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

Hydration status and associated dietary factors in children

[resumo][abstract

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 $f(R)$ 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