Dissertations / Theses on the topic 'Gravity'

The origin of the late-time accelerated expansion of the universe is still a great mystery. Numerous cosmological models have been proposed to explain this phenomenon. Modern days' technology and equipment have allowed scientists to successfully execute many observations in cosmology and astrophysics: space missions, large ground-based telescopes and gravitational-wave antennas have led to important discoveries and ruled out many models. The Lambda-Cold Dark Matter, model provides a coherent and satisfactory framework to accommodate all fundamental observations. Therefore it is called the "standard model of cosmology''. Despite its many successes, Lambda CDM requires the introduction of dark energy in the form of an unnaturally small cosmological constant and is plagued by fine-tuning problems ("why do dark energy, dark matter and baryons have comparable energy densities today?''). The elementary particle candidates which are assumed to form the cold dark matter component have never been directly detected. These facts can be taken as possible indications of a potential crisis. This has motivated the introduction of various alternative models, among which a novel class of modified gravity theories, called "mimetic gravity'' or "mimetic dark matter-theory'', which aims at explaining both the dark energy and (at least part of) the dark matter components as consequences of a suitable modification of the gravitational theory w.r.t. Einstein General Relativity. (Chapter 1 and 2) In this PhD thesis, we propose the "generalized mimetic gravity theory", which arises in full generality by means of a non-invertible disformal transformation of the most general single scalar field scalar-tensor theory of gravity and implemented our idea for Horndeski and beyond-Horndeski models. This novel class of models is a generalization of the so-called mimetic dark matter theory recently introduced by Chamseddine and Mukhanov, as discussed in Chapters 2 and 3. It can source the background evolution of the universe by mimicking any perfect fluid, including radiation, dark matter, and dark energy. In this chapter, we also show that very general single-scalar-field scalar-tensor theories of gravity are generically invariant under invertible disformal transformations. In Chapter 4 we analyze linear scalar perturbations around a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) background in mimetic Horndeski gravity and show that the sound speed is zero on all backgrounds and therefore the system does not have any wave-like scalar degrees of freedom. Further, we present mimetic vector-tensor theories. In particular, we establish that the non-invertible disformal transformation at the origin of the normalization constraint term in the Einstein-Aether theory, i.e., that the Einstein-Aether theory is also in the class of mimetic theories. We shall also show that an Einstein-Maxwell system sourced by dust can be recovered in the weak limit of a minimal Einstein-Aether theory and that vector field becomes rotation and acceleration free in such a limit (Chapter 5). Finally, in the concluding Chapter 6, we wind up the thesis by discussing some applications and future research directions in mimetic theories of gravity. % So far, it is not ruled out by any observations. The Chapters 3 and 4 are based on our published papers and Chapter 5 is based on the material which will appear in a forthcoming paper (P. Karmakar, T. Koivisto, D. Mota and S. Mukohyama)
L'origine dell' accelerazione con cui attualmente l' universo si sta espandendo è ancora uno dei più grandi misteri della cosmologia. Diversi modelli cosmologici sono stati proposti per spiegare questo fenomeno. Le tecnologie e gli strumenti di misura moderni hanno permesso agli scienziati di eseguire con successo molte osservazioni in cosmologia e astrofisica: missioni spaziali, grandi telescopi terrestri e antenne per misurare le onde gravitazionali hanno portato a importanti scoperte ed escluso molti modelli. Il modello cosmologico cosiddetto "Lambda-Cold Dark Matter" è il modello che meglio spiega in un quadro coerente e soddisfacente tutte le osservazioni fondamentali. Per questo è chiamato il modello "standard della cosmologia". Nonostante i suoi numerosi successi, il modello Lambda CDM richiede l'introduzione della cosiddetta energia oscura sotto forma di un'innaturale piccola costante cosmologica ed è afflitto da problemi di fine-tuning (perchè l'energia oscura, la materia oscura e i barioni hanno densità di energia paragonabili oggi?'). I candidati di particelle elementari che si presume possano formare la componente di materia oscura fredda non sono mai stati rilevati direttamente. Questi fatti possono essere presi come possibili indicazioni di una potenziale crisi. Ciò ha portato all'introduzione di vari modelli alternativi, tra cui una nuova classe di teorie di gravità modificata, detta "gravità mimetica" o "teoria della materia oscura mimetica", che mira a spiegare sia l'energia oscura e (almeno parte de) i componenti di materia oscura come conseguenza di un' opportuna modifica della teoria della gravità rispetto alla Teoria della Relatività Generale di Einstein. (Capitolo 1 e 2) In questa tesi di dottorato, proponiamo la teoria della "gravità mimetica generalizzata", che emerge in piena generalità per mezzo di una trasformazione disforme non-invertibile della teoria scalare-tensoriale della gravita a singolo campo scalare più generale possibile, implementandola poi al caso dei modelli di Horndeski e di modelli che vanno oltre Horndeski. Questa nuova classe di modelli è una generalizzazione della cosiddetta teoria della materia oscura "mimetica", recentemente introdotta da Chamseddine e Mukhanov, come discusso nei capitoli 2 e 3. Essa può far da sorgente all'evoluzione di background dell'universo mimando qualsiasi fluido perfetto, tra cui un fluido di radiazione, di materia oscura e l'energia oscura. In questi capitoli mostriamo anche che teorie scalari-tensoriali della gravita` molto generali a singolo campo scalare sono genericamente invarianti per trasformazioni disformi invertibili. Nel Capitolo 4 analizziamo le perturbazioni scalari lineari intorno ad un background di Friedmann-Lemaitre-Robertson-Walker (FLRW) spazialmente piatto nell'ambito della gravità mimetica di Horndeski e dimostriamo che la velocità del suono è nulla su qualsiasi background e pertanto il sistema non dispone di eventuali gradi di libertà scalari che si propagano. Inoltre, discutiamo teorie mimetiche vettoriali-tensoriali. In particolare, si stabilisce che la condizione di non-nvertibilità della trasformazione disforme è all'origine del termine di vincolo di normalizzazione nella teoria di Einstein-Aether, ovvero che la teoria di Einstein-Aether rientra anch'essa nella classe di teorie mimetiche. Si mostrerà anche che un sistema di Einstein-Maxwell con polvere può essere recuperato nel limite debole di una teoria minimale di Einstein-Ather e che il campo vettoriale di questa teoria diventa irrotazionale e senza accelerazione in tale limite (capitolo 5). Infine, nel Capitolo conclusivo 6, finiamo la tesi discutendo alcune applicazioni e le direzioni future della ricerca in teorie di gravità mimetica. I capitoli 3 e 4 si basano sulle nostre pubblicazioni e il Capitolo 5 si basa sul materiale che apparirà in un prossimo articolo (P. Karmakar, T. Koivisto, D. Mota e S. Mukohyama.

Descriveremo una teoria di gravità emergente, che ci permette sotto certe condizioni di identificare gli effetti gravitazionali della materia oscura come deformazioni del mezzo di energia oscura tramite elasticità lineare. Per fare ciò, descriveremo quantità necessarie e costanti in tre differenti capitoli. Nella prima parte rivisiteremo la teoria della gravitazione di Newton da un punto di vista classico della gravità basato sulla costante di gravitazione universale e sul potenziale gravitazionale. Nella seconda parte vedremo brevemente alcuni dei risultati principali in cosmologia moderna senza essere troppo specifici. Descriveremo alcuni fatti sperimentali come la velocità di rotazione delle galassie a spirale, che suggerisce l'esistenza della materia oscura. Spiegheremo in oltre il motivo per cui abbiamo bisogno di una scala di accelerazione come alternativa alle teorie sulla materia oscura e come le due possono congiungersi in gravità emergente. Nella terza parte daremo una rigorosa e al contempo elementare descrizione di elasticità lineare. Introdurremo i tensori di sforzo e stress, come sono legati fra di loro e vedremo anche alcuni importanti parametri che ci aiuteranno a descrivere il mezzo di energia oscura nei dettagli. Mostreremo l'esistenza di una direzione preferenziale nel mezzo di energia oscura che identifica una superficie di interfaccia dove tutte le identificazioni possono essere fatte. Nel capitolo finale uniremo finalmente tutte le nozioni date nei tre capitoli precedenti per formulare una semplice, ma allo stesso tempo completa descrizione della teoria, descrivendo i suoi punti forti e deboli. Una volta che abbiamo mostrato che le quantità gravitazionali ed elastiche sono relazionate sotto alcune condizioni nel mezzo di energia oscura, diventerà facile vedere la dualità fra certe leggi, specialmente dal punto di vista energetico.

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.

Gravity Fails is a collection of four short stories and two memoirs that explore the ways in which characters adjust and fit into to a world that is destructive, fragmented and sometimes alien. Many of these pieces deal not with the moment of crisis, but with the aftermath. In "Gravity Fails," the young Danielle struggles to feel safe after the violent murder of her mother. Eliza Morrison negotiates the disappearance of her husband in "More Colors." "Following Rebecca" chronicles a woman's return to normalcy after her alcoholic husband divorces her. These characters are not happy; they are not healthy. Their lives have, in some way, been fragmented. But they find ways to move on by whatever possible means, and at their core, they are searching not just for a way to survive, but for a way to put themselves back together and find wholeness.
M.A.
Department of English
Arts and Sciences
English

An intriguing feature of scalar-tensor theories is the emergence of different metrics, e.g. when matter is minimally coupled to a metric non-trivially related to the Einstein metric g[mu,nu] used to construct the Ricci scalar. Strong equivalence principle constraints then typically force permissible “many-metric” scenarios to reduce to a bimetric picture. In this thesis we first aim to construct the most general bimetric relation, where the two metrics are related by a single scalar degree of freedom [phi] and its derivatives. This results in the disformal metric relation and a natural extension which we present. In the context of primordial structure formation, disformal bimetric theories give rise to “general single field inflation” models of the P(X, [phi]) type. We investigate the perturbative properties of such disformally motivated models. The focus is on non-Gaussian phenomenology and we establish non-Gaussian fingerprints for inflationary single field models and non-inflationary bimetric setups, also going beyond the slow-roll approximation. Furthermore we show that various dualities exist between disformally motivated P(X, [phi]) theories and higher-form models. As an explicit example we use the dual picture to compute non-Gaussian signals for three-form theories. In the context of dark energy/modified gravity, we show that the conformal subgroup of the general disformal relation can be used to construct a generalized “derivative” Chameleon setup. We present and investigate this setup and study its phenomenology. Finally we show that a natural extension of the disformal relation can generate Galileon solutions from a single geometrical invariant - the first Lovelock term - in four dimensions. As such the over-arching theme of this thesis is to show that the disformal bimetric picture and its extensions present us with a geometrical understanding of scalar-tensor/single field models. That they provide a unified description of large classes of scenarios linked to accelerated space-time expansion and also point us towards new physically motivated setups.

Ce travail de thèse s'inscrit dans cadre de l'application de la Géométrie Différentielle pour la Relativité Générale, en particulier elle présente l'approche de la Géométrie Multisymplectique pour la formulation de plusieurs exemple de théorie de jauge, et de la théorie de gravitation. La Géométrie Multisymplectique nous offre un cadre géométrique pour formuler la théorie classique des champs de manière indépendante des coordonnées, sur des espace-temps généraux. L'idée clé est de construire une description Hamiltonienne de la théorie des champs compatible avec les Principes de la relativité restreinte et générale, des théories des cordes et plus généralement avec toute tentative de comprendre la gravitation. Lespace-temps émerge de la dynamique elle-même et il n'y a pas de séparation espace-temps/champs donnée a priori. N'y a pas de structure d'espace-temps donnée a priori. Les coordonnées d'espace-temps émergent de l'analyse des quantités observables et de la dynamique
RThis thesis is contributed to the topic of modern Mathematical Physics differential Geometry in General Relativity, more exactly, to a study of the multisymplectic geometry approach in formulation of various examples of gauge theories, including theory of gravitation. The multisymplectic geometry provides a geometrical framework to formulate classical field theory in a coordinate free manner on arbitrary space-time manifold. Main idea is to construct a Hamiltonian description of classical fields theory compatible with, Principles of special and general relativity and string theories and more generally any effort towards understanding gravitation. Since space¬time should merge out from the dynamics. We need a description without any space-time/field splitting a priori. There is no space-time structure given a priori. Space-time coordinates should merge out from the analysis of what are the observable quantities and from the dynamics

In this thesis, we review some basic properties of the Einstein-aether gravity. We derive the field equations from an action and study a subclass of this theory corresponding to the Einstein-Maxwell like theory. We also show that the Gö
del type metrics are also exact solutions of this theory. Furthermore, we determine the observational constraints on the dimensionless preferred parameters of this theory using the parametrized post- Newtonian formalism. We stress that none of calculations and discussions are original in this thesis.

In der vorliegenden Arbeit wird mit Hilfe der verallgemeinerten Eichtheorie/Gravitations-Dualität, welche stark gekoppelte Eichtheorien mit schwach gekrümmten gravitativen Theorien verbindet, stark korrelierte Quantenzustände der Materie untersucht. Der Schwerpunkt liegt dabei in Anwendungen auf Systeme der kondensierten Materie, insbesondere Hochtemperatur-Supraleitung und kritische Quantenzustände bei verschwindender Temperatur. Die Eichtheorie/Gravitations-Dualität entstammt der Stringtheorie und erlaubt eine Umsetzung des holographischen Prinzips. Aus diesem Grund wird eine kurze Einführung in die Konzepte der Stringtheorie und ihre Auswirkungen auf das holographische Prinzip gegeben. Für das tiefere Verständnis der effektiven Niederenergie-Feldtheorien wird zusätzlich die Supersymmetrie benötigt. Ausgestattet mit einem robusten Stringtheorie-Hintergrund wird die unterschiedliche Interpretation der Dirichlet- oder D-Branen, ausgedehnte Objekte auf denen offene Strings/Fäden enden können, diskutiert: Zum einen als massive solitonische Lösungen der Typ II Supergravitation und auf der anderen Seite, ihre Rolle als Quelle für supersymmetrische Yang-Mills Theorien. Die Verbindung dieser unterschiedlichen Betrachtungsweise der D-Branen liefert eine explizite Konstruktion der Eichtheorie/Gravitations-Dualität, genauer der AdS_5/CFT_4 Korrespondenz zwischen der N=4 supersymmetrischen SU(N_c) Yang-Mills Theorie in vier Dimensionen mit verschwindender beta-Funktion in allen Ordnungen, also eine echte konforme Theorie, und Type IIB Supergravitation in der zehn dimensionalen AdS_5 X S^5 Raumzeit. Darüber hinaus wird das Wörterbuch, das zwischen den Operatoren der konformen Feldtheorie und den gravitativen Feldern übersetzt, im Detail eingeführt. Genauer gesagt, die Zustandssumme der stark gekoppelten N=4 supersymmetrischen Yang-Mills Theorie im Grenzwert großer N_c, ist identisch mit der Zustandssumme der Supergravitation unter Berücksichtigung der zugehörigen Lösungen der Bewegungsgleichungen, ausgewertet am Rand des AdS-Raumes. Die Anwendung der perturbativen Quantenfeldtheorie und die Verbindungen zur quantenstatistischen Zustandssumme erlaubt die Erweiterung des holographischen Wörterbuchs auf Systeme mit endlichen Dichten und endlicher Temperatur. Aus diesem Grund werden alle Aspekte der Quantenfeldtheorie behandelt, die für die Anwendung der ``Linear-Response''-Theorie, der Berechnung von Korrelationsfunktionen und die Beschreibung von kritischen Phänomenen benötigt werden, wobei die Betonung auf allgemeine Zusammenhänge zwischen Thermodynamik, statistischer Physik bzw. statistischer Feldtheorie und Quantenfeldtheorie liegt. Des Weiteren wird der Renormierungsgruppen-Formalismus zur Beschreibung von effektiven Feldtheorien und kritischen Phänomene im Kontext der verallgemeinerten Eichtheorie/Gravitations-Dualität ausführlich dargelegt. Folgende Hauptthemen werden in dieser Arbeit behandelt: Die Untersuchung der optischen Eigenschaften von holographischen Metallen und ihre Beschreibung durch das Drude-Sommerfeld Modell, ein Versuch das Homes'sche Gesetz in Hochtemperatur-Supraleitern holographisch zu beschreiben indem verschiedene Diffusionskonstanten und zugehörige Zeitskalen berechnet werden, das mesonische Spektrum bei verschwindender Temperatur und schlussendlich holographische Quantenzustände bei endlichen Dichten. Entscheidend für die Anwendung dieses Rahmenprogramms auf stark korrelierte Systeme der kondensierten Materie ist die Renormierungsgruppenfluss-Interpretation der AdS_5/CFT_4 Korrespondenz und die daraus resultierenden emergenten, holographischen Duale, welche die meisten Beschränkungen der ursprünglichen Theorie aufheben. Diese sogenannten ``Bottom-Up'' Zugänge sind besonders geeignet für Anwendungen auf Fragestellungen in der Theorie der kondensierten Materie und der ``Linear-Response''-Theorie, mittels des holographischen Fluktuations-Dissipations-Theorem. Die Hauptergebnisse der vorliegenden Arbeit umfassen eine ausführliche Untersuchung der R-Ladungs-Diffusion und der Impulsdiffusion in holographischen s- und p-Wellen Supraleitern, welche durch die Einstein-Maxwell Theorie bzw. die Einstein-Yang-Mills Theorie beschrieben werden, und eine Vertiefung des Verständnisses der universellen Eigenschaften solcher Systeme. Als zweites wurde die Stabilität der kalten holographischen Quantenzustände der Materie untersucht, wobei eine zusätzliche Diffusions-Mode entdeckt wurde. Diese Mode kann als eine Art ``R-Spin-Diffusion'' aufgefasst werden, die der Spin-Diffusion in Systemen mit frei beweglichen ``itineranten'' Elektronen ähnelt, wobei die Entkopplung der Spin-Bahn Kopplung die Spin-Symmetrie in eine globale Symmetrie überführt. Das Fehlen der Instabilitäten und die Existenz einer ``Zero-Sound'' Mode, bekannt von Fermi-Flüssigkeiten, deuten eine Beschreibung der kalten holographischen Materie durch eine effektive hydrodynamische Theorie an.
In this dissertation strongly correlated quantum states of matter are explored with the help of the gauge/gravity duality, relating strongly coupled gauge theories to weakly curved gravitational theories. The main focus of the present work is on applications to condensed matter systems, in particular high temperature superconductors and quantum matter close to criticality at zero temperature. The gauge/gravity duality originates from string theory and is a particular realization of the holographic principle. Therefore, a brief overview of the conceptual ideas behind string theory and the ramifications of the holographic principle are given. Along the way, supersymmetry and supersymmetric field theories needed to understand the low energy effective field theories of superstring theory will be discussed. Armed with the string theory background, the double life of D-branes, extended object where open strings end, is explained as massive solitonic solutions to the type II supergravity equations of motion and their role in generating supersymmetric Yang-Mills theories. Connecting these two different pictures of D-branes will give an explicit construction of a gauge/gravity duality, the AdS_5/CFT_4 correspondence between N=4 supersymmetric SU(N_c) Yang-Mills theory in four dimensions with vanishing beta-function to all orders, describing a true CFT, and type IIB supergravity in ten-dimensional AdS_5 X S^5 spacetime. Furthermore, the precise dictionary relating operators of the conformal field theory to fields in the gravitational theory is established. More precisely, the partitions functions of the strongly coupled N=4 supersymmetric Yang-Mills theory in the large N_c limit is equal to the on-shell supergravity partitionevaluated at the boundary of the AdS space. Applying the knowledge of perturbative quantum field theory and its relation to the quantum partition function the dictionary may be extended to finite temperature and finite density states. Thus, all aspects of quantum field theory relevant for the application of linear response theory, the computation of correlation functions, and the description of critical phenomena are covered with emphasis on elucidating connections between thermodynamics, statistical physics, statistical field theory and quantum field theory. Furthermore, the renormalization group formalism in the context of effective field theories and critical phenomena will be developed explaining the critical exponents in terms of hyperscaling relations. The main topics covered in this thesis are: the analysis of optical properties of holographic metals and their relation to the Drude-Sommerfeld model, an attempt to understand Homes' law of high temperature superconductors holographically by computing different diffusion constants and related timescales, the mesonic spectrum at zero temperature and holographic quantum matter at finite density. Crucially for the application of this framework to strongly correlated condensed matter systems is the renormalization flow interpretation of the AdS_5/CFT_4 correspondence and the resulting emergent holographic duals relaxing most of the constraints of the original formulation. These so-called bottom up approaches are geared especially towards applications in condensed matter physics and to linear response theory, via the central operational prescription, the holographic fluctuation-dissipation theorem. The main results of the present work are an extensive analysis of the R-charge- and momentum diffusion in holographic s- and p-wave superconductors, described by Einstein-Maxwell theory and the Einstein-Yang-Mills model, respectively, and the lessons learned how to improve the understanding of universal features in such systems. Secondly, the stability of cold holographic quantum matter is investigated. So far, there are no instabilities detected in such systems. Instead, an interesting additional diffusion mode is discovered, which can be interpreted as an ``R-spin diffusion'', resembling spin diffusion in itinerant electronic systems where the spin decouples from the orbital momenta and becomes an internal global symmetry. The lack of instabilities and the existence of a zero sound and diffusion mode indicates that cold holographic matter is closely described by an effective hydrodynamic theory.

In this thesis we study higher spin theories in three dimensional gravity with a negative cosmological constant based on the gauge group SL(N,R)xSL(N,R). First we introduce the topic of semi-classical gravity in three dimensional anti de Sitter space andits conjectured duality with a two dimensional conformal field theory. We study black hole solutions for the case of N=3 characterized by their holonomies around the non-contractible cycles of space-time. The black hole solutions are shown to be gauge equivalent to a BTZ black hole which is charged under a set of U(1) Chern-Simons fields. Nevertheless, depending on the choice of embedding of the gravitational gauge group, the space-time geometry may be non-trivial. We also investigate in detail the physical sensibility of non principal embeddings of the gravitational SL(2,R) in SL(N,R) and we conclude that theonly embedding with a positive definite spectrum is the principal one.
Cette thèse s'intéresse aux théories gravitationnelles couplées à des spins entiers avec une constante cosmologiquenégative, dont l'origine provient de l'étude du groupe de jauge sl_N. La gravitation dans l'espace tridimensionnel anti de Sitter est d'abord décrite de manière semi-classique et la dualité avec la théorie conforme de champs en deux dimensions est ensuite présentée. L'exposition des solutions de la théorie N=3 permet la caractérisation des trous noirs par les propriétés des holonomies autour de boucles non-contractiles. L'invariance de jauge rapproche alors ces solutions aux trous noirs BTZ avec une charge d'un champ Chern-Simons. Néanmoins, la géometrie de l'espace-temps dépend du plongement des symétries gravitationnelles SL(2,R) dans le groupe de jauge SL(N,R). Le spectre des solutions est calculé pour tous ces plongements, et la condition de positivité exclut alors tous les cas sauf le plongement principal.

In dieser Dissertation untersuchen wir eine Vielzahl von Themen aus dem Bereich der Kosmologie und der Gravitation. Insbesondere behandeln wir Fragestellungen aus der Inflationstheorie, der Strukturbildung im neuzeitlichen Universum und massiver Gravitation, sowie Quantenaspekte schwarzer Löcher und Eigenschaften bestimmter skalare Theorien bei sehr hohen Energien. Im sogenannten "New Higgs Inflation"-Modell spielt das Higgs-Boson die Rolle des Inflaton-Felds. Das Modell ist kompatibel mit Messungen der Higgs-Masse, weil das Higgs-Boson nichtminimal an den Einstein-Tensor gekoppelt wird. Wir untersuchen das Modell in Hinblick auf die kürzlich veröffentlichten Resultate der BICEP2- und Planck-Experimente und finden eine hervorragende Übereinstimmung mit den gemessenen Daten. Desweiteren zeigen wir auf, dass die scheinbaren Widersprüche zwischen Planck- und BICEP2-Daten dank eines negativ laufenden Spektralindex verschwinden. Wir untersuchen außerdem die Unitaritätseigenschaften der Theorie und räsonieren, dass es während der gesamten Entwicklung des Universums nicht zu Unitaritätsverletzung kommt. Während der Dauer der inflationären Phase sind Kopplungen in den Higgs-Higgs und Higgs-Graviton-Sektoren durch eine großen feldabhängige Skala unterdrückt. Die W- und Z-Bosonen hingegen entkoppeln aufgrund ihrer sehr großen Masse. Wir zeigen eine Möglichkeit auf, die es erlaubt die Eichbosonen als Teil der Niederenergietheorie zu behalten. Dies wird erreicht durch eine gravitationsabhängige nichtminimale Kopplung des Higgs-Felds an die Eichbosonen. Im nächsten Abschnitt konzentrieren wir uns auf das neuzeitliche Universum. Wir untersuchen den sogenannten sphärischen Kollaps in Modellen gekoppelter dunkler Energie. Insbesondere leiten wir eine Formulierung des sphärischen Kollaps her, die auf den nichtlinearen Navier-Stokes-Gleichungen basiert. Im Gegensatz zu bekannten Beispielen aus der Literatur fließen alle wichtigen Fifth-Force Effekte in die Entwicklung ein. Wir zeigen, dass unsere Methode einfachen Einblick in viele Subtilitäten erlaubt, die auftreten wenn die dunkle Energie als inhomogen angenommen wird. Es folgt eine Einleitung in die Theorien von massiven Spin-2 Teilchen. Hier erklären wir die Schwierigkeiten der Formulierung einer nichtlinearen, wechselwirkenden Theorie. Wir betrachten das bekannte Problem des Boulware-Deser-Geists und zeigen zwei Wege auf, dieses No-Go-Theorem zu vermeiden. Insbesondere konstruieren wir die eindeutige Theorie eines wechselwirkenden massiven Spin-2 Teilchens, die auf kubischer Ordnung trunkiert werden kann, ohne dass sie zu Geist-Instabilitäten führt. Der zweite Teil dieser Arbeit widmet sich bekannten Problemen der Physik schwarzer Löcher. Hier liegt unser Fokus auf der Idee, das schwarze Löcher als Bose-Kondensate von Gravitonen aufgefasst werden können. Abweichungen von semiklassischem Verhalten sind Resultat von starken Quanteneffekten die aufgrund einer kollektiven starken Kopplung auftreten. Diese starke Kopplung führt in bekannten Systemen zu einem Quantenphasenübergang oder einer Bifurkation. Die quantenmechanischen Effekte könnten der Schlüssel zur Auflösung lang existierender Probleme in der Physik schwarzer Löcher sein. Dies umschließt zum Beispiel das Informationsparadox und das ``No-Hair''-Theorem. Außerdem könnten sie wertvolle Einblicke in die Vermutung liefern, dass schwarze Löcher die Systeme sind, die Informationen am schnellsten verschlüsseln. Als Modell für ein schwarzes Loch studieren wir ein System von ultrakalten Bosonen auf einem Ring. Dieses System ist bekannt als eines, dass einen Quantenkritischen Punkt besitzt. Wir demonstrieren, dass am kritischen Punkt Quanteneffekte sogar für sehr große Besetzungszahlen wichtig sein können. Hierzu definieren wir die Fluktuationsverschränkung, die angibt, wie sehr verschiedene Impulsmoden miteinander verschränkt sind. Die Fluktuationsverschränkung ist maximal am kritischen Punkt und ist dominiert von sehr langwelligen Fluktuationen. Wir finden daher Resultate die unabhängig von der Physik im ultravioletten sind. Im weiteren Verlauf besprechen wir die Informationsverarbeitung von schwarzen Löchern. Insbesondere das Zusammenspiel von Quantenkritikalität und Instabilität kann für ein sehr schnelles Wachstum von Ein-Teilchen-Verschränkung sorgen. Dementsprechend zeigen wir, dass die sogenannte "Quantum Break Time'', welche angibt wie schnell sich die exakte Zeitentwicklung von der semiklassischen entfernt, wie log(N) wächst. Hier beschreibt N die Anzahl der Konstituenten. Im Falle eines Gravitonkondensats gibt N ein Maß für die Entropie des schwarzen Lochs an. Dementsprechend interpretieren wir unsere Erkenntnisse als einen starken Hinweis, dass das Verschlüsseln von Informationen in schwarzen Löchern denselben Ursprung haben könnte. Das Verdampfen von schwarzen Löchern beruht in unserem Bild auf zwei Effekten. Kohärente Anregungen der tachyonischen radialen Mode führen zum Kollaps des Kondensats, während sich die inkohärente Streuung von Gravitonen für die Hawking-Strahlung verantwortlich zeigt. Hierfür konstruieren wir einen Prototyp, der einen bosonischen Freiheitsgrades mit impulsabhängigen Wechselwirkungen beschreibt. Im Schwinger-Keldysh-Formalismus untersuchen wir die Echtzeit-Evolution des Kondensats und zeigen, dass der Kollaps und die damit einhergehende Evaporation auf selbst-ähnliche Weise verläuft. In diesem Fall ist das Kondensat während des gesamten Kollapses an einem kritischen Punkt. Desweiteren zeigen wir Lösungen, die an einem instabilen Punkt leben, und daher schnelle Verschränkung erzeugen könnten. Der finale Teil der Arbeit befasst sich mit Renormierungsgruppenflüssen in skalaren Theorien mit impulsabhängigen Wechselwirkungen. Wer leiten die Flussgleichung für eine Theorie, die nur eine Funktion des kinetischen Terms enthält her. Hier zeigen wir die Existenz von Fixpunkten in einer Taylor-Entwicklung der Funktion auf. Wir diskutieren, inwiefern unsere Analyse für Einblick in allgemeinere Theorien mit Ableitungswechselwirkungen sorgen kann. Dies beinhaltet zum Beispiel Gravitation.
This thesis covers various aspects of cosmology and gravity. In particular, we focus on issues in inflation, structure formation, massive gravity, black hole physics, and ultraviolet completion in certain scalar theories. We commence by considering the model of New Higgs Inflation, where the Higgs boson is kinetically non-minimally coupled to the Einstein tensor. We address the recent results of BICEP2 and Planck and demonstrate that the model is in perfect agreement with the data. We further show how the apparent tension between the Planck and BICEP2 data can be relieved by considering a negative running of the spectral index. We visit the issue of unitarity violation in the model and argue that it is unitary throughout the evolution of the Universe. During inflation, couplings in the Higgs-Higgs and Higgs-graviton sector are suppressed by a large field dependent cutoff, while the W and Z gauge bosons acquire a very large mass and decouple. We point out how one can avoid this decoupling through a gravity dependent nonminimal coupling of the gauge bosons to the Higgs. We then focus on more recent cosmology and consider the spherical collapse model in coupled dark energy models. We derive a formulation of the spherical collapse that is based on the nonlinear hydrodynamical Navier-Stokes equations. Contrary to previous results in the literature, it takes all fifth forces into account properly. Our method can also be used to gain insight on subtleties that arise when inhomogeneities of the scalar field are considered. We apply our approach to various models of dark energy. This includes models with couplings to cold dark matter and neutrinos, as well as uncoupled models. In particular, we check past results for early dark energy parametrizations. Next, we give an introduction to massive spin-two theories and the problem of their non-linear completion. We review the Boulware-Deser ghost problem and point out the two ways to circumvent classic no-go theorems. In particular, we construct the unique theory of a massive spin-two particle that does not suffer from ghost instabilities when truncated at the cubic order. The second part of this dissertation is dedicated to problems in black hole physics. In particular, we focus on the proposal that black holes can be understood as quantum bound states of soft gravitons. Deviations from semiclassicality are due to strong quantum effects that arise because of a collective strong coupling, equivalent to a quantum phase transition or bifurcation. These deviations may hold the key to the resolution of long standing problems in black hole physics, such as the information paradox and the no hair theorem. They could also provide insights into the conjecture that black holes are the fastest information scramblers in nature. As a toy model for black holes, we study a model of ultracold bosons in one spatial dimension which is known to undergo a quantum phase transition. We demonstrate that at the critical point, quantum effects are important even for a macroscopic number of particles. To this end, we propose the notion of fluctuation entanglement, which measures the entanglement between different momentum modes. We observe the entanglement to be maximal at the critical point, and show that it is dominated by long wavelength modes. It is thus independent of ultraviolet physics. Further, we address the question of information processing in black holes. We point out that the combination of quantum criticality and instability can provide for fast growth of one-particle entanglement. In particular, we show that the quantum break time in a simple Bose-Einstein prototype scales like log(N), where N is the number of constituents. By noting that in the case of graviton condensates, N provides a measure for the black hole entropy, we take our result as as a strong hint that scrambling in black holes may originate in the same physics. In our picture, the evaporation of the black hole is due to two intertwined effects. Coherent excitation of the tachyonic breathing mode collapses the condensate, while incoherent scattering of gravitons leads to Hawking radiation. To explore this, we construct a toy model of a single bosonic degree of freedom with derivative self-interactions. In the Schwinger-Keldysh formalism, we consider the real-time evolution and show that evaporation and collapse occur in a self-similar manner. The condensate is at a critical point throughout the collapse. Moreover, we discover solutions that are stuck at an unstable point and may thus exhibit fast generation of entanglement. The final chapter of this thesis is dedicated to renormalization group (RG) flows in scalar theories with derivative couplings. We derive the exact flow equation for a theory that depends on a function of only the kinetic term. We demonstrate the existence of fixed points in a Taylor series expansion of the Lagrangian and discuss how our studies can provide insight into RG flows in more general theories with derivative couplings, for example gravity.

Thesis (M.S.)--University of California, San Diego, 2009.
Title from first page of PDF file (viewed February 6, 2009). Available via ProQuest Digital Dissertations. Includes bibliographical references (p. 61-63).

The aim of this thesis is to entertain the possibility of a quantum departure from the general relativistic description of compact sources in strong field regime and claim that a quantum understanding of the classical background could be necessary. We therefore develop an effective field theory providing a simplified framework to address the effects of non-linearities in strong gravitational backgrounds. Starting from a massless Fierz- Pauli-type lagrangian for the Newtonian potential and introducing the self-coupling terms, we arrive at a non-linear equation describing the effective gravitational potential of an arbitrarily compact homogeneous source. Unlike the general relativistic solutions no Buchdahl limit is found as the solutions display a regular behaviour in any compactness regime. Moreover, we provide a quantum interpretation of these results in terms of a quantum coherent state formalism. Such an approach proves to be widely capable of accounting for classical field configurations as well as providing some collective properties of the constituent soft quanta. The latter show a good agreement with some of the crucial relations of the corpuscular model. We do not interpret this approach as a model of phenomenological relevance but better as a simplified picture aimed at capturing novel quantum feature of black holes physics.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2008.
Includes bibliographical references (leaves 269-275).
The Mars Gravity Biosatellite is an unprecedented independent spaceflight platform for gravitational biology research. With a projected first launch after 2010, the low Earth orbit satellite will support a cohort of fifteen 14.5- to 25.5-week-old female BALB/cByJ mice for up to five weeks. During this time, the spacecraft will rotate at a rate of 31.6 rpm to generate Mars-equivalent artificial gravity of magnitude 0.38-g. Reentry capability will permit the return of live specimens to the Earth's surface at the culmination of the study. The proposed first mission aims to explore the physiological impacts on mice of 0.38-g. On board the Mars Gravity Biosatellite, a video acquisition and digitisation system will enhance in-flight collection of data on sensorimotor adaptation. As part of this thesis, a rotational ground control system has been designed and constructed at MIT. The apparatus incorporates a video processing module similar to that baselined for the mission. It also features the first custom-designed gondola centrifuge that accommodates up to four singlyhoused rodents in flight-equivalent habitat modules. At a rotation rate of 31.6 rpm, the centripetal acceleration experienced by each animal is less than 1.07-g. The 0.34 m radius of rotation is equivalent to that of the orbital vehicle. A behavioural study with four BALB/cByJ mice explores the effects of chronic rotation alone and confirms that they can be quantified and therefore decoupled from the anticipated on-orbit effects of rotation-induced Mars-equivalent gravity. The results provide justification for the scientific validity of the Mars Gravity Biosatellite as a rotating spaceflight platform. In addition, details are presented on the design, implementation, test and operation of a two-mouse closed-loop environmental control and life support system (ECLSS). The ground-based assembly is colocated with the centrifuge, and the entire apparatus is enclosed within a sealed zero-pressure urethane/polyethylene membrane. It incorporates scaled-down versions of a subset of flight-equivalent atmospheric reconditioning subassemblies together with sensors, actuators and a computer to perform autonomous feedback-driven supervisory control.
(cont.) Data is presented that validates a system that includes oxygen replenishment, carbon dioxide scrubbing via reaction with lithium hydroxide, ammonia removal using acidtreated activated charcoal, and humidity control with a custom-designed condensing heat exchanger. Results of a multi-week test represent an experimental proof-of-concept for the Mars Gravity Biosatellite's ECLSS strategy, showing good control of environmental parameters within specified ranges. The work presented in this thesis offers four primary contributions to aerospace biomedical engineering and rodent behavioural science: 1. Preliminary design and operations plans for the Mars Gravity payload. This thesis claims specific contributions in the areas of electronics, instrumentation, software and systems-level design of the payload module. 2. The first direct measurement of the influence of chronic rotation on mice in flight-like habitats at 31.6 rpm. The first in-centrifuge use of video-based behavioural analysis. 3. Proof-of-concept justification for the Mars Gravity Biosatellite ECLSS strategy. 4. The conception, design, implementation and operation of the first integrated ground test apparatus to combine chronic rotation capability with an ECLSS testbed.
by Thaddeus R. F. Fulford-Jones.
Ph.D.

The aim of this project is the study of an alternative class of wormholes, the thin-shell wormholes. We investigate the construction , the existence and stability of thin-shell wormhole solutions made of negative tension branes in Einstein gravity and Einstein-Gauss-Bonnet gravity. We impose Z2 symmetry and we work for spherical, planar and hyperbolic symmetric spacetimes. In Einstein gravity, we show that we usually obtain stable solutions with a proper combination of an electric charge and a negative cosmological constant. In Einstein-Gauss-Bonnet gravity, we are interested in the solutions that admit the general relativistic limit. We also investigate how the extra term in the action, the Gauss-Bonnet term, affects the stability of these wormholes.

Cette thèse est consacrée à l’étude du rôle des frontières en gravitation quantique pour une région compacte de l’espace-temps et explore en détail le cas en trois dimensions d'espace-temps. Cette étude s'inscrit dans le contexte du principe holographique qui conjecture que la géométrie d'une région de l’espace et sa dynamique peuvent être entièrement décrites par une théorie vivant sur la frontière de cette région. La réalisation la plus étudiée de ce principe est la correspondance AdS/CFT, où les fluctuations quantiques d'une géométrie asymptotiquement AdS sont décrites par une théorie conforme sur la frontière à l'infini, invariante sous le groupe de Virasoro. La philosophie appliquée ici diffère d’AdS/CFT. Je me suis intéressé à une holographie quasi- locale, c’est-à-dire pour une région bornée de l’espace avec une frontière à distance finie. L'objectif est de clarifier la relation bulk-boundary dans le cadre du modèle de Ponzano-Regge, qui définit la gravitation quantique euclidienne en 3D par une intégrale de chemin discrète.Je présente les premiers calculs approximatifs et exacts des amplitudes de Ponzano-Regge avec un état quantique de frontière 2D. Après présentation générale du calcul de l'amplitude 3d en fonction de l'état quantique de bord 2D, on se concentre sur le cas d'un tore 2D, qui trouve application dans l'étude de la thermodynamique des trous noirs BTZ. Dans un premier temps, la frontière 2D est décrite par des états de spin networks semi-classiques. L'approximation par phase stationnaire permet de retrouver dans la limite asymptotique la formule de l'amplitude de la gravité quantique 3D en tant que caractère du groupe BMS des symétries d'un espace-temps asymptotiquement plat. Puis dans un second temps, on introduit de nouveaux états quantiques cohérents, qui permettent une évaluation analytique exacte des amplitudes de gravité quantique 3D à distance finie sous la forme d'une régularisation complexe du caractère BMS. La possibilité de ce calcul exact suggère l’existence de structures (quasi-) intégrables liées aux symétries de la gravité quantique 3D en présence de frontières 2D bornées
This thesis is dedicated to the study of quasi-local boundary in quantum gravity. In particular, we focus on the three-dimensional space-time case. This research takes root in the holographic principle, which conjectures that the geometry and the dynamic of a space-time region can be entirely described by a theory living on the boundary of this given region. The most studied case of this principle is the AdS/CFT correspondence, where the quantum fluctuations of an asymptotically AdS space are described by a conformal field theory living at spatial infinity, invariant under the Virasoro group. The philosophy applied in this thesis however differs from the AdS/CFT case. I chose to focus on the case of quasi-local holography, i.e. for a bounded region of space-time with a boundary at a finite distance. The objective is to clarify the bulk-boundary relation in quantum gravity described by the Ponzano-Regge model, which defined a model for 3D gravity via a discrete path integral.I present the first perturbative and exact computations of the Ponzano-Regge amplitude on a torus with a 2D boundary state. After the presentation of the general framework for the 3D amplitude in terms of the 2D boundary state, we focus on the case of the 2D torus, that found an application in the study of the thermodynamics of the BTZ black hole. First, the 2D boundary is described by a coherent spin network state in the semi-classical regime. The stationary phase approximation allows to recover in the asymptotic limit the usual amplitude for 3D quantum gravity as the character of the symmetry of asymptotically flat gravity, the BMS group. Then we introduce a new type of coherent boundary state, which allows an exact evaluation of the amplitude for 3D quantum gravity. We obtain a complex regularization of the BMS character. The possibility of this exact computation suggests the existence of a (quasi)-integrable structure, linked to the symmetries of 3D quantum gravity with 2D finite boundary

The material consumption of the foundation of a WEC is a big part of the totalmaterial consumption of the entire power plant. To reduce these consumption, it isrequired to optimize the foundation. To find out how small the foundation could beconstructed and still be able to cope with the requirements, it is necessary to knowabout the various forces that affects the foundation. The focus in this thesis is on theforces acting on the buoy. The forces are calculated theoretical with experimentsconducted to reach conclusions about the size of the gravity foundation.Experiments will also be conducted to investigate experimentally whether there is anydifference between the suction force of sand and clay soil.Results from experiments with buoys show that the theoretical calculations arereasonably close with results from experiments. Conclusion of the differencesbetween theory and experiment is found that this may be due to the wave tank abilityto reproduce waves, and the angle between the buoy and the wave direction.Results from experiments of the suction force shows that there is no differencebetween sand and clay. The conclusion from this experiment is that the results applyonly to the clay that was investigated in this experiment and cannot be compared withother clay types with different properties.A discussion about the foundations is carried out where the conclusion is drawn thatthe foundations for the minimum mass to maintain equilibrium at 4 m wave height isapproximately 33 ton.

General Relativity (GR) has long been acclaimed for its elegance and simplicity, and has successfully passed many stringent observational tests since it was introduced a century ago. However, there are two regimes in which the theory has yet to be fully challenged. One of them is in the neighbourhood of very strong gravitational fields, and the other is the behaviour of gravity on cosmological scales. While strong field gravity has challenged theorists because of the desire to find consistency between GR and quantum mechanics, cosmology has motivated extensions to GR via the empirical discoveries of dark matter and dark energy. In this thesis, we study a diverse range of modifications to GR and confront them with observational data. We discuss how a generic theory of modified gravity can be parameterized for studies within cosmology, and we introduce a general parameterization that is simpler than those that have been previously considered. This parameterization is then applied to investigate a specific theory, known as ``gravitational aether''. The gravitational aether theory was created to solve one of the theoretical inconsistencies that exists between GR and quantum mechanics, namely the fact that vacuum fluctuations appear not to gravitate. Cosmology is unique in testing this theory, and we find that the gravitational aether solution is excluded when all of the available cosmological data are combined. Nevertheless, a generalization of this theory provides a consistent way to describe the strength of coupling between pressure and gravity, and we present the most accurate measurements of this coupling parameter. In addition, we discuss the constraints that can be placed on modified gravity models using the latest data from cosmic microwave background (CMB) anisotropies, combined with several other probes of large-scale structure. Currently the most accurate CMB anisotropy measurements come from the Planck 2015 CMB power spectra, which we use, along with other cosmological data sets, to perform an extensive study of modified theories of gravity. We find that GR remains the simplest model that can explain all of the data. We end with a discussion of the prospects for future experiments that can improve our understanding of gravity.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

The recent observational data in cosmology seem to indicate that the universe is currently expanding in an accelerated way. This unexpected conclusion can be explained assuming the presence of a non-vanishing yet extremely fine tuned cosmological constant, or invoking the existence of an exotic source of energy, dark energy, which is not observed in laboratory experiments yet seems to dominate the energy budget of the Universe. On the other hand, it may be that these observations are just signalling the fact that Einstein's General Relativity is not the correct description of gravity when we consider distances of the order of the present horizon of the universe. In order to study if the latter explanation is correct, we have to formulate new theories of the gravitational interaction, and see if they admit cosmological solutions which fit the observational data in a satisfactory way. Quite generally, modifying General Relativity introduces new degrees of freedom, which are responsible for the different large distance behaviour. On one hand, often these new degrees of freedom have negative kinetic energy, which implies that the theory is plagued by ghost instabilities. On the other hand, for a modified gravity theory to be phenomenologically viable it is necessary that the extra degrees of freedom are efficiently screened on terrestrial and astrophysical scales. One of the known mechanisms which can screen the extra degrees of freedom is the Vainshtein mechanism, which involves derivative self-interaction terms for these degrees of freedom. In this thesis, we consider two different models, the Cascading DGP and the dRGT massive gravity, which are candidates for viable models to modify gravity at very large distances. Regarding the Cascading DGP model, we consider the minimal (6D) set-up and we perform a perturbative analysis at first order of the behaviour of the gravitational field and of the branes position around background solutions where pure tension is localized on the 4D brane. We consider a specific realization of this set-up where the 5D brane can be considered thin with respect to the 4D one. We show that the thin limit of the 4D brane inside the (already thin) 5D brane is well defined, at least for the configurations that we consider, and confirm that the gravitational field on the 4D brane is finite for a general choice of the energymomentum tensor. We also confirm that there exists a critical tension which separates background configurations which possess a ghost among the perturbation modes, and background configurations which are ghost-free. We find a value for the critical tension which is different from the value which has been obtained in the literature; we comment on the difference between these two results, and perform a numeric calculation in a particular case where the exact solution is known to support the validity of our analysis. Regarding the dRGT massive gravity, we consider the static and spherically symmetric solutions of these theories, and we investigate the effectiveness of the Vainshtein screening mechanism. We focus on the branch of solutions in which the Vainshtein mechanism can occur, and we truncate the analysis to scales below the gravitational Compton wavelength, and consider the weak field limit for the gravitational potentials, while keeping all non-linearities of the mode which is involved in the screening. We determine analytically the number and properties of local solutions which exist asymptotically on large scales, and of local (inner) solutions which exist on small scales. Moreover, we analyze in detail in which cases the solutions match in an intermediate region. We show that asymptotically flat solutions connect only to inner configurations displaying the Vainshtein mechanism, while non asymptotically flat solutions can connect both with inner solutions which display the Vainshtein mechanism, or with solutions which display a self-shielding behaviour of the gravitational field. We show furthermore that there are some regions in the parameter space of the theory where global solutions do not exist, and characterize precisely in which regions the Vainshtein mechanism takes place.

Thesis (Ph.D.)--Indiana University, Dept. of Physics, 2007.
Source: Dissertation Abstracts International, Volume: 68-07, Section: B, page: 4556. Adviser: V. Alan Kostelecky. Title from dissertation home page (viewed Apr. 22, 2008).

This doctoral thesis deals with both infrared modifications of gravity and with the recently proposed microscopic picture of black holes. The former subject, i.e. infrared modifications of gravity, denotes a class of theories that typically weaken Einsteins theory of gravity at very large (usually cosmological) distance scales while preserving its successes at smaller distances (in particular within the solar system). Infrared modified theories of gravity allow to make progress with the cosmological constant problem since the cosmological constant literally corresponds to a space-time source of infinite extent. The results presented in this thesis concern two representatives of infrared modified theories of gravity: Massive Gravity and Brane Induced Gravity. Massive Gravity has been extensively studied for graviton propagation on a flat Minkowski background. What we will do in this thesis, however, is to study Massive Gravity on curved backgrounds such as cosmologically relevant FRW backgrounds. It actually turns out that the physics associated with the propagation of gravitons on curved spaces is enormously rich. In particular, we were able to show that the linear theory is protected from potential unitarity violations by generically entering a strong coupling regime before the unitary violation of the linear theory could have occurred. We coined this mechanism the self-protection mechanism. In fact, the self-protection mechanism can be understood as a striking example of the recently proposed classicalization mechanism, where the classicalon plays the role of the new background geometry that forms when entering the non-linear regime. Even though that the self-protection mechanism is very appealing from a theoretical perspective, it goes hand in hand with the destruction of the FRW background as soon as we enter the non-linear regime. This is phenomenological unacceptable as this always happens for early times in the universe. This led us to the construction of a completely new theory of massive deformations, where we supple- mented the known ’hard mass’ term with a new ’soft mass’ term. This new theory is both stable and consistent on the whole Friedman manifold. A particular interesting special case can be obtained when we set the hard mass identically equal to zero, since in this case we obtain a modification that is solely operative on curved backgrounds, whereas we still have standard massless graviton propagation for regions where the background curvature is small. This modification is thus completely orthogonal to known massive gravity theories. The other infrared modified theory of gravity this thesis deals with, i.e. Brane Induced Gravity, has been thought to contain a ghost within its spectrum of physical particles if we consider two or more additional spatial dimensions (whereas for one spatial dimension we would obtain the consistent DGP model). However, this ghost degree of freedom is completely unexpected physically, as we can think of Brane Induced Gravity simply as a higher dimensional Einstein gravity theory with a specific, healthy four dimensional source. Therefore, we performed a complete Dirac constraint analysis that actually showed that the Hamiltonian on the constraint surface is positive definite, and thus that Brane Induced Gravity is consistent, contrary to prior claims in the literature. By studying the system as well in the covariant language, we were able to understand that these previous derivations of the ghost degree of freedom did not take the 00-Einstein equation into account properly. This equation actually is a constraint that renders the would-be ghost mode non-dynamical. The other subject of this thesis deals with the microscopic picture of black holes recently proposed by Gia Dvali and Cesar Gomez. To be concrete, we invented a novel non-relativistic scalar theory that is supposed to mimic properties of general relativity relevant for black hole physics but is simple enough to make extensive quantitative calculations. In a first step, we analyzed the system perturbatively. This allowed us to show that there is indeed indication that the system dynamically secures to stay at the point of quantum phase transition. However, only a thorough nonlinear numerical analysis that is currently under investigation will yield a definite answer.

Massive gravity is a particular theoretical model that modifies gravity on cosmological scales and therefore could provide a dynamical explanation for the observed accelerated expansion of our Universe. In this thesis we investigate various theoretical problems of massive gravity, important for its consistency and phenomenological viability. It is known that the predictions from the linearized massive gravity contradict the predictions of General Relativity. It is, however, an artifact due to the breakdown of the perturbative expansion in the massless limit. In our work we investigate this problem in the diffeomorphism invariant formulation of massive gravity in which the graviton mass term is written in terms of four scalar fields. We determine the so-called Vainshtein scale below which the scalar modes of the massive graviton enter the non-perturbative regime for a wide class of non-linear mass terms. We find the asymptotic solutions of the spherically symmetric gravitational field below and above the Vainshtein radius, and show that massive gravity goes smoothly to the General Relativity below this scale. We also determine the corresponding corrections to the Newton potential. In general, any non-linear extension of the quadratic graviton mass term propagates the Boulware-Deser ghost. The only theory in which the ghost is not propagating in the high energy decoupling limit, is the de Rham-Gabadadze-Tolley theory. Here we show that the ghost arises in the fourth order of perturbations in this theory away from the decoupling limit. However, we further argue that the ghost can be avoided in the full non-linear theory if not all four scalar fields propagate independent degrees of freedom. In particular, we investigate the simple example of (1+1)-dimensional massive gravity and find that the theory exhibits a gauge symmetry, which reduces the number of degrees of freedom. We also generalize the diffeomorphism invariant formalism of massive gravity to arbitrary curved backgrounds. We find that, given a specific background metric, the resulting generally covariant massive gravity exhibits an internal symmetry in the configuration space of the scalar fields. The symmetry transformations of the scalar fields are given by the isometries of the reference metric. In particular, we investigate massive gravity on de Sitter space in this formalism. We confirm the known result that, in the case when the graviton mass is related to the cosmological constant as m^2=2\Lambda/3, the theory is partially massless and propagates only four degrees of freedom.
Massive Gravitation ist ein theoretisches Modell, welches Gravitation auf kosmologischen Längenskalen modifiziert, und das so eine dynamische Erklärung für die beobachtete Beschleunigung der Expansion des Universums liefern könnte. In dieser Arbeit untersuchen wir verschiedene theoretische Probleme der massiven Gravitation, die wichtig bezüglich der Konsistenz und phänomenologischen Viabilität der Theorie sind. Es ist bekannt, dass die Vorhersagen der massiven Gravitation auf linearer Ordnung den Vorhersagen der allgemeinen Relativitätstheorie widersprechen. Dies ist jedoch ein Artefakt, das vom Zusammenbruch der perturbativen Entwicklung im masselosen Limes verursacht wird. In unserer Arbeit untersuchen wir dieses Problem in der Diffeomorphismen-invarianten Formulierung der massiven Gravitation, in der der Graviton-Massenterm mit vier skalare Feldern ausgedrückt wird. Wir bestimmen die sogenannte Vainshtein-Skala, unterhalb derer sich die skalaren Moden des massiven Gravitons nichtperturbativ verhalten, für eine große Klasse möglicher Massenterme. Wir finden die asymptotischen Lösungen des sphärisch symmetrischen Gravitationsfeldes inner- und außerhalb des Vainshtein-Radiuses und zeigen, dass massive Gravitation sich unterhalb dieser Skala kontinuierlich der Allgemeinen Relativitätstheorie annähert. Außerdem bestimmen wir die resultierenden Korrekturen zum Newton-Potential. Im Allgemeinen propagiert in jeder Theorie mit einer nichtlinearen Erweiterung des quadratischen Graviton-Massenterms ein Boulware-Deser Geist. Die einzige solche Theorie, in der der Geist im Hochenergie-Entkopplungslimes nicht propagiert, ist das de Rham-Gabadadze-Tolley Modell. Hier zeigen wir, dass der Geist selbst in dieser Theorie außerhalb des Entkopplungslimes in vierter Ordnung Störungstheorie erscheint. Wir argumentieren dann jedoch, dass der Geist in der voll nichtlinearen Theorie vermeiden werden kann, wenn nicht alle Skalarfelder unabhängige Freiheitsgrade darstellen. In dieser Hinsicht untersuchen wir das einfache Beispiel (1+1)-dimensionaler massiver Gravitation und finden, dass diese Theorie eine Eichsymmetrie enthält, die die Anzahl der Freiheitsgrade reduziert. Schließlich verallgemeinern wir den Diffeomorphismen-invarianten Formalismus massiver Gravitation auf allgemeine gekrümmte Hintergründe. Wir finden, dass auf bestimmten Hintergründen die resultierende allgemein kovariante massive Gravitation eine Symmetrie im Konfigurationsraum der skalaren Felder aufweist. Die Symmetrietransformationen der skalaren Felder sind durch die Isometrien der Referenzmetrik gegeben. Insbesondere untersuchen wir massive Gravitation auf de Sitter-Raum in diesem Formalismus. Wir bestätigen das bekannte Ergebnis, dass, im Falle einer Gravitonmasse im Verhältnis zur kosmologischen Konstante von m^2=2\Lambda/3, die Theorie teilweise masselos ist. Dadurch propagieren in diesem Fall nur vier Freiheitsgrade.

We propose a Generalized Uncertainty Principle (GUP) consistent with String Theory, Black Hole Physics and Doubly Special Relativity. This modifies all quantum mechanical Hamiltonians and predicts quantum gravity corrections. We compute corrections to the Lamb shift, simple harmonic oscillator, Landau levels, and the tunneling current. When applied to an elementary particle, it suggests that the space must be quantized, and that all measurable lengths are quantized in units of a fundamental length. We investigated whether GUP can explain the violation of the equivalence principle at small length scales that was observed experimentally. We investigated the consequences of GUP on Liouville theorem. We examined GUP effect on post inflation preheating, and show that it predicts an increase or a decrease in parametric resonance and a change in particle production. The effect of GUP on the creation of black holes is investigated to justify the experimental results from the Large Hadron Collider.
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This thesis investigates two very different aspects of quantum gravity. In the first - and main - section, we examine the question of quantum gravitational contributions to the running of a coupling parameter alongside the various problems and issues that this raises. We treat quantum gravity as an e ective eld theory and use perturbative methods to address issues. Speci cally, we look at a '4-type scalar coupling. In a gauge-invariant way, we consider a non-minimally coupled, massive scalar eld, with non-constant background, in the presence of a cosmological constant and contrary to most of the literature, we also calculate all derivative terms. An e ective action is constructed, renormalization counterterms calculated, and we nd that, within certain bounds, gravity leads to asymptotic freedom of scalar eld theory. Furthermore, we investigate whether considering quadratic divergences in gravitational calculations can tell us anything useful. In this case we nd non-vanishing quadratic divergences. However, we also recognise the possibility that quadratic divergences are somewhat of a red herring and that by suitable eld rede nitions, we can eliminate these from our calculations. The second section of the thesis addresses the possibility of super uidity in a quark gluon plasma. We use the framework of AdS/CFT, with knowledge of black hole thermodynamics, to consider the duality between a black hole in anti-de Sitter space and a uid existing on the boundary. Initially, we look at a simple case of a black hole possessing only mass and charge in AdS spacetime and calculate such properties as the entropy, temperature and speci c heat capacity, identifying a telltale sign of a phase change (speci c heat capacity tending to in nity) and of points of vanishing viscosity (corresponding with a zero entropy). After con rming that such a boundary exists, we take a di erent approach where we calculate and interpret the solutions to a relativistic Gross-Pitaevskii equation on a sphere. On projection back to R3, the solutions are seen to be tori, which we choose to interpret as vortex rings in analogy to the expected feature of those which are known to appear in a real super uids.

Having as a starting point the problem of dark energy described before, this thesis studies modifications of General Relativity (GR), as possible gravitational scenarios for the early and late time Universe, motivated by both classical as well as quantum considerations. In particular, it focuses on modifications of GR of the type f(R) as well as the f(R;G) ones, where R and G is the Ricci scalar and Gauss-Bonnet term respectively. On the same time, a modification of GR based on the Renormalisation Group approach to quantum gravity is considered, as well as its link to f(R) gravity. The main goal of the investigations carried out in this thesis, is to understand the structure, as well as the phenomenological implications of non-linear modifications of GR for cosmology, at both the background as well as the linear perturbation level. In particular, chapter 2 presents a brief introduction to the dynamics of GR in the presence of a "dark component" at the background, as well as at the linear perturbation level, while chapter 3 is an introduction to the fundamental properties of non-linear modifications of GR, reviewing important results of the relevant literature. Chapter 4 elaborates with a fundamental property of non{linear gravity models, namely the study of different representations of vacuum actions proportional to f(R) as well as f(G), in view of Legendre transformations, for the case of spacetime manifolds with a boundary. As it is explicitly shown there, although the dynamical equivalence is always true in the bulk, it is not guaranteed on the boundary of the spacetime manifold. On the other hand, chapter 5 focuses on understanding the role of the effective anisotropic stress present in f(R;G) gravity models, attempting to construct particular models of the latter type, with a vanishingly small anisotropic stress, so as to agree with current observations. As it turns out, suppression of the effective anisotropic stress in this class of models is very difficult, highlighting the role of the effective anisotropic stress as a smoking gun for testing modified gravity models with current and future observations. Chapter 6 serves as an introduction to the idea of the Renormalisation Group (RG) and its applications in cosmology, while chapter 7 starts from an RG improved Einstein{Hilbert action and studies its connection with f(R) gravity, as well as its implications for the primordial and the late time acceleration of the Universe. It is shown that the effective f(R) model has some remarkable properties and interesting implications for both early and late time cosmology.

This thesis develops novel analytic models of scalar-tensor theories with quadratic coupling. In this framework, the coupling strength between scalar and matter is regulated in a way that allows the vacuum expectation value to vanish for low matter densities while becoming non-vanishingly large in the high-density regime. This results in significant deviations from the predictions of General Relativity in the strong-gravity regime. In astrophysics, we addressed the core-collapse supernova problem to account for the apparently missing energy required to explain the observed powerful explosions. We assumed a small, massless scalar gravitational field, thus allowing General Relativity to be recovered in the weak-gravity asymptotic limit. The non-trivial effects coming from the coupling function in the presence of a high-density field were analyzed at the instant of neutron star formation. Our results show that the scalar gravitational field evolves from a cosmological value to a new equilibrium via a Higgs-like mechanism. Additionally, the calculations associated with the gravitational binding energy shift and relevant relaxation timescale are explicitly shown. The full theory space of the model was also investigated for positive values of the coupling parameter. We studied a mechanism to address the stalled shock issue in core-collapse scenarios, which involved the application of sufficiently large positive values to the coupling parameter. Our results show that pulsating neutron stars act like optical cavities in which resonant scalar waves are parametrically amplified. It implies that the surface of a neutron star acts like an anti-phase reflector, releasing traveling scalar gravitational waves similar to an optical laser. In cosmology, the same framework was applied to a generic Friedman-Robertson-Walker universe involving general metric coupling and scalar potential functions. In cosmology, the same framework was applied to a generic Friedman-Robertson-Walker universe involving general metric coupling and scalar potential functions. We developed a mechanism which allowed the scalar field to be dynamically trapped, thus generating a potential capable of driving primordial inflation. Our results show that a trapped scalar field produces non-trivial dynamical consequences when applied to standard cosmology. Additionally, our analytic solutions for the generic inflationary behaviour, produce acceptable duration and e-foldings, thus recovering the Hubble parameter which is consistent with the present-day value. A feature of our cosmological model is that the universe can undergo several accelerating or decelerating phases, even though the scalar potential and metric coupling are monotonic functions overall. As this is important for the current dark energy problem, the quasi-static motion of the gravitational field induced by the scalar potential in the early universe, is investigated for a small value of the scalar field with normalized metric at the present time. Our results show that a variable Lambda Cold Dark Matter universe emerges naturally from the quadratic model.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2005.
"June 2005."
Includes bibliographical references (p. 77-79).
Artificial gravity provided by short radius centrifugation is considered a promising countermeasure to the deleterious physiological effects of microgravity during long-duration space flight. We investigated the feasibility of dual countermeasures to address space flight deconditioning of the musculoskeletal and cardiovascular systems, by combining centrifugation with lower-body exercise. The exercise device is a small stair-stepper with constant resistance provided by dampers beneath each foot, and is the first such device to be used in centrifuge studies. We modified the existing centrifuge to support the additional stresses due to exercise and added following structural elements: support struts on the rotation shaft, a redesigned footplate to which the exercise device was mounted, and horizontal support beams. We also added a sliding mattress with linear ball bearings on rails, so that the subject's body can move up and down while stepping. Design changes and exercise feasibility were validated by having subjects exercise during centrifugation at 23 rpm. We measured heart rate, blood pressure, forces on the feet, and knee deflection due to Coriolis accelerations, for up to four subjects. As expected, heart rate and blood pressure did increase normally with exercise on the centrifuge, relative to when not exercising. However, both heart rate and systolic blood pressure were higher for exercise on the non-spinning centrifuge than on the spinning centrifuge, attributable to the necessity of pulling against the stair-stepper's dampers in order to exercise while lying supine. Approximately half the subject's weight was exerted on the footplate when not exercising.
(cont.) This was expected: since the subject's head was at zero radius and thus at 0-g radially, the 100% artificial gravity gradient along the body's longitudinal axis gave an average effective gravity of about 0.5 g. More pressure (up to 80% body weight) was exerted when the subject was stair-stepping. The measured lateral deflection of the knee during normal stair-stepping and knee bend exercises increased up to three inches compared to deflections in a non-rotating environment. This issue must be further addressed to determine if stair-stepping or knee bend exercises are to be used safely in artificial gravity.
by Jessica Leigh Edmonds.
S.M.

In this thesis we discuss scalar field theories, and their applications to gravity. We provide a summary of why there is interest in modifying Einstein’s General Relativity, and discuss why scalar fields make a good candidate for a modification to make. We demonstrate their effects on the dynamics of matter, and discuss the necessity of screening mechanisms in order for these scalar fields to not be ruled out by current observations. We present discussion on two screening mechanisms in particular, the Chameleon and Vainshtein mechanisms. We then present work that aims to study the soft behaviour of scattering amplitudes belonging to single scalar field theories. We generalise current techniques in the literature such that the study of a much wider set of theories is possible. We use this technique to perform a detailed study of a particular family of theories, a so called (1, 2) theory, and demonstrate that the DBI symmetry is the unique way to enhance the soft behaviour of the scattering amplitudes of this family. We also identify the special Galileon as the unique way to maximally enhance the soft behaviour within the (1, 2) class, and verify the validity of recursion techniques to calculate scattering amplitudes. We then move on to studying the Chameleon in more detail. We provide motivation for modifying its high energy behaviour by studying the ‘surfer solution’, and use this to propose the DBI-Chameleon. We demonstrate that this theory avoids the problems the Chameleon suffers in the early Universe and forms a good effective field theory in this regime. Finally we present a UV complete theory describing a massive Galileon, and study its dynamics to verify if it exhibits Vainshtein screening. Theories with Vainshtein screening are usually unable to be UV completed in a Wilsonian way. We present our preliminary findings which suggest screening is possible for at least some parameter values.

This thesis is concerned with the theory of spinors, embeddings and everywhere invariance with applications to general relativity. The approach is entirely geometric with particular emphasis on the use of natural structures. A clear indication of the interaction between the above topics is given; this Interaction then sheds light on various aspects of general relativity theory. The main ideas discussed are:- (i) Spinors, conformal structure and the spacetime projective null bundle framework. (ii) Spaces of embeddings. (ill) Embeddings and spin structure. (iv) Null embeddings and the null limit (a technique for obtaining differential equations on null hypersurfaces). (v) Quasi-local momentum. (vi) The space of metrics, natural group actions and generalized conformal structure. (vii) Everywhere invariance and the invariance equation as a method for obtaining spacetime symmetries. Three appendices are also provided:- These give comprehensive summaries of the theories of principal bundles, conformal structure and asymptotic simplicity.

Cette thèse concerne principalement, sans toutefois s'y limiter, le problème de la gravité quantique dans le contexte de la gravité quantique en boucle. Les deux aspects fondamentaux et les conséquences physiques de la gravité à boucles sont étudies dans ce travail. Nous étudions l'invariance de Lorentz de la gravité quantique de la boucle, à la fois dans l'approche canonique et dans le modelé de mousse de spin. Nous présentons une description de jauge su(1,1) de la théorie de la gravité en quatre dimensions, au lieu de la description habituelle su(2). Nous étudions la quantification de boucle au niveau cinématique, où nous avons constaté il n'y a pas d'anomalie entre la description su(1,1) et la description su(2). Dans le même temps, nous effectuons l'analyse semi-classique du modelé de mousse de spin de Conrady-Hnybida dans une situation très générale dans laquelle des tétraèdres de type temps avec des triangles de type temps sont pris en compte. Nous identifions la contribution dominante avec des géométries simplicales discrètes et nous retrouvons l'action de gravité de Regge. Dans une seconde partie de cette thèse, nous étudions le lien entre la gravité mimétique étendue, une classe de théories scalaires-tenseurs, et la dynamique effective de la cosmologie quantique à boucles ainsi que les modèles de trous noirs polymères sphériques inspirés de la gravité quantique à boucles. En attendant, nous résolvons explicitement les équations d'Einstein modifiées dans le cadre de modèles de polymères effectifs à symétrie sphérique. La métrique effective pour une géométrie de trou noir intérieure statique décrivant la région piégée est donnée
This thesis mostly involves, but not restricts to, the problem of quantum gravity in the context of loop quantum gravity. Both fundamental aspects and the physical consequences of loop gravity are investigated in this work. We study the Lorentzian invariance of loop quantum gravity, in both the canonical approach and the spin foam model approach. We introduce an su(1, 1) gauge description of gravity theory in four dimensions, instead of the usual su(2) description. We investigate the loop quantization at the kinematical level, where we show that there is no anomaly between the su(1, 1) description and the su(2) description of space-like areas. Meanwhile, we perform the semi-classical (large-j asymptotic) analysis of the spin foam model (Conrady-Hnybida extension) in the most general situation, in which timelike tetrahedra with timelike triangles are taken into account. We identify the dominant contribution to the discrete simplicial geometries and recover the Regge action of gravity. On a second part of this thesis we focus on the problem of the high energy effective dynamics of loop gravity in cosmology and black holes through simplified models. We investigate the link between the family of extended Mimetic gravity, a class of scalar-tensor theories, and the effective dynamics of loop quantum cosmology as well as the spherical polymer black hole models inspired from loop quantum gravity. Futhermore, we solve the modified Einstein's equations explicitly in the framework of effective spherically symmetric polymer models. The effective metric for a static interior Black Hole geometry describing the trapped region is given

Regardless of new mining technologies and environmental regulations, the minerals we extract from the earth’s crust will eventually run out. Likewise, our society demands a constant increase of technology to improve our quality of life. Mining in Zero Gravity is a speculative design project that offers a vision of our first attempt at mining platinum group metals from asteroids by the year 2040. Kolibri is designed within the boundaries of the future challenges facing the mining industry and the development of our space industry.

Includes bibliographical references.
The key element of modern cosmology is the assumption that the General Theory of Relativity (GR) is the correct theory of gravitation. It replaced the Newtonian theory of gravity which was presented in the Principia in 1687. The fundamental idea in GR is that gravity is a manifestation of the curvature of the spacetime, while in Newton’s theory gravity acts directly as a force between bodies. Many of the predictions of GR, such as the bending of star light by gravity and a tiny shift in the orbit of the planetMercury, have been quantitatively confirmed by experiment