40 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
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
Gaussian superpositions in scalar-tensor quantum cosmological models
A free scalar field minimally coupled to gravity model is quantized and the
Wheeler-DeWitt equation in minisuperspace is solved analytically, exhibiting
positive and negative frequency modes. The analysis is performed for positive,
negative and zero values of the curvature of the spatial section. Gaussian
superpositions of the modes are constructed, and the quantum bohmian
trajectories are determined in the framework of the Bohm-de Broglie
interpretation of quantum cosmology. Oscillating universes appear in all cases,
but with a characteristic scale of the order of the Planck scale. Bouncing
regular solutions emerge for the flat curvature case. They contract classically
from infinity until a minimum size, where quantum effects become important
acting as repulsive forces avoiding the singularity and creating an
inflationary phase, expanding afterwards to an infinite size, approaching the
classical expansion as long as the scale factor increases. These are
non-singular solutions which are viable models to describe the early Universe.Comment: 14 pages, LaTeX, 3 Postscript figures, uses graficx.st
The Consistency of Causal Quantum Geometrodynamics and Quantum Field Theory
We consider quantum geometrodynamics and parametrized quantum field theories
in the framework of the Bohm-de Broglie interpretation. In the first case, and
following the lines of our previous work [1], where a hamiltonian formalism for
the bohmian trajectories was constructed, we show the consistency of the theory
for any quantum potential, completing the scenarios for canonical quantum
cosmology presented there. In the latter case, we prove the consistency of
scalar field theory in Minkowski spacetime for any quantum potential, and we
show, using this alternative hamiltonian method, a concrete example where
Lorentz invariance of individual events is broken.Comment: Final version. See also http://www.cosmologia.cbpf.b
Spiral Galaxies Rotation Curves with a Logarithmic Corrected Newtonian Gravitational Potential
We analyze the rotation curves of 10 spiral galaxies with a newtonian
potential corrected with an extra logarithmic term, using a disc modelization
for the spiral galaxies. There is a new constant associated with the extra term
in the potential. The rotation curve of the chosen sample of spiral galaxies is
well reproduced for a given range of the new constant. It is argued that this
correction can have its origin from string configurations. The compatibility of
this correction with local physics is discussed.Comment: Latex file, 6 pages, 20 figure