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J. Bartlett (Strasbourg)
In principle, the Sunyaev--Zel'dovich (SZ) effect offers a vision of clusters completely analogous to that provided by X--ray observations, with the important exceptions that its surface brightness is independent of redshift and that its magnitude is proportional to gas density, rather than density squared. This means that clusters can be found at high redshift and that observations are less sensitive than X--ray measures to intracluster gas inhomogeneities. Observations have, up til the present, only concerned X--ray selected clusters, although the possibility of true SZ surveys, i.e., over otherwise blank fields, may soon become a possibility. Such surveys would permit the construction of SZ source counts, the possible detection of clusters at very high redshift, and analyses of fluctuations in the SZ background. We will discuss these possibilities with a particular eye on ways of constraining the density parameter; for example, the existence of clusters at high redshifts represents a ``smoking gun'' for a low density parameter.
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