Detailed analysis of a large, dedicated sample of HST clusters

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While virtually all cosmological observations support the standard, ΛCDM cosmological model, galaxy clusters pose yet unanswered puzzles. Among them and related with each other are the arc-statistics problem, the overconcentration problem and the problem of large Einstein radii. All of them refer to the unexpectedly strong gravitational-lensing effects produced by massive galaxy clusters. It appears as if such clusters were much more efficient lenses than theory can explain. One major difficulty with the assessment of how relevant these problems are is the significance of the claims. Strong gravitational lensing being a rare phenomenon, they rest on small samples of well-studied cases. Testing the substance of these problems is the paramount goal of a 525 orbit proposal to the Space Telecope Science Institute that has recently been approved. In its course, 25 well-selected galaxy clusters will be observed in 14 wave bands. It will be one of the largest observing campaigns ever conducted with the Hubble Space Telescope. We are involved in this programme and propose to address all of its gravitational-lensing aspects. We wish to construct accurate mass maps including error bars based on joint analyses of strong and weak lensing, to derive concentration parameters and effective Einstein radii, to compare those to theoretical predictions and analyse the significance of possible deviations from them, and to test geometrically the relation between angular-diameter distance and redshift in order to constrain the expansion rate of the Universe. The upcoming cluster sample will be the ideal observational basis for the research proposed.