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Black holes solutions of modified gravity theories
No full textL’intérêt majeur des travaux exposés dans cette thèse est d’explorer la chevelure des trous noirs dans des cadres plus généraux que celui de la Relativité Générale en tenant compte de la présence d’une constante cosmologique, de dimensions supplémentaires, de champs de matière exotiques ou de termes de courbure de rang plus élevé. Ces extensions de la Relativité Générale peuvent s’inscrire dans le cadre de la théorie des cordes. C’est en étudiant des extensions naturelles de la Relativité Générale que nous pouvons aussi mieux comprendre la théorie d’Einstein. Dans un premier temps, nous exposerons la théorie de la Relativité Générale avec notamment les principes sur lesquelles elle s’appuie et nous donnerons les éléments mathématiques dont nous avons besoin pour la suite. Puis, une première extension sera présentée avec l’introduction de dimensions supplémentaires et de champs de p-formes qui constituent la généralisation naturelle de l’interaction électromagnétique. Nous construirons dans ce cadre de nouvelles solutions statiques de trous noirs où les p-formes permettent de modeler la géométrie de l’horizon. Nous exposerons ensuite l’extension la plus générale de la théorie d’Einstein en dimension quelconque qui génère des équations du second ordre en la métrique : la théorie de Lovelock. Nous déterminerons dans ce contexte une large classe de solutions en dimension 6 pour laquelle la théorie se réduit à celle d’Einstein-Gauss-Bonnet avec toujours la présence de p-formes. Enfin, nous étudierons une généralisation de la Relativité Générale en dimension 4 dont la modification est induite par un champ scalaire couplé conformément à la gravitation. Nous exhiberons notamment une nouvelle solution de trou noir avec un horizon plat dans cette théorie en présence de champs axioniques. Pour clore cette thèse, l’aspect thermodynamique de ces théories gravitationnelles sera étudié ; ce qui permettra de déterminer la masse et les charges de ces nouvelles solutions et d’étudier des phénomènes de transitions de phase en présence d’un champ scalaire conforme.The main interest of the work exposed in this thesis is to explore hairy black holes in a more general framework than General Relativity by taking into account the presence of a cosmological constant, of higher dimensions, of exotic matter fields or of higher curvature terms. These extensions to General Relativity can be derived in the context of String Theory. It is also by studying natural extensions to General Relativity that we can more deeply understand the theory of Einstein. Firstly, we will display the theory of General Relativity with its building blocks in particular and we will give the mathematical tools that we need afterwards. Then, a first extension will be detailed with the introduction of higher dimensions and p-form fields which constitute the natural generalization of the electromagnetic interaction. We will build in this framework new static black hole solutions where p-form fields allow to shape the geometry of the horizon. Secondly, we will present the general extension of Einstein theory in any dimension which produces second order field equations: Lovelock theory. We will determine in this context a large class of solutions in dimension 6 for which the theory is reduced to Einstein-Gauss-Bonnet theory with the presence of p-form fields. Thirdly, we will study a generalization of General Relativity in dimension 4 whose modification is induced by a conformally coupled scalar field. We will namely exhibit a new black hole solution with a flat horizon in the presence of axionic fields. To conclude this thesis, thermodynamical aspects of these gravitational theories will be studied. In this way, we will be able to determine the mass and the charges of these new solutions and we will examine phase transition phenomena in the presence of a conformally scalar field
Solutions exactes de trous noirs en théories scalaire-tenseur
No full textLa Relativité Générale admet une unique solution de trou noir, caractérisée par sa masse M, son moment angulaire J, et sa charge électrique Q. On dit donc que les trous noirs en Relativité Générale n'ont pas de cheveux, c'est-à-dire pas d'autre quantité physique indépendante (théorème de calvitie). Malgré les innombrables succès de la Relativité Générale, des problèmes subsistent, comme celui de la singularité au centre des trous noirs, où la courbure de l'espace-temps devient infinie. Les théories de gravité modifiée tentent de résoudre ces limitations. Cette thèse teste le théorème de calvitie dans une modification populaire de la gravitation, appelée théories scalaire-tenseur, où un unique degré de liberté (un champ scalaire) est ajouté à l'habituelle métrique de l'espace-temps de la Relativité Générale. En exploitant diverses symétries, de nouveaux trous noirs, dits chevelus, sont obtenus. Certains contournent véritablement le théorème de calvitie, en étant caractérisés par une nouvelle quantité, distincte de M, J ou Q. Un progrès intéressant est également réalisé, puisque dans certains cas, la singularité disparaît : la courbure de l'espace-temps demeure finie même au cœur du trou noir. Des liens théoriques sont établis entre les théories scalaire-tenseur (qui prennent place dans les quatre dimensions usuelles de l'espace-temps), et les théories de gravité en dimensions supérieures. Enfin, des propriétés propres aux théories scalaire-tenseur permettent de transformer des solutions initiales de trous noirs en d'autres solutions de géométrie très différente, comme des trous de ver.General Relativity allows for a unique black hole solution, characterized by its mass M, angular momentum J, and electric charge Q. Black holes in General Relativity are thus said to have no hair, that is, no other independent physical quantity (no-hair theorem).Despite the numerous successes of General Relativity, some limitations remain, like the central singularity possessed by black holes, where the curvature of spacetime becomes infinite. Modified theories of gravity try to solve some of these shortcomings.This thesis tests the no-hair theorem in a popular modification of gravity, called scalar-tensor theories, where a unique degree of freedom (a scalar field) is added on top of the usual metric of spacetime of General Relativity. Using various symmetries, new black holes, called hairy black holes, are obtained. Some of them evade strongly the no-hair theorem, being characterized by a new quantity, distinct from M, J or Q. An interesting progress is also achieved, since in certain cases, the usual singularity disappears: the curvature of spacetime remains bounded even at the core of the black hole. Moreover, theoretical links are established between scalar-tensor theories (which take place in the usual four dimensions of spacetime), and theories of gravity in higher dimensions. Finally, certain particular properties of scalar-tensor theories enable to transform initial black hole solutions into other solutions with very distinct geometry, like wormholes
Compact astrophysical objects in modified gravity
No full textVingt années se sont écoulées depuis la découverte de l'expansion accélérée de l'Univers, ravivant l'intérêt pour les théories alternatives de la gravité. Ajouter un champ scalaire à la métrique habituelle de la relativité générale est l'une des manières les plus simples de modifier notre théorie de la gravité. En parallèle, nos connaissances sur les trous noirs et les étoiles à neutrons sont en plein essor, grâce notamment au développement de l'astronomie par ondes gravitationnelles. Cette thèse se situe au carrefour entre les deux domaines : elle étudie les propriétés des objets compacts dans les théories tenseur-scalaire généralisées. Je commence par rappeler les théorèmes d'unicité essentiels établis depuis les années soixante-dix. Après avoir présenté le théorème d'unicité pour les trous noirs en théorie de Horndeski, je l'étends aux étoiles. La deuxième partie de cette thèse détaille les différentes manières de contourner ce théorème. Parmi elles, je présente des solutions où la dépendance temporelle du champ scalaire permet de le raccorder à une solution cosmologique, mais aussi des trous noirs statiques et asymptotiquement plats. Dans la troisième partie, j'établis un critère important pour la stabilité de ces solutions, qui s'appuie sur leur structure causale. C'est aussi l'occasion d'étudier la propagation des ondes gravitationnelles au voisinage de trous noirs, et de sélectionner les théories dans lesquelles les ondes gravitationnelles se propagent à la même vitesse que la lumière.Twenty years have passed since the discovery of the accelerated expansion of the Universe, reviving the interest for alternative theories of gravity. Adding a scalar degree of freedom to the usual metric of general relativity is one of the simplest ways to modify our gravitational theory. In parallel, our knowledge about black holes and neutron stars is booming, notably thanks to the advent of gravitational wave astronomy. This thesis is at the crossroads between the two fields, investigating the properties of compact objects in extended scalar-tensor theories. I start by reviewing essential no-hair results established since the seventies. After discussing the no-hair theorem proposed for black holes in Horndeski theory, I present its extension to stars. The second part of the thesis investigates in detail the various ways to circumvent this theorem. These notably include solutions with a time-dependent scalar field in order to match cosmological evolution, but also static and asymptotically flat configurations. In a third part, I establish an important stability criterion for these solutions, based on their causal structure. It is also the occasion to study the propagation of gravitational waves in black hole environments, and to select the theories where gravitational waves travel at the same speed as light
Higher order gravity theories and their black hole solutions
No full text44 pages, 2 figures, To appear in the proceedings of the 4th Aegean Summer School on Black Holes, 17-22 September 2007, Mytilene, Island of Lesvos, GreeceInternational audienceWe discuss a particular higher order gravity theory, Lovelock theory, that generalises in higher dimensions, general relativity. After briefly motivating modifications of gravity, we will introduce the theory in question and we will argue that it is a unique, mathematically sensible, and physically interesting extension of general relativity. We will see, by using the formalism of differential forms, the relation of Lovelock gravity to differential geometry and topology of even dimensional manifolds. We will then discuss a generic staticity theorem, which will give us the charged static black hole solutions. We will examine their asymptotic behavior, analyse their horizon structure and briefly their thermodynamics. We will then examine the distributional matching conditions for Lovelock theory. We will see how induced 4 dimensional Einstein-Hilbert terms result on the brane geometry from the higher order Lovelock terms. With the junction conditions at hand, we will go back to the black hole solutions and give applications for braneworlds: perturbations of codimension 1 braneworlds and the exact solution for braneworld cosmology as well as the determination of maximally symmetric codimension 2 braneworlds. In both cases, the staticity theorem evoked beforehand will give us the general solution for braneworld cosmology in codimension 1 and maximal symmetry warped branes of codimension 2. We will then end with a discussion of the simplest Kaluza-Klein reduction of Lovelock theory to a 4 dimensional vector-scalar-tensor theory which has the unique property of retaining second order field equations. We will conclude by listing some open problems and common difficulties
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ernst equation and spheroidal coordinates with a cosmological constant term
No full text9 pages, no figures, some additional results to gr-qc/0610091. Prepared for 12th Conference on Recent Developments in Gravity (NEB XII), Nafplio, Greece, 29 Jun - 2 Jul 2006We discuss solution generating techniques treating stationary and axially symmetric metrics in the presence of a cosmological constant. Using the recently found extended form of Ernst's complex equation, which takes into account the cosmological constant term, we propose an extension of spheroidal coordinates adapted to asymptotically de-Sitter and anti de-Sitter static spacetimes. In the absence of a cosmological constant we show in addition that any higher dimensional metric parametrised by a single angular momentum can be given by a 4 dimensional solution and Weyl potentials parametrising the extra Killing directions. We explicitly show how a stationary, and a static axially symmetric spacetime solution in 4 dimensions, can be {\it added} together to give a 5 dimensional stationary and axisymmetric solution
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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