Quasar Lensing Statistics and Omega_Lambda: What Went Wrong?

Abstract

In the pre-WMAP, pre-Supernova-Ia-Hubble-diagram era, quasar lensing statistics stubbornly indicated low values of Omega_Lambda. In contrast, a number of recent lensing statistics studies either find the data support the standard Lambda-CDM picture, or simply take the standard cosmological parameters as a given. Have the data or the analyses changed or improved, and how? I review several of the ``historical'' and the more recent studies, and show that there is no particular measurement, assumption, or model parameter in the old studies that was grossly wrong. Instead, at least several effects, operating together, are likely required in order to achieve agreement between the observations and the currently standard cosmology. Most likely among these effects are: a somewhat lower lensing cross section for elliptical galaxies than assumed in the past; some loss of lensed quasars in optical samples due to extinction by the lenses; and a somewhat lower-than-standard value of Omega_Lambda ~ 0.6. The agreement between recent model calculations and the results of radio lens surveys may be fortuitous, and due to a cancellation between the errors in the input parameters for the lens population and the cosmology, on the one hand, and for the source population, on the other hand.