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We show that X-ray and microwave observations of the hot gas in a rich cluster of galaxies can be combined to give a direct and model-free measurement of the angular diameter distance to the cluster. Such observations thus provide an evolution-independent global method for the determination of both H/sub o/ and q/sub o/. As long as the intracluster gas is reasonably homogeneous and the effects of discrete sources can be subtracted, the method is limited by the accuracy of the observations rather than by any inherent dispersion in cluster properties. The observations required to obtain q/sub o/ are feasible, but at the limit of present techniques.
Silk et al. (Fri,) studied this question.