosmological constant (ΩΛ), the Hubble constant (H0), and ages of the oldest measured objects (t0) are summarized. Many recent, independent dynamical measurements are yielding a low value for the matter density (Ωm ~ 0.3). New evidence from type Ia supernovae suggests that ΩΛ may be non-zero. Many recent Hubble constant measurements appear to be converging in the range of 65-75 km/sec/Mpc. Eliminating systematic errors lies at the heart of accurate measurements for all of these parameters; as a result, a wide range of cosmological parameter space is currently still open. Fortunately, the prospects for accurately measuring cosmological parameters continue to increase and there is good reason for optimism that success may shortly be forthcoming.

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