Submitted by Leonardo Cavalcante (leo.ocavalcante@gmail.com) on 2018-04-30T14:18:12Z No. of bitstreams: 1 Arquivototal.pdf: 1310917 bytes, checksum: 54733929b9cb3c26ca0a0dc1adb0ae42 (MD5) / Made available in DSpace on 2018-04-30T14:18:12Z (GMT). No. of bitstreams: 1 Arquivototal.pdf: 1310917 bytes, checksum: 54733929b9cb3c26ca0a0dc1adb0ae42 (MD5) Previous issue date: 2015-02-27 / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / Modifed theories of gravity has the objective to elucidate some open issues in the context of general relativity. From this perspective many theories have been proposed over the years. In this paper we deal with some of these theories and, for each of them, we gave our contribution proposing new scenarios that showed promise. So in the context of braneworld theory we held the first studies in three dimensional DGP theory, with terms of induced gravity of the new massive gravity type. The study of the propagators of disturbed theory showed unit propagation modes without the presence of tachyons. As regards to the Horava-Lifshitz theory, we studied the structure of membranes with scalar fields in 4+1 dimensions and the formation of black holes in 1+1 dimensions has been obtained for the first time. In the two previous cases we deal with nonprojectable version of this theory. Have also studied braneworld models in a bimetric theory where gravity is coupled to the scalar field that connects the two metrics. Although the scalar field having a linear term kinetic the theory remained stable and gravity localization was obtained. Finally, we apply the theory of gravitation with nondynamical fields to a cosmological model and found that for a certain specific stage of the Universe Expansion is possible to have a mechanism that generates a self tuning of the cosmological constant problem. / Teorias de gravidade modificada tem como objetivo elucidar alguns problemas em aberto no âmbito da Relatividade Geral. Dentro desta perspectiva muitas teorias foram propostas ao longo dos anos. Neste trabalho abordamos algumas destas teorias e, para cada uma delas, demos a nossa contribuição propondo novos cenários que se mostraram promissores. Assim, no contexto das teorias de mundo-brana estudamos pela primeira vez teorias do tipo DGP em 2+1 dimensões, com termos de gravidade induzida do tipo nova gravidade massiva. O estudo dos propagadores da teoria perturbada mostrou a propagação de modos unitários e sem a presença de táquions. No que diz respeito à teoria de Horava-Lifshitz, estudamos a estrutura de branas com campos escalares em 4+1 dimensões e a formação de buracos em 1+1 foi obtida pela primeira vez. Nos dois casos anteriores tratamos da versão não projetável desta teoria. Estudamos ainda modelos de mundo-brana em uma teoria com bimétrica onde a gravidade se acopla ao campo escalar que conecta as duas métricas. Apesar de o campo escalar possuir um termo cinético linear a teoria se mostrou estável e localização da gravidade foi obtida. Por fim, aplicamos a teoria da gravitação com campos não dinâmicos a um modelo cosmológico e constatamos que para certa fase específica da expansão do Universo é possível termos um mecanismo que gera um auto ajuste da constante cosmológica.

Neste trabalho aplicamos métodos espectrais para a determinação da configuração inicial de três espaços-tempos contendo buracos negros. Para isto apresentamos primeiro a foliação do espaço-tempo em hipersuperfícies tridimensionais espaciais parametrizadas pela função temporal t. Este processo é chamado de decomposição 3+1 [2] [5]. O resultado deste processo são dois conjuntos de equações classificadas em equações de vínculo e evolução [4]. As equações de vínculo podem ser divididas em vínculos Hamiltoniano e dos momentos. Para a obtenção dos dados iniciais dos problemas estudados aqui, apenas a equação de vínculo Hamiltoniano será resolvida numericamente, pois as equações de vínculo dos momentos possuem solução analítica nestes casos. Uma pequena descrição dos métodos espectrais é apresentada, destacando-se os método de Galerkin, método pseudoespectral ou de colocação e método de Tau, que são empregados na resolução das equações de vínculo Hamiltoniano dos problemas estudados. Verificamos que os resultados obtidos neste trabalho superam aqueles produzidos por Kidder e Finn [15], devido a uma escolha diferente das funções de base, que aqui satisfazem uma das condições de contorno. / In this work we apply spectral methods for determining the initial configuration of three spacetimes containing black holes. For this we present first the foliation of spacetime into three-dimensional spacelike hypersurfaces parameterized by the time function t. This process is called 3 + 1 decomposition [2] [5]. The result of this process are two sets of equations classified into constraint and evolution equations [4]. The constraint equations can be divided into Hamiltonian and momentum constraints.To obtain the initial data of the problems studied here, only the Hamiltonian constraint is solved numerically, since the momentum constraint of these cases have analytical solution. A short description of spectral methods is presented, highlighting Galerkin method, pseudospectral or collocation method and Tau method, which are employed in solving the constraint equations Hamiltonian of the problems studied. We found that the results obtained in this work outperform those produced by Kidder and Finn [15], due to a different choice of basis functions, which meet here one of the boundary conditions.

Orientador: Prof. Dr. Roldão da Rocha Jr. / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2017. / Neste trabalho foram investigadas algumas conseguências da física de buracos negros em teorias cujo domínio está além do domínio da relatividade geral, em especial em teorias efetivos com dimensões extras. A investigação foi em substancialmente conduzida baseando-se em três efeitos gravitacionais, a saber, a radiação Hawking, o regime de deflexão forte de lentes gravitacionais e a formação de buracos negros quânticos. Uma solução de modelo cosmológico imerso em uma brana espessa foi também investigada. Modelos e teorias efetivas fornecem meios para testar os limites de validade de teorias conhecidas e indicam o que deveríamos esperar além desses limites. Baseado nessa ideia foram usados alguns modelos efetivos para estudar efeitos não previstos pela relatividade geral, associados a cada um dos fenômenos mencionados. / This work is devoted to investigate some consequences of black holes physics beyond the domain of general relativity, mainly in effective extra dimensional models. The investigation is carried along three gravitational effects, namely the Hawking radiation, the strong deflection of gravitational lensing and the formation of quantum black holes. A cosmological thick brane solution is also investigated. Effective theories and models provide a prominent approach for testing the limits of known theories and show what would be expected beyond that. Based on such idea we have used effective models for finding deviations of general relativity associated to each of the mentioned phenomena.

Orientador: Prof. Dr. Vilson Tonin Zanchin / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, Santo André, 2018.

Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2015-06-09T18:28:55Z No. of bitstreams: 0 / Made available in DSpace on 2015-06-09T18:28:55Z (GMT). No. of bitstreams: 0 / A Terapia por Captura de Nêutrons pelo Boro (BNCT) é uma técnica radioterapêutica, que visa o tratamento de alguns tipos de câncer, em que sua energia útil é proveniente da reação nuclear promovida pela incidência de nêutrons térmicos no isótopo de 10B. No Brasil existe uma instalação, localizada junto ao canal de irradiação número 3 do Reator de Pesquisas IEA-R1 do IPEN, que foi projetada para o desenvolvimento de pesquisas em BNCT. Para uma aplicação adequada da técnica é necessário que o feixe de irradiação na posição de amostra, seja composto predominantemente por nêutrons térmicos com reduzida contaminação dos componentes do feixe, correspondente aos nêutrons epitérmicos e rápidos e à radiação gama. Este trabalho tem como objetivo monitorar e avaliar o feixe de irradiação na posição de irradiação de amostras, através do uso de detectores de ativação (folhas de ativação), e a partir de simulações utilizando o código de transporte de radiação, MCNP, avaliar mudanças na instalação, mais especificamente no conjunto de filtros e moderadores, para que se aprimore as condições de irradiação na instalação. O trabalho propos uma nova metodologia de cálculo para estudos de otimização do feixe a partir do recurso de redução de variância presente no MCNP, o wwg (weight window generation). Com os resultados obtidos através da adoção de um conjunto maior de folhas de ativação, foi possível a discriminação experimental do feixe de nêutrons em 5 faixas de energia e concluir que a instalação para estudos em BNCT do IPEN possui fluxo de nêutrons térmicos de 108 n/cm2.s, intensidade suficiente para que os estudos na área possam ser realizados com grande potencial de alteração de suas componentes conforme demanda experimental. / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Établir une théorie de gravité quantique qui décrit de manière cohérente les propriétés quantiques de la matière et de l'espace-temps est l'un des défis majeurs de la physique théorique. Malgré plusieurs décennies de recherches, de nombreux problèmes conceptuels et techniques doivent encore être résolus. L'étude de modèles simplifiés donne des idées de résolution. La première partie de la thèse traite de la gravité quantique bidimensionnelle. À deux dimensions, la gravité quantique est beaucoup mieux comprise et de nombreux calculs peuvent être faits exactement. Si la gravité quantique bidimensionnelle a été largement étudiée quand elle est couplée à de la matière conforme, le cas de la matière non-conforme était très peu connu jusque récemment. Nous calculons d'abord l'action gravitationnelle pour un champ scalaire massif sur une surface de Riemann avec bords puis pour un fermion de Majorana massif sur une variété compacte. Ce dernier cas correspond à une CFT perturbée par une perturbation conforme et est d'ordinaire étudié grâce à l'ansatz de DDK, mais les résultats sont différents. Finalement, on calcule le spectre de l'action de Mabuchi dans l'approximation du minisuperespace. La seconde partie étudie les propriétés thermales des trous noirs dans le contexte de la correspondance AdS/CFT. On construit un modèle de mécanique quantique fondé sur les principes holographiques pour simuler la dynamique des trous noirs quantiques. Ce modèle permet d'obtenir des résultats numériques exacts. / Finding a theory of quantum gravity describing in a consistent way the quantum properties of matter and spacetime geometry is one of the greatest challenges of modern theoretical physics. However after several decades of research, many conceptual and technical issues are still to be resolved. Insights on these questions can be given by simplified toy models that allow for exact computations. The first part of the thesis deals with two-dimensional quantum gravity. In two dimensions quantum gravity is much better understood and many computations can be carried out exactly. Whereas two-dimensional quantum gravity coupled to conformal matter has been widely studied and is now well understood, much less was known until recently when matter is non-conformal. First we compute the gravitational action for a massive scalar field on a Riemann surface with boundaries and then for a massive Majorana fermion on a manifold without boundary. The latter case corresponds to a CFT perturbed by a conformal perturbation and is usually tackled through the DDK ansatz, but the results do not seem to match. Finally we give a minisuperspace computation of the spectrum of the Mabuchi action, a functional that appears in the gravitational action for a massive scalar field. In the second part we focus on black hole thermal behaviour which provides a lot of insight of how a theory of quantum gravity should look like. In the context of string theory the AdS/CFT correspondence provides powerful tools for understanding the microscopic origin of black holes thermodynamics. We construct a quantum mechanical toy model based on holographic principles to study the dynamics of quantum black holes.

Les trous noirs supermassifs résident dans les centres de la plupart des galaxies massives et on observe des corrélations entre leurs masses et les propriétés de leurs galaxies hôtes. De plus, on observe des trous noirs de plus d’un milliard de masses solaires quelques centaines de millions d’années seulement après le Big Bang. Ces trous noirs supermassifs présents dans l’univers jeune ne sont que le sommet de l’iceberg de l’ensemble de la population de trous noirs, mais ils mettent en question notre compréhension de la formation et de la croissance des premiers trous noirs. Notre nouvelle méthode améliorant le calcul de la densité de colonne de H2 donne des probabilités pour former des graines massives de trous noirs qui sont plus d’un ordre de grandeur plus élevées que prédit précédemment. Nous trouvons que CR7 pourrait être le premier candidat à héberger un tel trou noir formé par effondrement direct et nous démentons l’existence initialement revendiquée d’une population stellaire massive primordial dans CR7. Nous calculons la densité des taux de fusion des trous noirs binaires des premières étoiles et leurs taux de détection avec aLIGO. Notre modèle démontre que les détections des ondes gravitationnelles à venir au cours des prochaines décennies permettront d’imposer des contraintes plus strictes sur les propriétés des premières étoiles et donc sur les scénarios de formation des premiers trous noirs. Nous développons un modèle analytique en 2D de la rétroaction des noyaux actifs de galaxie pour démontrer qu’un profil de disque plus réaliste réduit la quantité de gaz qui est éjectée du halo par rapport aux modèles 1D existants. La rétroaction empêche l’accretion de gaz sur le trou noir central pendant seulement ∼1 million d’année environ, ce qui permet une accretion de gaz presque continue dans le plan du disque. Avec cette thèse, je contribue à une meilleure compréhension de la formation et la croissance des premiers trous noirs supermassifs. / Supermassive black holes reside in the centres of most massive galaxies and we observe correlations between their mass and properties of the host galaxies. Besides this correlation between a galaxy and its central black hole (BH), we see BHs more massive than one billion solar masses already a few hundred million years after the Big Bang. These supermassive BHs at high redshift are just the tip of the iceberg of the entire BH population, but they challenge our understanding of the formation and growth of the first BHs. Our improved method to calculate H2 self-shielding yields probabilities to form massive seed BHs that are more than one order of magnitude higher, than previously expected. We find that CR7 might be the first candidate to host such a direct collapse BH and we disprove the initially claimed existence of a massive metal-free stellar population in CR7. We calculate the merger rate density of binary BHs from the first stars and their detection rates with aLIGO. Our model demonstrates that upcoming detections of gravitational waves in the next decades will allow to put tighter constraints on the properties of the first stars and therefore on formation scenarios of the first BHs. We develop a 2D analytical model of active galactic nuclei-driven outflows to demonstrate that a more realistic disc profile reduces the amount of gas that is ejected out of the halo, compared to existing 1D models. The outflow prevents gas accretion on to the central BH for only about ∼1Myr, which permits almost continuous gas inflow in the disc plane. With this thesis, I contribute to a better understanding of the formation and growth of the first supermassive BHs.

We present a catalog of the neutral atomic hydrogen structures (H I holes) and the analysis of their properties in nearby (≤ 10.3 Mpc) gas-rich dwarf galaxies of the LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, The H I Nearby Galaxy Survey) group. We used high sensitivity (≤ 1.1 mJy beam-1 channel-1), high velocity resolution (1.3 km s-1 to 2.6 km s-1) and high linear resolution (average ~110 pc; angular resolution ~6”) H I data of 37 dwarf irregulars and four blue compact dwarf galaxies. We cataloged H I holes in the entire sample and studied the of the properties of holes. We also investigated the effect of H I porosity on star formation, and the correlation of the star formation rate (SFR) calculated from H I holes with standard star formation tracers Hα and FUV. We detected 306 H I holes in LITTLE THINGS galaxies. We confirmed 22 kpc-sized holes, the largest and the smallest hole diameters are about 2.3 kpc and 38 pc (resolution limit) respectively. The expansion velocities of the holes range from 5 km s-1 (upper limit) to 30 km s-1, and the rotational velocities range from 6 km s-1 to 77 km s-1. The H I disk radii of the galaxies range from about 0.5 kpc to 6.7 kpc. The kinetic ages of the holes range from about 1 to 127 Myr, and the estimated scale heights are varying from 61 pc to 653 pc. The percentage distribution of the holes outside and inside the V-band break radius is nearly uniform, 49% and 51% respectively. In LITTLE THINGS galaxies, we found no obvious correlation between the surface and volume porosities, and SFR. However, two highest and two lowest porosity galaxies have no star formation at present. The holes are consistent with the SFR estimated from the energy required to create a hole and the star formation rates measured from Hα and FUV, indicating that the holes are consistent with a star formation origin.

This thesis concerns the use of observations of gravitational waves as tools for astronomy and fundamental physics. Gravitational waves are small ripples in spacetime produced by rapidly accelerating masses; their existence has been predicted for almost 100 years, but the first direct evidence of their existence came only very recently with the announcement in February 2016 of the detection by the LIGO and VIRGO collaborations. Part I of this thesis presents an introduction to gravitational wave astronomy, including a detailed discussion of a wide range of gravitational wave sources, their signal morphologies, and the experimental detectors used to observe them. Part II of this thesis concerns a particular data analysis problem which often arises when trying to infer the source properties from a gravitational wave observation. The use of an inaccurate signal model can cause significant systematic errors in the inferred source parameters. The work in this section concerns a proposed technique, called the Gaussian process marginalised likelihood, for overcoming this problem. Part III of this thesis concerns the possibility of testing if the gravitational field around an astrophysical black hole conforms to the predictions of general relativity and the cosmic censorship hypothesis. It is expected that the gravitational field should be well described by the famous Kerr solution. Two approaches for testing this hypothesis are considered; one using X-ray observations and one using gravitational waves. The results from these two approaches are compared and contrasted. Finally, the conclusions and a discussion of future prospects are presented in part IV of this thesis.

Gravitation is the dominant influence in most astrophysical interactions. Weak-field interactions have been extensively studied, but the strong-field regime remains largely unexplored. Gravitational waves (GWs) are an excellent means of accessing strong-field regions. We investigate what we can learn about both astrophysics and gravitation from strong-field tests and, in particular, GWs; we focus upon extreme-mass-ratio (EMR) systems where a small body orbits a much more massive one. EMR bursts, a particular class of GW signals, could be used to determine the properties of massive black holes (MBHs). They could be detectable with a space-borne interferometer from many nearby galaxies, as well as the Galactic centre. Bursts could provide insightful constraints on the MBHs' parameters. These could elucidate the formation history of the MBHs and, by association, their host galaxies. The Galactic centre is the most promising source. Its event rate is determined by the stellar distribution surrounding the MBH; the rate is not high, but we still expect to gain useful astronomical information from bursts. Strong-field tests may reveal deviations from general relativity (GR). We calculate modifications that could be observed assuming metric f(R)-gravity as an effective alternative theory. Gravitational radiation is modified, as are planetary precession rates. Both give a means of testing GR. However, existing laboratory measurements already place tighter constraints on f(R)-gravity, unless there exists a screening effect, such as the chameleon mechanism, which suppresses modifications on small scales. To make precision measurements of astrophysical systems or place exacting bounds on deviations from GR, we must have accurate GW templates. Transient resonances are currently not included in the prescription for generating EMR inspiral waveforms. Their effects can be estimated from asymptotic expansions of the evolving orbital parameters. The quantitative impact on parameter estimation has yet to be calculated, but it appears that it shall be necessary to incorporate resonances when creating inspiral waveforms.