The mean density of the Universe from cluster evolution

The determination of the mean density of the Universe is a long-standing pr oblem of modern cosmology. The number density evolution of X-ray clusters a t a fixed temperature is a powerful cosmological test, new in nature, somew hat different from standard analyses based on the dynamical measurement of individual objects. However, the absence of any available sample of X-ray s elected clusters with measured temperatures at high redshift has prevented this test from being applied earlier. Recently, temperature measurements of ten EMSS clusters at 0.3 less than or equal to z less than or equal to 0.4 have allowed the application of this test. In this work, we present the fi rst results of a new analysis we have performed of this data set as well as a new estimation of the local temperature distribution function of cluster s: a likelihood analysis of the temperature distribution functions gives a preferred value for the mean density of the Universe which corresponds to 7 5 % of the critical density. An open model with a density smaller than 30 % of the critical density is rejected with a level of significance of 95%. ( C) Academie des sciences/Elsevier, Paris.