TIME DEPENDENT COSMOLOGICAL TERM AND HOLOGRAPHY

Abstract

The physical space is believed to contain a kind of energy whose gravitational effect resembles the cosmological Λ-term introduced by Einstein. We discuss how the holographic conjecture, relating the universe entropy with the horizon area, may fix the late time variation of this effective Λ-term and the induced time dependence of the gravitational constant. The resulting cosmological solutions at late times present a constant ratio between the matter density and the vacuum energy density. It is then argued that, among the non-decelerating solutions, the coasting expansion is the only acceptable from a thermodynamic viewpoint. This solution is characterized by a universe age given by Ht = 1, and by density parameters Ω m = 1/3 and Ω Λ = 2/3, as required by the cosmic concordance model.