Classical and quantum quintessence cosmology

This paper implements the idea of considering the instantonic creation of brane worlds whose five-dimensional bulk contains a negative cosmological constant and a scalar quintessence field with time-dependent equation of state, restricting to the case that the quintessence field couples minimally to Hilbert-Einstein gravity. We construct an Euclidean formalism, both for the four- and five-dimensional cases, singling out a Hamiltonian constraint that depends on the parameter defining the quintessence state equation. Specializing at several particular values of that parameter, we obtain solutions to the constraint equation and analyse them both classically and quantum mechanically. It is found that these solutions can represent either asymptotically anti-de Sitter wormholes or pure anti-de Sitter spaces whose quantum states are obtained by means of the Wheeler de Witt equation. Starting with the different five-dimensional solutions, an instantonic procedure is applied to describe the creation of geometrically equivalent inflating de Sitter branes whose quantum states are also evaluated in some cases. We thus consider the quantum state of the universe to be contributed by all the instantonic paths that correspond to these particular brane worlds.Comment: 12 pages, RevTex, 1 figure incorporate

Unified Model for Dark Energy

A new model for the universe filled with a generalized Chaplygin fluid is considered which unitarily describes as a single vacuum entity both a quintessence scalar field and a cosmological constant, so unifying the notion of dark energy. While the evolution of the universe filled with such a fluid does not obviously contradict the present cosmic acceleration, the introduced single dark-energy component, for equations of state with characteristic parameter $\omega\geq -1$, behaves like an usual quintessence fluid with constant equation of state at early high densities, and like a pure cosmological constant at late cosmological times.Comment: 5 pages, some misprints corrected, a comment on the initial equation of state inserted, one reference adde

Brane russian doll

It is shown that an (n-1)-dimensional inflating brane world instantonically created from nothing can exist in the region beyond the Rindler horizon of the Lorentzian spacetime associated with another inflating brane world in n dimensions which is also instantonically created from nothing. Generalizing this construction we obtain an unbounded from above tower of successive brane worlds, each having one more dimension than the one which it nests and one less dimension than the one which nests it.Comment: 10 pages, LaTe

Eternally accelerating universe without event horizon

Increasing astronomical evidence indicates that the expansion of the universe is accelerating. By simply solving Einstein equations we show in this letter that a wide class of generic quintessence models leading to eternal acceleration is associated with spacetime static metrics which do not exhibit future event horizons, and therefore do not pose the same problems for string theory as asymptotic de Sitter space.Comment: 10 pages, LaTe