Cosmological Parameters from Statistics of Strongly Lensed Radio Sources
  We calculate the expected number of strongly lensed radio sources in a sample of 6500 sources observed with the Very Large Array as part of the Cosmic Lens All Sky Survey (CLASS) during the first two sessions of its observations. A comparison between the predicted and the observed number of lensed radio sources allows a determination of the current value of Omega-Lambda. If there are six strongly lensed sources in this sample, our 95% confidence upper limit on Lambda in a flat universe is 0.79. Instead of considering a simple cosmological constant, we also consider the possibility of a quintessence scalar field responsible for the additional energy density of the universe. We present our constraints on the Omega_x-w plane. We discuss the region allowed by combined gravitational lensing statistics, high redshift Type Ia supernovae distances, and globular cluster ages.   Instead of a cosmological model, we can constrain the redshift distribution of faint radio sources based on the observed gravitational lensing rate and an assumed cosmological model. If there are six strongly lensed sources, the 68% confidence upper limit on the average redshift of radio sources with flux densities less than 150 mJy at 8.4 GHz is <z> < 1.4 + (Omega-Lambda) \pm 0.1. In order to obtain a much tighter estimate on the cosmological parameters, it is essential that the redshift distribution for radio sources at the faint flux density levels be observationally determined. We strongly recommend that statistically complete optical spectroscopic programs be carried out to obtain redshifts for a representative subsample of faint background radio sources. 
Asantha R. Cooray (Univ. of Chicago)