Strong Gravitational Lenses in a Cold Dark Matter Universe
  We present theoretical and observational studies of strong gravitational lenses produced by clusters of galaxies. Our purpose is to test the Cold Dark Matter (CDM) model at small and highly non-linear scales where it has been claimed that the CDM model may confront several difficulties. We concentrate our attention on the statistics of strong gravitational lenses. We use two complementary statistics, lensed arcs and quasars, to probe the mass distributions. First, we construct a triaxial lens model, and develop a new method to include triaxiality of dark halos in the lens statistics. We find that the effect of triaxiality is significant; it enhances lensing probabilities by a factors of a few to ten, assuming the degree of triaxiality predicted in the CDM model. In particular, we argue that both central concentration and large triaxiality of dark halos are required to reproduce the observed number of arcs in clusters; thus the result can be interpreted as a strong evidence for the cold and collisionless dark matter. One of the most notable advantages of the triaxial modeling is that the triaxial modeling allows us to predict image multiplicities. We find that the CDM halos predict significant fraction (more than 20%) of naked cusp lenses. In addition, we point out the image multiplicities depend strongly on the central concentration of dark halos. Therefore we propose image multiplicities as a new powerful test of the CDM model. Finally we searched for large-separation lensed quasars from the data of the Sloan Digital Sky Survey, and succeeded in discovering the first large-separation lensed quasar SDSS J1004+4112. We find that the discovery of the large-separation quadruple lens SDSS J1004+4112 is quite consistent with the theoretical predictions based on the CDM model. (Abridged) 
Masamune Oguri (University of Tokyo)