9,376 research outputs found
Qualitative analysis of a scalar-tensor theory with exponential potential
A qualitative analysis of a scalar-tensor cosmological model, with an
exponential potential for the scalar field, is performed. The phase diagram for
the flat case is constructed. It is shown that solutions with an initial and
final inflationary behaviour appear. The conditions for which the scenario
favored by supernova type Ia observations becomes an attractor in the space of
the solutions are established.Comment: Latex file, 9 pages, 1 figur
Does Quantum Cosmology Predict a Constant Dilatonic Field?
Quantum cosmology may permit to determine the initial conditions of the
Universe. In particular, it may select a specific model between many possible
classical models. In this work, we study a quantum cosmological model based on
the string effective action coupled to matter. The Schutz's formalism is
employed in the description of the fluid. A radiation fluid is considered. In
this way, a time coordinate may be identified and the Wheeler-DeWitt equation
reduces in the minisuperspace to a Schr\"odinger-like equation. It is shown
that, under some quite natural assumptions, the expectation values indicate a
null axionic field and a constant dilatonic field. At the same time the scale
factor exhibits a bounce revealing a singularity-free cosmological model. In
some cases, the mininum value of the scale factor can be related to the value
of gravitational coupling.Comment: Latex file, 14 page
Quantum cosmological perfect fluid model and its classical analogue
The quantization of gravity coupled to a perfect fluid model leads to a
Schr\"odinger-like equation, where the matter variable plays the role of time.
The wave function can be determined, in the flat case, for an arbitrary
barotropic equation of state ; solutions can also be found for
the radiative non-flat case. The wave packets are constructed, from which the
expectation value for the scale factor is determined. The quantum scenarios
reveal a bouncing Universe, free from singularity. We show that such quantum
cosmological perfect fluid models admit a universal classical analogue,
represented by the addition, to the ordinary classical model, of a repulsive
stiff matter fluid. The meaning of the existence of this universal classical
analogue is discussed. The quantum cosmological perfect fluid model is, for a
flat spatial section, formally equivalent to a free particle in ordinary
quantum mechanics, for any value of , while the radiative non-flat case
is equivalent to the harmonic oscillator. The repulsive fluid needed to
reproduce the quantum results is the same in both cases.Comment: Latex file, 13 page
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