A New Era in Determining the Matter Density

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Abstract
For decades, the determination of the mean density of matter has been tied to observations of the distribution of light. This has led to a ``bias,'' perhaps as large as a factor of 2, in determining a key cosmological parameter, Omega_M. Recent measurements of the physical properties of clusters, CMB anisotropy and the power spectrum of mass inhomogeneity have ushered in a new era where the mass density can be measured without ``visual bias.'' The early data lead to a consistent picture of the matter and baryon densities, from which I infer: Omega_B = 0.039 +/- 0.0075 and Omega_M = 0.33 +/- 0.035. Over the next few years knowledge of Omega_M will improve significantly, enabling many ``secondary uses,'' from determining the equation-of-state of the dark energy to probing star-formation history.

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