Cosmological Interpretation from High Redshift Clusters Observed Within   the XMM-Newton $\Omega$-Project
  During the last ten years astrophysical cosmology has brought three remarkable results of deep impact for fundamental physics: the existence of non-baryonic dark matter, the (nearly) flatness of space, the domination of the density of the universe by some gravitationally repulsive fluid. This last result is probably the most revolutionizing one: the scientific review Sciences has considered twice results on this question as Breakthrough of the Year (for 1998 and 2003). However, direct evidence of dark energy are still rather weak, and the strength of the standard scenario relies more on the "concordance" argument rather than on the robustness of direct evidences. Furthermore, a scenario can be build in an Einstein-de Sitter universe, which reproduces as well as the concordance model the following various data relevant to cosmology: WMAP results, large scale structure of the universe, local abundance of massive clusters, weak lensing measurements, most Hubble constant measurements not based on stellar indicators. Furthermore, recent data on distant x-ray clusters obtained from XMM and Chandra indicates that the observed abundances of clusters at high redshift taken at face value favors an Einstein de Sitter model and are hard to reconcile with the concordance model. It seems wise therefore to consider that the actual existence of the dark energy is still an open question. 
Alain Blanchard (LATT)