STRONG GRAVITATIONAL LENSING STATISTICS AS A TEST OF COSMOGONIC SCENARIOS

Gravitational lensing statistics can provide a direct and powerful tes t of cosmic structure formation theories. Since lensing tests, directl y, the magnitude of the nonlinear mass density fluctuations on lines o f sight to distant objects, no issues of '' bias '' (of mass fluctuati ons with respect to galaxy density fluctuations) exist here, although lensing observations provide their own ambiguities of interpretation. We develop numerical techniques for generating model density distribut ions with the very large spatial dynamic range required by lensing con siderations and for identifying regions of the simulations capable of multiple image lensing in a conservative and computationally efficient way that should be accurate for splittings significantly larger than 3''. Applying these techniques to existing standard Cold dark matter ( CDM) (OMEGA = 1) and Primeval Baryon Isocurvature (PBI) (OMEGA = 0.2) simulations (normalized to the COBE amplitude), we find that the CDM m odel predicts large splitting (>8'') lensing events roughly an order-o f-magnitude more frequently than the PBI model. Under the reasonable b ut idealized assumption that lensing structures can be modeled as sing ular isothermal spheres (SIS), the predictions can be directly compare d to observations of lensing events in quasar samples. Several large s plitting (DELTAtheta > 8'') cases are predicted in the standard CDM mo del (the exact number being dependent on the treatment of amplificatio n bias), whereas none is observed. In a formal sense, the comparison e xcludes the CDM model at high confidence (essentially for the same rea son that CDM predicts excessive small-scale cosmic velocity dispersion s.) A very rough assessment of low-density but flat CDM model (OMEGA = 0.3, LAMBDA/3H-0(2) = 0.7) indicates a far lower and probably accepta ble level of lensing. The PBI model is consistent with, but not strong ly tested by, the available lensing data, and other open models would presumably do as well as PBI. These preliminary conclusions and the as sumptions on which they are based can be tested and the analysis can b e applied to other cosmogonic models by straightforward extension of t he work presented