1,131 research outputs found
Spectra of primordial fluctuations in two-perfect-fluid regular bounces
We introduce analytic solutions for a class of two components bouncing
models, where the bounce is triggered by a negative energy density perfect
fluid. The equation of state of the two components are constant in time, but
otherwise unrelated. By numerically integrating regular equations for scalar
cosmological perturbations, we find that the (would be) growing mode of the
Newtonian potential before the bounce never matches with the the growing mode
in the expanding stage. For the particular case of a negative energy density
component with a stiff equation of state we give a detailed analytic study,
which is in complete agreement with the numerical results. We also perform
analytic and numerical calculations for long wavelength tensor perturbations,
obtaining that, in most cases of interest, the tensor spectral index is
independent of the negative energy fluid and given by the spectral index of the
growing mode in the contracting stage. We compare our results with previous
investigations in the literature.Comment: 11 pages, 5 figure
The accelerated expansion of the Universe as a quantum cosmological effect
We study the quantized Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) model
minimally coupled to a free massless scalar field. In a previous paper,
\cite{fab2}, solutions of this model were constructed as gaussian
superpositions of negative and positive modes solutions of the Wheeler-DeWitt
equation, and quantum bohmian trajectories were obtained in the framework of
the Bohm-de Broglie (BdB) interpretation of quantum cosmology. In the present
work, we analyze the quantum bohmian trajectories of a different class of
gaussian packets. We are able to show that this new class generates bohmian
trajectories which begin classical (with decelerated expansion), undergo an
accelerated expansion in the middle of its evolution due to the presence of
quantum cosmological effects in this period, and return to its classical
decelerated expansion in the far future. We also show that the relation between
luminosity distance and redshift in the quantum cosmological model can be made
close to the corresponding relation coming from the classical model suplemented
by a cosmological constant, for . These results suggest the posibility of
interpreting the present observations of high redshift supernovae as the
manifestation of a quantum cosmological effect
Covariant Bardeen Perturbation Formalism
In a previous work we obtained a set of necessary conditions for the linear
approximation in cosmology. Here we discuss the relations of this approach with
the so called covariant perturbations. It is often argued in the literature
that one of the main advantages of the covariant approach to describe
cosmological perturbations is that the Bardeen formalism is coordinate
dependent. In this paper we will reformulate the Bardeen approach in a
completely covariant manner. For that, we introduce the notion of pure and
mixed tensors, which yields an adequate language to treat both perturbative
approaches in a common framework. We then stress that in the referred covariant
approach one necessarily introduces an additional hyper-surface choice to the
problem. Using our mixed and pure tensors approach, we were able to construct a
one-to-one map relating the usual gauge dependence of the Bardeen formalism
with the hyper-surface dependence inherent to the covariant approach. Finally,
through the use of this map, we define full non-linear tensors that at first
order correspond to the three known gauge invariant variables ,
and , which are simultaneously foliation and gauge invariant. We then
stress that the use of the proposed mixed tensors allows one to construct
simultaneously gauge and hyper-surface invariant variables at any order.Comment: 15 pages, no figures, revtex4-1, accepted for publication in PRD,
typos fixed, improved discussion about higher order gauge and foliation
invarianc
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