Repositório RCAAP

The dynamics of closed LFRW universes with a massive inflaton field is examined where Friedmann equations are corrected by the introduction of a potential term arising from quantum gravity corrections to cosmological scenarios near the singularity. This extra term implements nonsingular bounces in the early evolution of the universe. For certain windows in the parameter space (labeled by the scalar field mass and the conserved Hamiltonian), phenomena of nonlinear resonance take place. Nonlinear resonance may induce the destruction of KAM tori that trap the inflaton, leading to a rapid growth of the scale factor and consequent escape of the universe into inflation. We make a complete analysis of the nonlinear resonance phenomena and show that windows of parametric resonance, characterized by an integer n > 2, are the ones that strongly favour inflation in the system. We discuss how generic is this behaviour for inflationary models.

We present some cosmological solutions of Brans-Dicke theory, characterized by a decaying vacuum energy density and by a constant relative matter density. With these features, they shed light on the cosmological constant problems, leading to a presently small vacuum term, and to a constant ratio between the vacuum and matter energy densities. By fixing the only free parameter of our solutions, we obtain cosmological parameters in accordance with observations of the relative matter density, the universe age and redshift-distance relations.

We obtain exact solutions for a static and charged cosmic string in a Einstein-Maxwell-Dilaton theory of a scalar-tensor type in (3+1)-Dimensions. This theory is specified by the dilaton field phi, the graviton field gµn and the electromagnetic field Fµn, and one post-Newtonian parameter alpha(phi). It contains three different cases, each of them corresponding to a particular solution of the Rainich algebra for the Ricci tensor.

We discuss the gravitomagnetism in the context of scalar-tensor theories of gravity. We obtain the equation of motion of a particle in terms of gravitoelectric and gravitomagnetic fields. We discuss the gravitomagnetic time delay and the Lense-Thirring effect in the context of scalar-tensor theories of gravity. In the particular case of Brans-Dicke Theory, we compare the results obtained with those predicted by general relativity and show that within the accuracy of experiments designed to measure these effects, both theories predict essentially the same results.

Given the wealth of increasingly accurate cosmological observations, especially the recent results from the WMAP, and the development of methods and strategies in the search for cosmic topology, it is reasonable to expect that we should be able to detect the spatial topology of the Universe in the near future. Motivated by this, we examine to what extent a possible detection of a nontrivial topology of positively curved universe may be used to place constraints on the matter content of the Universe. We show through concrete examples that the knowledge of the spatial topology allows to place constraints on the density parameters associated to dark matter (omegam) and dark energy (<FONT FACE=Symbol>W L</FONT>).

We discuss Gödel's universe in the context of the induced-matter theory. We show that the problem of generating Gödel's metric from an extra dimension is equivalent to finding an embedding of Gödel's universe in a Ricci-flat five-dimensional space. On the other hand, according to the Campbell-Magaard theorem, any spacetime can be locally embedded into a five-dimensional pseudo-Riemannian Ricci-flat manifold. We obtain explicitly a global embedding of Gödel's universe which is Ricci-flat and has a non-Lorentzian signature of type (++---).

We study the evolution of cosmological magnetic fields in FRW models with curved spatial sections and outline a geometrical mechanism for their superadiabatic amplification on large scales. The mechanism operates within standard electromagnetic theory and applies to FRW universes with open spatial sections. We discuss the general relativistic nature of the effect and show how it modifies the adiabatic magnetic evolution by reducing the depletion rate of the field. Assuming a universe that is only marginally open today (i.e. for 1-omega0 ~ 10-2), we estimate the main features of the superadiabatically amplified residual field and find that is of astrophysical interest.

We have developed an algorithm that numericaly computes the dimension of an extremely inhomogeneous matter distribution, given by a discrete hierarchical metric. With our results it is possible to analise how the dimension of the matter density tends to d = 3 , as we consider larger samples.

In spite of the macroscopic character of the fluctuation amplitudes, we show that the standard inflationary distribution of primordial density fluctuations still exhibits inherently quantum mechanical correlations (which cannot be mimicked by any classical stochastic ensemble). To this end, we propose a Gedanken experiment for which certain Bell inequalities are violated. We also compute the effect of decoherence and show that the violation persists provided that the decoherence lies below a certain non-vanishing threshold. Moreover, there exists a higher threshold above which no violation of any Bell inequalities can occur, so that the corresponding distributions can be interpreted as stochastic ensembles of classical fluctuations.

We consider an electric charge rotating around a Schwarzschild black hole. We compute, using quantum field theory in curved spacetime at the tree level, the power emitted by the rotating charge minimally coupled to the Maxwell field. We also compute how much of the radiation emitted by the swirling charge is absorbed by the black hole.

We analyze the scalar radiation emitted by a source in uniform circular motion in Minkowski spacetime interacting with a massive Klein-Gordon field. We assume the source rotating around a central object due to a Newtonian force. By considering the canonical quantization of this field, we use perturbation theory to compute the radiation emitted at the tree level. Regarding the initial state of the field as being the Minkowski vacuum, we compute the emission amplitude for the rotating source, assuming it as being minimally coupled to the massive Klein-Gordon field. We then compute the power emitted by the swirling source as a function of its angular velocity, as measured by asymptotic static observers.

Renormalization Group (RG) is a powerful method for investigating quantum effects of matter fields on curved background. The formalism of RG in curved space is well known since 1984, but its applications to cosmology and black hole physics require more knowledge and opens a new interesting field of study. We review recent results about the derivation of renormalization group in a mass-dependent scheme and also consider ambiguities of conformal anomaly using dimensional and covariant Pauli-Villars regularizations.

Using a gauge-invariant formalism we find the density contrast equation in a cosmological scenario with particle production at the expenses of the gravitational field (open system cosmology). First, we find the modes for the density contrast considering that the particle production process participate of the inhomogeneities formation, and in a second phase the creation process mimics the inclusion of the a smooth L term , in the sense that it is not affects small deviations from homogeneity. The cosmic background has an accelerated regime, where an additional pressure due to the creation process is responsible for the cosmic acceleration. We study in this work if the creation process of particles in the cosmic fluid contributes to the inhomogeneities formation in an accelerated universe.

We study the behavior of a non-relativistic quantum particle interacting with different potentials, in the background space-time generated by a cosmic string. We find the energy spectra for the quantum systems under consideration and discuss how they differ from their flat Minkowski space-time values.

We investigate the role of the Gauss-Bonnet term for the n = 4 and n = 4-e renormalization group, for both conformal and general versions of the theory. The cancellation of the quantum effects of the Gauss-Bonnet term in the n = 4 limit represents an efficient test for the correctness of previous calculations and also resolves two long-standing problems concerning quantum corrections in quantum gravity. In the case of n = 4-epsilon renormalization group there is a number of new nontrivial fixed points, that may indicate to a rich nonperturbative structure of the theory. At the same time, if we do not treat e as a small parameter, the renormalization group is spoiled by an extensive gauge fixing ambiguity.

We calculate the Casimir energy associated with abelian gauge fields in real compact hyperbolic spaces. The cosmological applications of the vacuum energies are briefly considered.

In the present work, we use the formalism of quantum general relativity in order to quantize a Friedmann-Robertson-Walker model in the presence of a negative cosmological constant and radiation. The model has spatial sections with positive constant curvature. The wave-function of the model satisfies a Wheeler-DeWitt equation, for the scale factor, which has the form of the Schrödinger's equation for the quartic anharmonic oscillator. We find the eigenvalues and eigenfunctions by using a method first developed by Chhajlany and Malnev. After that, we use the eigenfunctions in order to construct wave-packets for evaluating the time-dependent, expected value of the scale factor. We find that, the expected value of the scale factor oscillates between maximum and minimum values. Since the scale factor never vanishes, we conclude that the model does not have a singularity.

We study the behaviour of relativistic quantum particles in the space-time generated by a moving mass current, in the weak field approximation. We solve the Dirac equation in this gravitational field and calculate the current associated with the particles.

Black holes acting as gravitational lenses produce, besides the primary and secondary weak field images, two infinite sets of relativistic images. These images can be studied using the strong field limit, an analytic method based on a logarithmic asymptotic approximation of the deflection angle. In this work, braneworld black holes are analyzed as gravitational lenses in the strong field limit and the feasibility of observation of the images is discussed.

We consider a six-dimensional braneworld model and we study the cosmological evolution of a (4+1) brane-universe. Introducing matter on the brane we show that the scale factor of the physical three-dimensional brane-universe is related to the scale factor of the fourth dimension on the brane, and the suppression of the extra dimension compared to the three dimensions requires the presence of dark energy.