20 research outputs found
(An)Isotropic models in scalar and scalar-tensor cosmologies
We study how the constants and may vary in different
theoretical models (general relativity with a perfect fluid, scalar
cosmological models (\textquotedblleft quintessence\textquotedblright) with and
without interacting scalar and matter fields and a scalar-tensor model with a
dynamical ) in order to explain some observational results. We apply
the program outlined in section II to study three different geometries which
generalize the FRW ones, which are Bianchi \textrm{V}, \textrm{VII} and
\textrm{IX}, under the self-similarity hypothesis. We put special emphasis on
calculating exact power-law solutions which allow us to compare the different
models. In all the studied cases we arrive to the conclusion that the solutions
are isotropic and noninflationary while the cosmological constant behaves as a
positive decreasing time function (in agreement with the current observations)
and the gravitational constant behaves as a growing time function
Bianchi {VI} in Scalar and Scalar-Tensor Cosmologies
We study several cosmological models with Bianchi \textrm{VI}
symmetries under the self-similar approach. In order to study how the
\textquotedblleft constants\textquotedblright\ and may vary, we
propose three scenarios where such constants are considered as time functions.
The first model is a perfect fluid. We find that the behavior of and
are related. If behaves as a growing time function then
is a positive decreasing time function but if is decreasing then
is negative. For this model we have found a new solution. The second model is a
scalar field, where in a phenomenological way, we consider a modification of
the Klein-Gordon equation in order to take into account the variation of .
Our third scenario is a scalar-tensor model. We find three solutions for this
models where is growing, constant or decreasing and is a positive
decreasing function or vanishes. We put special emphasis on calculating the
curvature invariants in order to see if the solutions isotropize.Comment: Typos corrected. References added, minor corrections. arXiv admin
note: text overlap with arXiv:0905.247
The Similarity Hypothesis in General Relativity
Self-similar models are important in general relativity and other fundamental
theories. In this paper we shall discuss the ``similarity hypothesis'', which
asserts that under a variety of physical circumstances solutions of these
theories will naturally evolve to a self-similar form. We will find there is
good evidence for this in the context of both spatially homogenous and
inhomogeneous cosmological models, although in some cases the self-similar
model is only an intermediate attractor. There are also a wide variety of
situations, including critical pheneomena, in which spherically symmetric
models tend towards self-similarity. However, this does not happen in all cases
and it is it is important to understand the prerequisites for the conjecture.Comment: to be submitted to Gen. Rel. Gra
A Kinematical Approach to Conformal Cosmology
We present an alternative cosmology based on conformal gravity, as originally
introduced by H. Weyl and recently revisited by P. Mannheim and D. Kazanas.
Unlike past similar attempts our approach is a purely kinematical application
of the conformal symmetry to the Universe, through a critical reanalysis of
fundamental astrophysical observations, such as the cosmological redshift and
others. As a result of this novel approach we obtain a closed-form expression
for the cosmic scale factor R(t) and a revised interpretation of the space-time
coordinates usually employed in cosmology. New fundamental cosmological
parameters are introduced and evaluated. This emerging new cosmology does not
seem to possess any of the controversial features of the current standard
model, such as the presence of dark matter, dark energy or of a cosmological
constant, the existence of the horizon problem or of an inflationary phase.
Comparing our results with current conformal cosmologies in the literature, we
note that our kinematic cosmology is equivalent to conformal gravity with a
cosmological constant at late (or early) cosmological times. The cosmic scale
factor and the evolution of the Universe are described in terms of several
dimensionless quantities, among which a new cosmological variable delta emerges
as a natural cosmic time. The mathematical connections between all these
quantities are described in details and a relationship is established with the
original kinematic cosmology by L. Infeld and A. Schild. The mathematical
foundations of our kinematical conformal cosmology will need to be checked
against current astrophysical experimental data, before this new model can
become a viable alternative to the standard theory.Comment: Improved version, with minor changes. 58 pages, including 7 figures
and one table. Accepted for publication in General Relativity and Gravitation
(GERG
The growth factor of matter perturbations in an f(R) gravity
The growth of matter perturbations in the model proposed by
Starobinsky is studied in this paper. Three different parametric forms of the
growth index are considered respectively and constraints on the model are
obtained at both the and confidence levels, by using the
current observational data for the growth factor. It is found, for all the
three parametric forms of the growth index examined, that the Starobinsky model
is consistent with the observations only at the confidence level.Comment: 15 pages, 5 figure