Invited Review given at the 18th Texas Symposium held in Chicago, December 1996. To be published by World Scientific Press, eds. A. Olinto, J. Frieman, and D. Schramm.

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MEASURING COSMOLOGICAL PARAMETERS

Wendy L. Freedman

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

ABSTRACT. In this review, the status of measurements of the matter density ( Omega _m), the vacuum energy density or cosmological constant ( Omega ), the Hubble constant (H₀), and ages of the oldest measured objects (t₀) are summarized. Measurements of the statistics of gravitational lenses and strong gravitational lensing are discussed in the context of limits on Omega . Three separate routes to the Hubble constant are considered: the measurement of time delays in multiply-imaged quasars, the Sunyaev-Zel'dovich effect in clusters, and Cepheid-based extragalactic distances. Globular-cluster ages plus a new age measurement based on radioactive dating of thorium in a metal-poor star are briefly summarized. Limits on the product of H₀ t₀ are also discussed. Many recent, independent dynamical measurements are yielding a low value for the matter density ( Omega _m ~ 0.2-0.3). A wide range of Hubble constant measurements appear to be converging in the range of 60-80 km/sec/Mpc. Areas where future improvements are likely to be made soon are highlighted; in particular, measurements of anisotropies in the cosmic microwave background. Particular attention is paid to sources of systematic error and the assumptions that underlie many of the measurement methods.