Multiple-Images in the Cluster Lens Abell 2218: Constraining the   Geometry of the Universe ?
  In this Letter we present a detailed study of the lensing configuration in the cluster Abell 2218. Four multiple-images systems with measured spectroscopic redshifts have been identified in this cluster. These multiple images are very useful to constrain accurately the mass distribution in the cluster core, but they are also sensitive to the value of the geometrical cosmological parameters of the Universe. Using a simplified maximum likelihood analysis we find 0 < Omega_M < 0.30 assuming a flat Universe, and 0 < Omega_M < 0.33 and w < -0.85 for a flat Universe with dark energy. Interestingly, an Einstein-de Sitter model is excluded at more than 4sigma. These constraints are consistent with the current constraints derived with CMB anisotropies or supernovae studies. The proposed method constitutes an independent test of the geometrical cosmological parameters of the Universe and we discuss the limits of this method and this particular application to Abell 2218. Application of this method with more sophisticated tools and to a larger number of clusters or with more multiple images constraints, will put stringent constraints on the geometrical cosmological parameters. 
G. Soucail, J.P. Kneib, G. Golse