Gravitational Lensing and Tests of the Cosmological Constant

Abstract

The case for a flat Cold Dark Matter model with a positive cosmological constant $Λ$ has been recently strongly advocated by some theoreticians. In this paper we give the observers point of view to the light of the most recent observations with a special emphasis on lensing tests. We confirm the apparent cosmic concordance for a flat Universe with $Ω_Λ$ close to 0.6 but we note that a low mass density open universe with no cosmological constant is still quite acceptable for most of the reliable observational tests, including lensing tests as well.