The probability that high-redshift quasars are gravitationally-lensed by intervening galaxies increases rapidly with the cosmological constant, ΩΛ0
(whilst being only weakly dependent on the density parameter, Ωm0
), and the low number of lenses observed implies that ΩΛ0
≲ 0.7 (e.g. Kochanek 1996). One of many uncertainties has been the (small) core radii of elliptical galaxies, which, at least naively, reduce their lensing cross-section. However, if ellipticals are normalised relative to their observed line-of-sight velocity dispersion, σ||, then increasing the core radius must result in an increased mass normalisation (specified by the assymptotic velocity dispersion, σ∞).