We propose a new cosmological model with a time-dependent cosmological constant (Λ ∝ 1/t), which starting at the Planck time as ΛP l ∼ M P l, evolves to the present-day allowed value of Λ0 ∼ 10M P l. This scenario is supported by non-critical string theory considerations. We compute the age of the Universe and the time-dependence of the scale factor in this model, and find general agreement wit...

In this paper we present exactly solved extended quintessence models; furthermore, through a dynamical effective Q-cosmological ”constant”, we recover some of the Λ decaying cases found in the literature. Finally we introduce a sort of complementarity between the Q-dominated or Λdominated expansions of the Universe. e-mail addresses: [email protected] [email protected]

SUMMARY A diverse set of observations now compellingly suggest that Universe possesses a nonzero cosmological constant. In the context of quantum-field theory a cosmological constant corresponds to the energy density of the vacuum, and the wanted value for the cosmological constant corresponds to a very tiny vacuum energy density. We discuss future observational tests for a cosmological constan...

We prove that Weyl invariant theories of gravity possess a remarkable property which, under very general assumptions, explains the stability of flat space-time. We show explicitly how conformal invariance is broken spontaneously by the vacuum expectation value of an unphysical scalar field; this process induces general relativity as an effective long distance limit. We show that all ghost degre...

A contribution of quantum vacuum to the energy momentum tensor is inevitably experienced in the present universe. One requires the presence of non-zero cosmological constant (Λ) to make the various observations consistent. A case of Λ in the Schwarzschild de Sitter space-time shows that precession of perihelion orbit provides a sensative solar test for non-zero Λ. Application of the relations i...

We review Coleman’s wormhole mechanism for the vanishing of the cosmological constant. We show that in a minisuperspace model wormhole-connected universes dominate the path integral. We also provide evidence that the Euclidean path integral over geometries with spherical topology is unstable with respect to formation of infinitely many wormhole-connected 4-spheres. Consistency is restored by su...

We present a cosmological model containing a cosmological constant Λ and a component with an inhomogeneous equation of state. We study the form of the inhomogeneous equation of state for which the model exhibits the relaxation of the cosmological constant, i.e. it asymptotically tends to the de Sitter regime characterized by a small positive effective cosmological constant. The effect of the re...

Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(G~/c3) ≈ 10−123. This review discusses several aspects of the cosmological constant both from the cosmological (sections 1–6) and field theoretical (sections 7–11) perspectives. The first section introduces the key issues related to cosmological constant and provides a brief histori...

It is traced out a parallel between the cosmological constant problem and the polymer physics. The time evolution of the universe world line is compared with the growing of a polymer chain. An equivalent Flory free energy and a modification of it are used as a means to evaluate two versions for the “radius of gyration” of the universe. It is proposed a link between this radius of gyration and t...