Deflationary cosmology with decaying vacuum energy density

A nonsingular cosmological scenario based on a new phenomenological decay law for the effective Λ term is proposed. In such a model, the cosmic history began from an instability of the de Sitter space-time rather than a singularity. The process is characterized by an arbitrary time scale HI^-1 giving the largest value of the cosmological constant (ΛI=3HI²) and the initial temperature of the Universe. Subsequently, the Universe evolved continuously toward a slightly modified Friedmann-Robertson-Walker cosmology. Among the "remnants" of the primordial inflationary period are the present values of the cosmological term Λ₀~3βH₀², the density parameter Ω₀~1-β, and the deceleration parameter q₀~(1-3β)2, where β is smaller than unity. In addition, the age parameter of the vacuum-matter-dominated phase is H₀t₀~2/3(1-β). It is also shown that, if HI^-1 is of the order of the Planck time, the Universe started with the Planck temperature and this model is consistent with the theoretically suggested value Λ₁Λ₀~10¹¹⁸.