1,371 research outputs found
Large classical universes emerging from quantum cosmology
It is generally believed that one cannot obtain a large Universe from quantum
cosmological models without an inflationary phase in the classical expanding
era because the typical size of the Universe after leaving the quantum regime
should be around the Planck length, and the standard decelerated classical
expansion after that is not sufficient to enlarge the Universe in the time
available. For instance, in many quantum minisuperspace bouncing models studied
in the literature, solutions where the Universe leave the quantum regime in the
expanding phase with appropriate size have negligible probability amplitude
with respect to solutions leaving this regime around the Planck length. In this
paper, I present a general class of moving gaussian solutions of the
Wheeler-DeWitt equation where the velocity of the wave in minisuperspace along
the scale factor axis, which is the new large parameter introduced in order to
circumvent the abovementioned problem, induces a large acceleration around the
quantum bounce, forcing the Universe to leave the quantum regime sufficiently
big to increase afterwards to the present size, without needing any classical
inflationary phase in between, and with reasonable relative probability
amplitudes with respect to models leaving the quantum regime around the Planck
scale. Furthermore, linear perturbations around this background model are free
of any transplanckian problem.Comment: 8 pages, 1 figur
The Unruh effect under the de Broglie-Bohm perspective
We investigate the Minkowski ground state associated with a real massless
scalar field as seen by an accelerated observer under the perspective of the de
Broglie-Bohm quantum theory. We use the Schr\"odinger picture to obtain the
wave functional associated with the Minkowski vacuum in Rindler coordinates,
and we calculate the field trajectories through the Bohmian guidance equations.
The Unruh temperature naturally emerges from the calculus of the average
energy, but the Bohmian approach precisely distinguishes between its quantum
and classical components, showing that they periodically interchange their
roles as the dominant cause for the temperature effects, with abrupt jumps in
the infrared regime. We also compute the power spectra, and we exhibit a very
special Bohmian field configuration with remarkable physical properties.Comment: 32 pages, 6 figure
Quantum Cosmology in Scalar-Tensor Theories With Non Minimal Coupling
Quantization in the minisuperspace of non minimal scalar-tensor theories
leads to a partial differential equation which is non separable. Through a
conformal transformation we can recast the Wheeler-DeWitt equation in an
integrable form, which corresponds to the minimal coupling case, whose general
solution is known. Performing the inverse conformal transformation in the
solution so found, we can construct the corresponding one in the original
frame. This procedure can also be employed with the bohmian trajectories. In
this way, we can study the classical limit of some solutions of this quantum
model. While the classical limit of these solutions occurs for small scale
factors in the Einstein's frame, it happens for small values of the scalar
field non minimally coupled to gravity in the Jordan's frame, which includes
large scale factors.Comment: latex, 18 page
Metodologia para tratamento de clorofórmio comercial para uso em separações cromatográficas.
bitstream/item/74946/1/pub-145.pd
Energy-momentum Density of Gravitational Waves
In this paper, we elaborate the problem of energy-momentum in general
relativity by energy-momentum prescriptions theory. Our aim is to calculate
energy and momentum densities for the general form of gravitational waves. In
this connection, we have extended the previous works by using the prescriptions
of Bergmann and Tolman. It is shown that they are finite and reasonable. In
addition, using Tolman prescription, exactly, leads to same results that have
been obtained by Einstein and Papapetrou prescriptions.Comment: LaTeX, 9 pages, 1 table: added reference
Dynamical Casimir effect with Dirichlet and Neumann boundary conditions
We derive the radiation pressure force on a non-relativistic moving plate in
1+1 dimensions. We assume that a massless scalar field satisfies either
Dirichlet or Neumann boundary conditions (BC) at the instantaneous position of
the plate. We show that when the state of the field is invariant under time
translations, the results derived for Dirichlet and Neumann BC are equal. We
discuss the force for a thermal field state as an example for this case. On the
other hand, a coherent state introduces a phase reference, and the two types of
BC lead to different results.Comment: 12 page
A Born-Infeld-like f(R) gravity
Several features of an theory in which there is a maximum value for
the curvature are analyzed. The theory admits the vaccuum solutions of GR, and
also the radiation evolution for the scale factor of the standard cosmological
model. Working in the Jordan frame, a complete analysis of the phase space is
performed, and its results supported with examples obtainted by numerical
integration. In particular, we showed that theory has nonsingular cosmological
solutions which after the bounce enter a phase of de Sitter expansion and
subsequently relax to a GR-like radiation-dominated evolution.Comment: Latex file, 14 pages, 7 figures (jpg format), including more detailed
discussions than previous version, accepted for publication in Physical
Review
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