The Invisible Cosmological Constant

We discuss the possibility of measuring the density parameter Ω0 and the cosmological constant λ0 = Λ/(3H 2 0) from dynamical tests using linear and non-linear spherical models. While information on both parameters can be obtained by looking at clustering at different redshifts, in practice the other observables also depend on the cosmology, and in some cases conspire to give a weak dependence on λ0. We also show that a repulsive Λ gives a smaller final radius of a virialized cluster than a vanishing Λ.