Measuring Cosmological Parameters

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 (Ω_m), the vacuum energy density or cosmological constant (Ω_Λ), 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 Ω_Λ. 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 (Ω_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.