Search

Only 5% of what makes up the Universe is well understood, and it consists of baryonic matter and radiation. Dark matter and energy correspond to the remaining 95% of the Universe, and their origin and evolution have not yet been satisfactorily explained. Dark matter, supposedly present in the galaxies' halo region, appears to be the mechanism that causes the unusual behavior of the stars' tangential velocity, which is higher than that predicted by the interaction with visible matter. With the solutions of the equations of motion for spacetime generated by a spinning cosmic string with an internal structure in Brans-Dicke gravitation, the present work aimed to evaluate whether this type of string can play the role currently defined as that of dark matter, which is to originate the typical rotational curves of galaxies, responsible for maintaining the tangential velocities of the stars that form these galaxies, whose behavior cannot be justified solely by the observed baryonic matter. For this, the model was used to obtain the velocities of the stars of 4 Sc-type galaxies, to be compared with their respective experimentally observed values., Comment: 11 pages, 2 figures, text in Portuguese, work presented at the 23a. Mostra de Ensino, Pesquisa e Extens\~ao, IFRS, C\^ampus Porto Alegre, 2023

This review presents an introduction to Quantum Cosmology, including the mathematical methods essential to the canonical approach, some of the existing conceptual problems and the connection of the models to possible observables., Comment: 11 pages, in Portuguese, 3 figures, version accepted for publication in Cadernos de Astronomia

In this work the evolution of two fields of matter in planar symmetric black holes D-dimensional with symmetry of Lifshitz whose dynamic exponent is $z$ were analyzed. The fields investigated were a scalar field non-minimal coupled to the Einstein tensor and the Ricci scalar and the electromagnetic field. Two black holes were chosen, one with 5-dimensions and z=1 and another with 6-dimensions and $z=0$. The equations of motion for both fields were developed in general and applied to each black hole mentioned. In some cases exact solutions of the equations were obtained in terms of hypergeometric functions and confluent Heun functions. The quasinormal modes (MQN) of evolution of the analyzed fields were calculated numerically using two different approaches: HH and AIM. In all the black holes studied the NSM's did not indicate instabilities neither regarding the couplings nor the space-times. In general, the quasinormal modes behave like the oscillation modes of a damped harmonic oscillator, presenting three regimes: underdamped $(\omega_R \neq0, \omega_I <0) $, critically damped $ (\omega_R=0,\omega_I^{crit} <0) $ and overdamped $ (\omega_R=0,\omega_I <0) $. In the analyzed cases an interesting behavior was found for $\omega_I $ when $ k \neq0 $ and $ r_+<10 $. In general, $\omega_I $ increase with $k$, but here we find a quadratic growth for $ k\sim r_+$ and then a decrease when $k>>r_+$. This behavior may be relevant in the context of gravity/gauge duality. Additionally, we also analyzed the evolution of the same fields in two black holes with symmetry of Lifshtiz in $ 2 + 1 $ dimensions., Comment: Master dissertation (in Portuguese), Advisor: Prof. Dr. Alan Pavan, 117 pages, 37 figures

In this thesis, we approached the applications of the John sector of the Horndeski gravity in the scenarios of black brane, braneworld and cosmology. Specially, we study AdS/CFT correspondence in the context of the Horndeski gravity to evaluate the viscosity/entropy density ratio, as a function of the fixed exponent and parameters of theory. In the braneworld, we address the issue of finding braneworld solutions in a five-dimensional Horndeski gravity and discuss gravity localization. Finally, in the cosmological scenario we finding solutions by using first-order formalism in the Horndeski gravity that constrains the superpotential and implies that no free choice of scalar potential is allowed. The scalar field equation of state reflects well the cosmological scenario at the present time and allows us to conclude that it can simulate the dark energy in the Horndeski gravity., Comment: 83 pages, in Portuguese, 20 figures

Is it allowed, in the context of the Lagrange multiplier formalism, to assume that nonholonomic constraints are already in effect while setting up Lagrange's function? This procedure is successfully applied in a recent book [L. N. Hand and J. D Finch, {\it Analytical Mechanics}] to the problem of the rolling penny, but it does not work in general, as we show by means of a counterexample. It turns out that in many cases the use of nonholonomic constraints in the process of construction of the Lagrangian is allowed, but the correct equations of motion are the little known Voronec's equations., Comment: Translation of the paper "Vinculos Nao-Holonomos e Equacoes de Voronec", to be published in Portuguese in Revista Brasileira de Ensino de Fisica

In this work we consider a model for gravity in 4-dimensional space-time originally proposed by A. Chamseddine which may be derived by a 5-dimensional Chern-Simons theory. Its topological origin makes it an interesting candidate for an easier quantization, e.g., in loop quantization framework. The present work is dedicated to classical physical consequences and canonical analysis of the model. Cosmological solutions as well as wave-like solutions were obtained and compared with the corresponding Einstein's General Relativity with cosmological constant., Comment: Doctoral thesis, in Portuguese

The Lounesto spinor classification is an important tool in fundamental physics, because it makes explicit the pleiade of spinors types, beyond the used in quantum field theory (QFT). In this work, we show how the classification emerges in two topics: first we show that the bilinear covariants arise naturally in the equation of motion for a fermionic field in a Riemann-Cartan background (with gravitational dynamics f(R)) and we identify a singular spinor field (a flag-dipole one) as a solution of the presented model. This is the first solution flag-dipole founded in the literature. We investigated the behavior of the bilinear covariants in the context of the called standard model extension of Colladay and Kostelecky and we present examples of transformations that violate the Lorentz symmetry, providing an extension of spinors classification, according to bilinear covariants, to scenarios with broken Lorentz symmetry. Finally, we proved that singular spinors of type flagpole are less susceptible to effects of Lorentz violation., Comment: 105 pages, Ph.D. Thesis, Federal University of ABC, Brazil, February 13th, 2017, in Portuguese. Two chapters expand the results in our arXiv:1302.2262 and arXiv:1607.08670. arXiv admin note: text overlap with arXiv:1007.0999 by other author

The recent observation of gravitational waves confirms one of the most interesting predictions in general relativity: the black holes. Because the gravitational waves detected by LIGO fit very well within general relativity as a phenomenon produced by two colliding black holes. Then the reality of black holes seems almost undoubted today. However, a stronger proof on the reality of black holes would be indicated by the observation of the event horizon, which is what defines it. In this article, it is indicated that only the event horizon defines a black hole. There is no mention to the singularity in its definition. Thus, it will be shown that black holes without a singularity are possible. Such black holes are called regular black holes., Comment: 7 pages. Introductory article in Portuguese. Published in Revista Brasileira de Ensino de Fisica

General Relativity is the modern theory of gravitation. It has replaced the newtonian theory in the description of the gravitational phenomena. In spite of the remarkable success of the General Relativity Theory, the newtonian gravitational theory is still largely employed, since General Relativity, in most of the cases, just makes very small corrections to the newtonian predictions. Moreover, the newtonian theory is much simpler, technically and conceptually, when compared to the relativistic theory. In this text, we discuss the possibility of extending the traditional newtonian theory in order to incorporate typical relativistic effects, but keeping the simplicity of the newtonian framework. We denominate these extensions neo-newtonian theories. These theories are discussed mainly in the contexts of cosmology and compact astrophysical objects., Comment: Latex file, 18 pages, 5 figures in eps format. To appear in Mens Agitat journal, proceedings of the Encontro de F\'isica da Amaz\^onia Caribenha, held in Boa Vista, Roraima, Brazil, from 28 to 31 october 2014. In portuguese

In this Thesis we will explicitly construct Hadamard states with different features. In the two first cases, the states will be constructed in static and in cosmological spacetimes. In one of the cases, the states will be constructed in such a way as to minimize the expectation value of the energy-momentum tensor of a free massive scalar field, multiplied by a suitable smooth function of compact support, in a cosmological spacetime. The final form of the state clearly reduces to the vacuum in a static spacetime. Besides, we will show that these are Hadamard states. They do not contradict the above mentioned nonexistence of a unique prescription because they depend on the chosen smooth function. In the other case, we will calculate the positive part of the spectral decomposition of the commutator function in a specific subset of the spacetime and will show that, in static and in cosmological spacetimes, the kernel of this function, once again multiplied by a suitable smooth function of compact support, corresponds to the kernel of Hadamard states. In both cases, the construction of the states does not rely on invariance under the action of a group of symmetries. Such a group may not even exist, in these cases. In the last case we will construct a state in the Schwarzschild-de Sitter spacetime, which describes a universe with a black hole and a cosmological constant. This state is invariant under the action of the group of symmetries of this spacetime, although it is not defined in its whole Kruskal extension. In this way, we avoid the situation in which the authors of (KayWald91) showed that a Hadamard state cannot exist. We will show that the state we construct is Hadamard and that it is not thermal, differently from the case of states constructed in spacetimes containing only one event horizon (DappiaggiMorettiPinamonti09,GibbonsHawking77)., Comment: PhD Thesis. 129+16 pages. Cataloging in Publication and cover information in Portuguese. Abstract in both Portuguese and English. arXiv admin note: text overlap with arXiv:0907.1034 by other authors

This work deals with the analysis of cylindrically symmetric and stationary space-times $\mathcal{C}_{t}$ with closed timelike curves. The equation of motion describing the evolution of a massive scalar field in a $\mathcal{C}_{t}$ space-time is obtained. A class of space-times with closed timelike curves describing cosmic strings and cylinders is studied in detail. In such space-times, both massive particles as well as photons can reach the non-causal region. Geodesics and closed timelike curves are calculated and investigated. We have observed that massive particles and photons describe, essentially, two kinds of trajectories: confined orbits and scattering states. The analysis of the light cones show us clearly the intersection between future and past inside the non-causal region. Exact solutions for the equation of motion of massive scalar field propagating in cosmic strings and cylinder space-times are presented. Quasinormal modes for the scalar field have been calculated in static and rotating cosmic cylinders. We found unstable modes in the rotating cases. Rotating as well as static cosmic strings, i.e., without regular interior solutions, do not display quasinormal modes for the scalar field. We conclude presenting a conjecture relating closed timelike curves and space-time instability., Comment: PhD thesis (in Portuguese), Advisor: Prof. Dr. Elcio Abdalla, 155 pages, 31 figures, May 2010

First and second order corrections for the scattering of different types of particles by a weak gravitational field, treated as an external field, are calculated. These computations indicate a violation of the Equivalence Principle: to first order, the cross-sections are spin dependent; if the calculations are pushed to the next order, they become dependent upon energy as well. Interesting enough, the aforementioned results are equivalent to those obtained by means of the so-called Effective Theory of Gravitation, in the limit in which one of the masses is much greater than all the other energies involved. We discuss also some applications of our research, such as the determination of an upper bound for the photon mass, and the possible detection, in the foreseeable future, of these violations of the Equivalence Principle., Comment: 58 pages, 3 figures, Ph.D. Thesis (Advisor: Antonio Accioly), in portuguese, uses axodraw

We elaborate a detailed study of certain aspects of (a version of) the AdS/CFT correspondence, conjectured by Maldacena and Witten, between quantum field theories in a gravitational background given by an asymptotically anti-de Sitter (AAdS) spacetime, and conformally covariant quantum field theories in the latter's conformal infinity (in the sense of Penrose), aspects such that: (a) are independent from (the pair of) specific models in Quantum Field Theory, and (b) susceptible to a recast in a mathematically rigorous mould. We adopt as a starting point the theorem demonstrated by Rehren in the context of Local Quantum Physics (also known as Algebraic Quantum Field Theory) in anti-de Sitter (AdS) spacetimes, called algebraic holography or Rehren duality. The main body of the present work consists in extending Rehren's result to a reasonably general class of d-dimensional AAdS spacetimes (d>3), scrutinizing how the properties of such an extension are weakened and/or modified as compared to AdS spacetime, and probing how non-trivial gravitational effects manifest themselves in the conformal infinity's quantum theory. The language employed for the quantum theories relevant for our generalization of Rehren duality follows the functorial formulation of Local Quantum Physics due to Brunetti, Fredenhagen and Verch, extended afterwards by Sommer in order to incorporate boundary conditions., Comment: English translation (with corrections) of the author's Ph.D. thesis (University of S. Paulo, Brazil, September 2007), originally written in Portuguese - see comments in page ii for more details. 201 pages, 7 figures. V3: an incorrect and ultimately irrelevant construction in Subsection 2.3.1 replaced by appropriate amendments, 1 extra reference and erratum

Research on black holes and their physical proprieties has been active on last 90 years. With the appearance of the String Theory and the Braneworld models as alternative descriptions of our Universe, the interest on black holes, in these context, increased. In this work we studied black holes in Braneworld models. A class of spherically symmetric black holes is investigaded as well its stability under general perturbations. Thermodynamic proprieties and quasi-normal modes are discussed. The black holes studied are the SM (zero mass) and CFM solutions, obtained by Casadio {\it et al.} and Bronnikov {\it et al.}. The geometry of bulk is unknown. However the Campbell-Magaard Theorem guarantees the existence of a 5-dimensional solution in the bulk whose projection on the brane is the class of black holes considered. They are stable under scalar perturbations. Quasi-normal modes were observed in both models. The tail behavior of the perturbations is the same. The entropy upper bound of a body absorved by the black holes studied was calculated. This limit turned out to be independent of the black hole parameters., Comment: Master Degree thesis (in portuguese), Advisor: Prof. Dr. Elcio Abdalla, 126 pages, 16 figures

This dissertation aims at studying the braneworld models in the context proposed by Randall and Sundrum. The focus is on the spin-0 perturbations in the Kerr space-time as a 4-dimensional braneworld. The work deals the main aspects of Einstein General Relativity as well as perturbations of black holes metrics. We also review the Randall-Sundrum models and their motivations and attempts to describe braneworld black holes. In the end the Kerr-Randall-Sundrum black string scalar perturbation and superradiance are obtained., Comment: Master Degree Thesis (Advisor: Elcio Abdalla), 110 pages, 7 figures, in portuguese

In this thesis the Bohm-de Broglie interpretation of quantum mechanics is applied to canonical quantum gravity. It is shown that, irrespective of any regularization or choice of factor ordering of the Wheeler-DeWitt equation, the unique relevant quantum effect which does not break spacetime is the change of its signature from lorentzian to euclidean. The other quantum effects are either trivial or break the four-geometry of spacetime. A Bohm-de Broglie picture of quantum geometrodynamics is constructed, which allows the investigation of these latter structures. For instance, it is shown that any real solution of the Wheeler-De Witt equation yields a generate four-geometry compatible with the strong gravity limit of General Relativity and the Carroll group. We prove that quantum geometrodynamics in the Bohm-de Broglie interpretation is consistent for any quantum potential. As a previous step to introduce our metodology, we study the quantum theory of fields in Minkowski spacetime in the Bohm-de Broglie interpretation and exhibit a concrete example where Lorentz invariance of individual events is broken., Comment: PhD Thesis at CBPF-Rio de Janeiro-May 2000 (Advisor: N. Pinto-Neto, 95 pages, in Portuguese)

We study the gravi-dilaton field of a Schwarzschild black hole pierced by a thin cosmic string in both massless and massive dilatonic gravity. We conclude that in the thin vortex approximation the string's spacetime is asymptotically flat with a conical deficit angle and that the inertial mass of the black hole is different from the gravitational one. We generalize our results to charged black holes., Comment: Poster. 5 pages in Portuguese. Published in Actas VIII National Meeting in Astronomy and Astrophysics, IST - Porto, 1998