Constraints on Ω_m, 0 and Ω_Λ, 0 from the cosmic microwave background (CMB, orange contours, WMAP, Komatsu et al. 2011), supernovae (blue contours, Union 2 SN Ia compilation, Amanullah et al. 2010), and baryon acoustic oscillations (green contours, SDSS, Eisenstein et al. 2005; from Amanullah et al. 2010, Fig. 10). The red and purple contours correspond to projected 1, 2, and 3σ constraints implied by Eqs. (8) and (10), respectively, using a presumed high-accuracy measurement of the evolution of Ω_ta at the low-redshift Universe (∼42 000 galaxy clusters at z ≤ 0.3, with an individual-cluster Ω_ta uncertainty of 50%, see Sect. 3), yielding a 1.5% accuracy estimate of Ω_ta0 and a 3.5% estimate of dΩ_ta/dz|₀. We have assumed that Ω_ta evolves with z as predicted by flat Ω_m, 0 = 0.315, ΛCDM cosmology. At this level of accuracy, Ω_Λ, 0 >  0 could be established at a 14σ confidence level from the turnaround density data alone.