60 research outputs found
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
Quintessence in brane cosmology
In order to reconcile the non conventional character of brane cosmology with
standard Friedmann cosmology, we introduce in this paper a slowly-varying
quintessence scalar field in the brane and analyse the cosmological solutions
corresponding to some equations of state for the scalar field. Different
compensation mechanisms between the cosmological constant in the bulk and the
constant tension resulting from the combined effect of ordinary matter and the
quintessence scalar field are derived or assumed. It has been checked that the
Randall-Sundrum approach is not necessarily the best procedure to reconcile
brane and standard cosmologies, and that there exists at least another
compensating mechanism that reproduces a rather conventional behaviour for an
accelerating universe.Comment: 10 pages, additional physical motivation and connections to high
energy physics and observations, to appear in Phys. Lett.
Two-point Padé-type approximation to the Cauchy transform of certain strong distributions
AbstractIn this paper we are mainly concerned with the Cauchy transform of certain strong distributions satisfying a type of symmetric property introduced by A.S. Ranga. Algebraic properties of the corresponding two-point Padé-type approximants are given along with results about convergence for sequences of such approximant
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 , 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
Interplay Between Gravity and Quintessence: A Set of New GR Solutions
A set of new exact analytical General Relativity (GR) solutions with
time-dependent and spatially inhomogeneous quintessence demonstrate 1) a static
non-empty space-time with a horizon-type singular surface; 2) time-dependent
spatially homogeneous `spheres' which are completely different in geometry from
the Friedmann isotropic models; 3) infinitely strong anti-gravity at a `true'
singularity where the density is infinitely large. It is also found that 4) the
GR solutions allow for an extreme `density-free' form of energy that can
generate regular space-time geometries
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