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3.2 Strong Gravitational Lenses

A number of strong (elliptical galaxy) gravitational lens systems are known that may offer the potential of constraining the value of Omegam and OmegaLambda through modeling of the lens properties. This method is less sensitive to OmegaLambda than the statistics of lensing, and again it is sensitive to a number of possible systematic effects: possible perturbations by cluster potentials, uncertainties in the underlying properties of the lensing galaxies, and model-dependent corrections due to evolution. The objects are faint and the errors in the luminosities and velocity dispersions are potentially very significant. A recent analysis of 7 strong lenses has been undertaken by Im et al. (1996). Their current results yield OmegaLambda = 0.64+0.15-0.26 (i.e., this measurement sits almost at the end of the range excluded by Kochanek (1996) at 95% confidence. Im et al. exclude Omegam = 1.0 at 97% confidence.