Dissertations / Theses on the topic 'Pseudo-Riemannian manifolds'

Thesis (Ph. D.)--University of Oregon, 2006.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 146-147). Also available for download via the World Wide Web; free to University of Oregon users.

This thesis generalises the theory of harmonic vector fields to the non-compact pseudo- Riemannian case. After introducing the required background theory we consider the first variation of the local energies to find the Euler-Lagrange equations for this new case. We then introduce a natural closed conformal gradient field on pseudo-Riemannian warped products and find the Euler-Lagrange equations for harmonic closed conformal vector fields of this sort. We then give examples of such harmonic closed conformal fields, this leads to a harmonic vector fields on a 2-sphere with a rotationally symmetric singular metric. The harmonic conformal gradient fields on all hyperquadrics are then categorised up to con- gruence. The harmonic Killing fields on the 2-dimensional hyperquadrics are found, and shown to be unique up to congruence.

We present an extension of Geodesics in Lorentzian Manifolds (Semi-Riemannian Manifolds or pseudo-Riemannian Manifolds ). A geodesic on a Riemannian manifold is, locally, a length minimizing curve. On the other hand, geodesics in Lorentzian manifolds can be viewed as a distance between ``events''. They are no longer distance minimizing (instead, some are distance maximizing) and our goal is to illustrate over what time parameter geodesics in Lorentzian manifolds are defined. If all geodesics in timelike or spacelike or lightlike are defined for infinite time, then the manifold is called ``geodesically complete'', or simply, ``complete''. It is easy to show that the magnitude of a geodesic is constant, so one can characterize geodesics in terms of their causal character: if this magnitude is negative, the geodesic is called timelike. If this magnitude is positive, then it is spacelike. If this magnitude is 0, then it is called lightlike or null. Geodesic completeness can be considered by only considering one causal character to produce the notions of spacelike complete, timelike complete, and null or lightlike complete. We illustrate that some of the notions are inequivalent.

In the present thesis we construct a new class of holonomy algebras in pseudo-Riemannian geometry. Starting from a smooth connected manifold M, we consider its (1;1)-tensor fields acting on the tangent spaces. We then prove that there exists a class of pseudo- Riemannian metrics g on M such that the (1;1)-tensor fields are g-self adjoint and their centralisers in the Lie algebra so(g) are holonomy algebras for the Levi-Civita connection of g. Our construction is elaborated with the aid of Manakov operators and holds for any signature of the metric g.

Die Arbeit untersucht lokale Geometrien, die Twistorspinoren zulassen auf pseudo-Riemannschen Mannigfaltigkeiten beliebiger Signatur. Hierzu entwickeln wir die benötigten Methoden, nämlich das konforme Traktorkalkül, welches eine konform-invariante Beschreibung von Twistorspinoren als parallele Objekte ermöglicht, weiter. In diesem Zusammenhang ist unser erstes zentrales Resultat ein Klassifikationssatz für konforme Strukturen, deren Holonomiegruppen einen total ausgearteten Unterraum beliebiger Dimension invariant lassen. Hierauf aufbauend können wir einen partiellen Klassifikationssatz für konforme Strukturen mit Twistorspinoren beweisen. Weiterhin studieren wir die Nullstellenmenge eines Twistorspinors unter Nutzung der Theorie der Orbitzerlegungen für parabolische Geometrien. Wir können die lokale geometrische Struktur der Nullstellenmenge vollständig beschreiben und zeigen, dass lokal jeder Twistorspinor mit Nullstelle konform äquivalent zu einem parallelem Spinor ist. Eine Anwendung dieser Resultate auf niedrig-dimensionale Split-Signaturen führt zu einer vollständigen geometrischen Beschreibung von Mannigfaltigkeiten mit nicht-generischen Twistorspinoren in den Signaturen (3,2) und (3,3) durch parallele Spinoren, was die schon bekannte Analyse des generischen Falls komplementiert. Darüberhinaus wenden wir das Traktorkalkül an, um einer konformen Spin- Mannigfaltigkeit auf natürliche Weise eine konforme Superalgebra zuzuordnen. Dieser Zugang führt zu verschiedenen Resultaten, die algebraische Eigenschaften dieser Superalgebra mit speziellen Geometrien auf der zugrundeliegenden Mannigfaltigkeit in Verbindung bringen. Weiterhin erhält man so neue Konstruktionsprinzipien für Twistorspinoren und konforme Killingformen. Zuletzt führen wir den Begriff der konformen Spin-c-Geometrie ein. Unter anderem liefern spezielle Spin-c-Twistorspinoren eine neue Charakterisierung von Fefferman-Räumen.
The present thesis studies local geometries admitting twistor spinors on pseudo- Riemannian manifolds of arbitrary signature. To this end, we refine and extend the necessary machinery of first prolongation of conformal structures and conformal tractor calculus which allows a conformally-invariant description of twistor spinors as parallel objects. In this context, our first main theorem is a classification result for conformal geometries whose conformal holonomy group admits a totally degenerate invariant subspace of arbitrary dimension. Based on this we are able to prove a partial classification result for conformal structures admitting twistor spinors. Moreover, we study the zero set of a twistor spinor using the theory of curved orbit decompositions for parabolic geometries. We can completely describe the local geometric structure of the zero set and show that locally every twistor spinor with zero is equivalent to a parallel spinor off the zero set. An application of these results in low-dimensional split-signatures leads to a complete geometric description of manifolds admitting non-generic twistor spinors in signatures (3,2) and (3,3) in terms of parallel spinors which complements the well-known analysis of the generic case. Moreover, we apply tractor calculus for the construction of a conformal superalgebra naturally associated to a conformal spin structure. This approach leads to various results linking algebraic properties of the superalgebra to special geometric structures on the underlying manifold. It also exhibits new construction principles for twistor spinors and conformal Killing forms. Finally, we introduce and elaborate on the notion of conformal Spin-c-geometry. Among other aspects, this gives rise to a new characterization of Fefferman spaces in terms of distinguished Spin-c-twistor spinors.

In dieser Arbeit untersuchen wir die Interaktion von Holonomie und der globalen Geometrie von Lorentzmannigfaltigkeiten und pseudo-Riemannschen Untermannigfaltigkeiten in Räumen konstanter Krümmung. Insbesondere konstruieren wir schwach irreduzible, reduzible Lorentzmetriken auf den Totalräumen von gewissen Kreisbündeln, was zu einer Konstruktionsmethode von Lorentzmannigfaltigkeiten mit vorgegebener Holonomiedarstellung führt. Danach führen wir eine Bochnertechnik für die Lorentzmannigfaltigkeiten ein, die ein nirgends verschwindendes, paralleles, lichtartiges Vektorfeld zulassen, dessen orthogonale Distribution kompakte Blätter hat. Schließlich klassifizieren wir normale Holonomiedarstellungen von raumartigen Untermannigfaltigkeiten in Räumen konstanter Krümmung und verallgemeinern die Klassifikation eine größere Klasse von Untermannigfaltigkeiten.
In this thesis we study the interaction of holonomy and the global geometry of Lorentzian manifolds and pseudo-Riemannian submanifolds in spaces of constant curvature. In particular, we construct weakly irreducible, reducible Lorentzian metrics on the total spaces of certain circle bundles leading to a construction of Lorentzian manifolds with specified holonomy representations. Then we introduce a Bochner technique for Lorentzian manifolds admitting a nowhere vanishing parallel lightlike vector field whose orthogonal distribution has compact leaves. Finally, we classify normal holonomy representations of spacelike submanifolds in spaces of constant curvature and extend the classification to more general submanifolds.

[No abstract supplied with this thesis - The first page (of three) of the Introduction follows] ¶ This thesis is largely concerned with the changing representations of 'boundary' or 'ideal' points of a pseudo-Riemannian manifold -- and our primary interest is in the space-times of general relativity. In particular, we are interested in the following question: What assumptions about the 'nature' of 'portions' of a certain 'ideal boundary' construction (essentially the 'abstract boundary' of Scott and Szekeres (1994)) allow us to define precisely the topological type of these 'portions', i.e., to show that different representations of this ideal boundary, corresponding to different embeddings of the manifold into others, have corresponding 'portions' that are homeomorphic? ¶ Certain topological properties of these 'portions' are preserved, even allowing for quite unpleasant properties of the metric (Fama and Scott 1995). These results are given in Appendix D, since they are not used elsewhere and, as well as representing the main portion of work undertaken under the supervision of Scott, which deserves recognition, may serve as an interesting example of the relative ease with which certain simple results about the abstract boundary can be obtained. ¶ An answer to a more precisely formulated version of this question appears very diffcult in general. However, we can give a rather complete answer in certain cases, where we dictate certain 'generalised regularity' requirements for our embeddings, but make no demands on the precise functional form of our metrics apart from these. For example, we get a complete answer to our question for abstract boundary sets which do not 'wiggle about' too much -- i.e., they satisfy a certain Lipschitz condition -- and through which the metric can be extended in a manner which is not required to be differentiable (C[superscript1]), but is continuous and non--degenerate. We allow similar freedoms on the interior of the manifold, thereby bringing gravitational wave space-times within our sphere of discussion. In fact, in the course of developing these results in progressively greater generality, we get, almost 'free', certain abilities to begin looking at geodesic structure on quite general pseudo-Riemannian manifolds. ¶ It is possible to delineate most of this work cleanly into two major parts. Firstly, there are results which use classical geometric constructs and can be given for the original abstract boundary construction, which requires differentiability of both manifolds and metrics, and which we summarise below. The second -- and significantly longer -- part involves extensions of those constructs and results to more general metrics.

Ce travail étudie les variétés pseudo-riemanniennes compactes localement homogènes à travers le prisme des (G,X)-structures, introduites par Thurston dans son programme de géométrisation. Nous commençons par présenter la problématique générale et discutons notamment du rapport entre la complétude géodésique de ces variétés et une autre notion de complétude propre aux (G,X)-structures. Nous donnons également dans le chapitre 1 une nouvelle preuve d’un théorème de Bromberg et Medina qui classifie les métriques lorentziennes invariantes à gauche sur SL(2,R) dont le flot géodésique est complet. Conjecturalement, toute (G,X)-structure pseudo-riemannienne sur une variété compacte est complète. Nous prouvons ici que cela est vrai pour certaines géométries, sous l’hypothèse que la (G,X)-structure est a priori kleinienne. On en déduit que, pour ces géométries, la complétude est une condition fermée. Lorsque X est un groupe de Lie de rang 1 muni de sa métrique de Killing, ce résultat complète un théorème de Guéritaud–Guichard–Kassel–Wienhard selon lequel la complétude est une condition ouverte. Nous nous tournons ensuite vers l’étude des représentations d’un groupe de surface à valeurs dans les isométries d’une variété riemannienne M complète simplement connexe de courbure sectionnelle inférieure à -1. Étant donnée une telle représentation ρ, nous montrons que l’ensemble des représentations fuchsiennes j telles qu’il existe une application (j,ρ)-équivariante et contractante de H2 dans M est un ouvert non vide et contractile de l’espace de Teichmüller (sauf lorsque ρ est elle-même fuchsienne). Ce résultat nous permet de décrire l’espace des métriques lorentziennes de courbure constante -1 sur un fibré en cercle au-dessus d’une surface compacte. Nous montrons que cet espace possède un nombre fini de composantes connexes classifiées par un invariant que nous appelons longueur de la fibre. Nous prouvons également que le volume total de ces métriques ne dépend que de la topologie du fibré et de la longueur de la fibre
In this work, we study closed locally homogeneous pseudo-Riemannian manifolds through the notion of (G,X)-structure, introduced by Thurston in his geometrization program. We start by presenting the general problem. In particular, we discuss the link between geodesical completeness of those manifolds and another notion of completeness specific to (G,X)-structures. In chapter 1, we also give a new proof of a theorem by Bromberg and Medina which classifies left invariant Lorentz metrics on SL(2,R) that are geodesically complete. Conjecturally, every pseudo-riemannian (G,X)-structure on a closed manifold is complete. Here we prove that it holds for certain geometries, provided that the (G,X )-structure is a priori Kleinian . This implies that, for such geometries, completeness is a closed condition. When X is a Lie group of rank 1 handled with its Killing metric, this result complements a theorem of Guéritaud–Guichard–Kassel–Wienhard, acording to which completeness is an open condition. We then turn to the study of representations of surface groups into the isometry group of a complete simply connected Riemannian manifold M of curvature less than or equal to -1. Given such a representation ρ, we prove that the set of Fuchsian representations j for which there exists a (j,ρ)-equivariant contracting map from H2 to M is a non-empty open contractible subset of the Teichmüller space (unless ρ itself is Fuchsian). This result allows us to describe the space of Lorentz metrics of constant curvature -1 on a circle bundle over a closed surface. We show that this space has finitely many connected components, classified by an invariant that we call the length of the fiber. We also prove that the total volume of those metrics only depends on the topology of the bundle and on the length of the fiber

In the first part of this thesis, we derive comparison formulas relating the zeta-regularized determinant of an arbitrary self-adjoint extension of the Laplace operator with domain consisting of smooth functions compactly supported on the complement of a point $P$, to the zeta-regularized determinant of the Laplace operator on $X$. Here $X$ is a compact Riemannian manifold of dimension 2 or 3; $P\in X$. In the second part, we provide a proof of a conjecture by Jakobson, Nadirashvili, and Toth stating that on an n-dimensional flat torus, the Fourier transform of squares of the eigenfunctions $|phi_j|^2$ of the Laplacian have uniform $l^n$ bounds that do not depend on the eigenvalue $\lambda_j$. The thesis is based on two published papers that can be found in the bibliography.

This thesis is largely concerned with the changing representations of 'boundary' or 'ideal' points of a pseudo-Riemannian manifold -- and our primary interest is in the space-times of general relativity. In particular, we are interested in the following question: What assumptions about the 'nature' of 'portions' of a certain 'ideal boundary' construction (essentially the 'abstract boundary' of Scott and Szekeres (1994)) allow us to define precisely the topological type of these 'portions', i.e., to show that different representations of this ideal boundary, corresponding to different embeddings of the manifold into others, have corresponding 'portions' that are homeomorphic?

Tese de doutoramento em Matemática (Pré-Bolonha), Especialidade de Matemática Pura apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra
O objetivo fundamental desta dissertação é apresentar uma visão abrangente sobre rolamentos, sem deslize nem torção, de variedades diferenciáveis, contribuindo para aprofundar o conhecimento teórico nesta área e evidenciar potenciais aplicações. Começamos por apresentar uma definição de aplicação rolamento para o caso mais geral em que o movimento acontece dentro de espaços ambiente que são variedades pseudo-Riemannianas. Isto generaliza a definição clássica de Sharpe. A seguir, provamos algumas propriedades essenciais dos rolamentos e fazemos a ligação destes com o transporte paralelo de vetores. Dentro do contexto geral, analisamos os rolamentos das hiperquádricas de espaços pseudo-Euclidianos, com enfoque no caso dos espaços pseudo-hiperbólicos H_k^n (r). Apresentamos as equações da cinemáticas do rolamento de H_k^n (r) sobre o espaço afim associado ao espaço tangente num ponto. A obtenção de soluções explícitas destas equações é alcançada em dois casos particulares, destacando-se a situação em que o rolamento é feito ao longo de geodésicas. Rolamentos de um espaço pseudo-hiperbólico sobre outro e de pseudoesferas são igualmente tratados. Investigamos os rolamentos de grupos de Lie quadráticos sobre um espaço afim tangente. Também nestes casos se deduzem as equações da cinemática e se procuram soluções explícitas. A abordagem usada neste caso tem a preocupação de não destruir a estrutura matricial que caracteriza os elementos destes grupos matriciais. Estudamos a controlabilidade de rolamentos nos casos da hiperquádrica H_k^n (r) e dos grupos de Lie quadráticos principais, os grupos pseudo-ortogonais e os grupos simpléticos. Seguimos uma abordagem algébrica que passa por reescrever as equações da cinemática como um sistema de controlo afim a evoluir num grupo de Lie. Aplicamos os resultados obtidos anteriormente na resolução de problemas de interpolação suave em variedades e apresentamos um algoritmo interpolador. As propriedades dos rolamentos permitem transformar um problema de interpolação complicado, formulado numa variedade, num outro mais simples de resolver. São ainda fornecidos os ingredientes necessários para a implementação prática do algoritmo nos casos particulares de H_0^n (r) e H_1^n (r).
The primary goal of this dissertation is to present a comprehensive overview about rolling motions, subject to non-slip and non-twist constraints, of differentiable manifolds, contributing to deepen the theoretical knowledge in this area and to point out potential applications. We first present a definition of rolling map for the situation when the motion occurs inside an ambient space which is a pseudo-Riemannian manifold. This generalizes the classical definition of Sharpe. We then present several essential properties of rolling and make the connection between rolling motions and parallel transport of vectors. Within this general framework, we analyze the rolling of hyperquadrics embedded in pseudo-Euclidean spaces, focusing on the case of pseudo-hyperbolic spaces H_k^n (r). The kinematic equations of rolling H_k^n (r) on the affine space associated to the tangent space at a point is presented. Explicit solutions of these equations are obtained in two particular cases, with emphasis when the rolling is done along geodesics. Rolling of a pseudo-hyperbolic space on another and rolling of pseudo-spheres are equally treated. We investigate the rolling of quadratic Lie groups on an affine space tangent. We also derive the corresponding kinematic equations and look for explicit solutions. The approach used here is chosen so that the matrix structure that characterizes the elements of these matrix groups is not destroyed. We also address the controllability issue of rolling motions in the cases of hyperquadrics H_k^n (r) and of the most important quadratic Lie groups, pseudo-orthogonal groups and symplectic groups. We used an algebraic approach to controllability that requires rewriting the kinematic equations as a control system evolving on a Lie group. We apply the results previously obtained to solve problems of smooth interpolation on manifolds and present an interpolating algorithm. The properties of rolling enable to transform a complicated interpolation problem, formulated on a manifold, on another much simpler to solve. Ingredients needed to implement the algorithm are provided in the specific cases of H_0^n (r) and H_1^n (r).