79 research outputs found
Constraints on Non-Singular Cosmological Models with Quadratic Lagrangians
We consider the generalized set of theories of gravitation whose Lagrangians
contain the term : . Inserting the RW
metric with an imposed non-singular and inflationary behaviour of the scale
factor , and using a arbitrary perfect fluid, we study the properties of
and in this context. By requiring the positivity of the energy
density, as well as real and finite velocity of sound, we can obtain the range
of values of that ensure the inflationary behaviour and absence of
singularity.Comment: 11 pages, RevTeX, 3 Postscript figure
Quantum Isotropization of the Universe
We consider minisuperspace models constituted of Bianchi I geometries with a
free massless scalar field. The classical solutions are always singular (with
the trivial exception of flat space-time), and always anisotropic once they
begin anisotropic. When quantizing the system, we obtain the Wheeler-DeWitt
equation as a four-dimensional massless Klein-Gordon equation. We show that
there are plenty of quantum states whose corresponding bohmian trajectories may
be non-singular and/or presenting large isotropic phases, even if they begin
anisotropic, due to quantum gravitational effects. As a specific example, we
exhibit field plots of bohmian trajectories for the case of gaussian
superpositions of plane wave solutions of the Wheeler-DeWitt equation which
have those properties. These conclusions are valid even in the absence of the
scalar field.Comment: 10 pages, RevTeX, 3 Postscript figures, uses graficx.st
Graceful exit from inflation using quantum cosmology
A massless scalar field without self interaction and string coupled to
gravity is quantized in the framework of quantum cosmology using the Bohm-de
Broglie interpretation. Gaussian superpositions of the quantum solutions of the
corresponding Wheeler-DeWitt equation in minisuperspace are constructed. The
bohmian trajectories obtained exhibit a graceful exit from the inflationary
Pre-Big Bang epoch to the decelerated expansion phase.Comment: 8 pages, RevTeX, 4 Postscript figures, uses graficx.sty. Added more
text and reference
Nonminimal Scalar-Tensor Theories and Quantum Gravity
Recentely, it is shown that the quantum effects of matter determine the
conformal degree of freedom of the space-time metric. This was done in the
framework of a scalar-tensor theory with one scalar field. A point with that
theory is that the form of quantum potential is preassumed. Here we present a
scalar-tensor theory with two scalar fields, and no assumption on the form of
quantum potential. It is shown that using the equations of motion one gets the
correct form of quantum potential plus some corrections.Comment: 15 page
On the consistency of a repulsive gravity phase in the early Universe
We exploit the possibility of existence of a repulsive gravity phase in the
evolution of the Universe. A toy model with a free scalar field minimally
coupled to gravity, but with the "wrong sign" for the energy and negative
curvature for the spatial section, is studied in detail. The background
solutions display a bouncing, non-singular Universe. The model is well-behaved
with respect to tensor perturbations. But, it exhibits growing models with
respect to scalar perturbations whose maximum occurs in the bouncing. Hence,
large inhomogeneties are produced. At least for this case, a repulsive phase
may destroy homogeneity, and in this sense it may be unstable. A newtonian
analogous model is worked out; it displays qualitatively the same behaviour.
The generality of this result is discussed. In particular, it is shown that the
addition of an attractive radiative fluid does not change essentially the
results. We discuss also a quantum version of the classical repulsive phase,
through the Wheeler-de Witt equation in mini-superspace, and we show that it
displays essentially the same scenario as the corresponding attractive phase.Comment: Latex file, 15 pages, 7 figures. There is a new figure, a new section
and some other minor correction
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