Vacuum Energy in Cosmic Dynamics

The analysis of the Th/U ratio in meteorites and the evolutionary ages of globular clusters favour values of the cosmic age of (19±5)×10⁹ yr. This evidence together with a Hubble parameterH₀>70 km s^‑1 Mpc^‑1=(14×10⁹ yr)^‑1 cannot be reconciled in a Friedmann model with Λ=0. It requires a cosmological constant in the order of 10^‑56 cm^‑2, equivalent to a vacuum densityρ_v=10^‑29 g cm^‑3 The Friedmann-Lemaître models (Λ>0) with a hot big-bang have been calculated. They are based on a present value of the baryonic matter density ofρ₀=0.5×10^‑30 g cm^‑3 as derived from the primordial⁴He and²H abundances. For a Hubble parameter ofH₀=75 km s^‑1 Mpc^‑1, our analysis favours a set of models which can be represented by a model with Euclidean metric (density parameter Ω₀=1.0, deceleration parameterq₀=‑0.93, aget₀=19.7×10⁹ yr) and by a closed model with perpetual expansion (Ω₀=1.072,q₀=‑1.0, aget₀=21.4×10⁹ yr). A present density parameter close to one can indeed be expected if the conjecture of an exponential inflation of the very early universe is correct. The possible behaviour of the vacuum density is demonstrated with the help of Streeruwitz' formula in the context of the closed model with an inflationary phase at very early times.