Upper panel: evolution with scale factor a of ρ_ta in units of the present-day critical density, ρ_c, 0. Black solid line: flat ΛCDM cosmology with Ω_m, 0 = 0.315 (Planck Collaboration VI 2020). Red dashed line: matter-only open cosmology, with Ω_m, 0 = 0.315, Ω_Λ, 0 = 0. Black dotted line: present-day (a = 1) tangent to the black solid line, with slope ρ_ta ∼ a^−1.5, shallower than the asymptotic behavior of an Ω_Λ, 0 = 0 Universe. Lower panel: evolution of ρ_ta with redshift z, for Ω_m, 0 = 0.315 and different values of Ω_Λ, 0. The magenta shaded box shows the accuracy that can be achieved by measuring ρ_ta in 100 clusters at z = 0.3 with fractional uncertainty of 50% in each, and is indicative of the discriminating power of such an experiment.