Дисертації з теми "Phenomenology of quantum gravity"
Щоб переглянути інші типи публікацій з цієї теми, перейдіть за посиланням: Phenomenology of quantum gravity.
Оформте джерело за APA, MLA, Chicago, Harvard та іншими стилями
Ознайомтеся з топ-50 дисертацій для дослідження на тему "Phenomenology of quantum gravity".
Біля кожної праці в переліку літератури доступна кнопка «Додати до бібліографії». Скористайтеся нею – і ми автоматично оформимо бібліографічне посилання на обрану працю в потрібному вам стилі цитування: APA, MLA, «Гарвард», «Чикаго», «Ванкувер» тощо.
Також ви можете завантажити повний текст наукової публікації у форматі «.pdf» та прочитати онлайн анотацію до роботи, якщо відповідні параметри наявні в метаданих.
Переглядайте дисертації для різних дисциплін та оформлюйте правильно вашу бібліографію.
1
Morgan, Dean Robert. "Quantum gravity phenomenology and high-energy neutrinos." Thesis, University of Sheffield, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.425199.
2
Blair, G. A. "Superstring inspired phenomenology." Thesis, University of Oxford, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.375220.
3
Waldron, Alison. "Quantum gravity induced decoherence and phenomenology of discrete symmetries." Thesis, King's College London (University of London), 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.435195.
4
Hersent, Kilian. "Field theories on quantum space-times : towards the phenomenology of quantum gravity." Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASP031.
Анотація:
La géométrie noncommutative est un formalisme mathématique qui exprime la structure de l’espace-temps avec des algèbres d’opérateurs. On s’attend à ce que les espace-temps noncommutatifs fassent émerger des effets de gravité quantiques, au moins dans un certain régime, notamment parce qu’ils utilisent les outils de la mécanique quantique pour décrire la géométrie. Ce manuscrit se concentre sur les aspects physiques de ces espace-temps quantiques, tout particulièrement à travers le formalisme des théories de champs et de jauge. Il est montré que les théories de champs scalaires engendrent possiblement des divergences dans l’infra-rouge et l’ultra-violet pour la fonction 2-point à une boucle. Ce phénomène s’appelle génériquement le mélange UV/IR et découle de la divergence du propagateur. L’analyse de ces divergences diffèrent du cas commutatif car l’espace des moments y est noncommutatif. D’autre part, une théorie de jauge sur κ-Minkowski, une déformation quantique de l’espace de Minkowski, est construite. Un premier calcul perturbatif produit une brisure de l’invariance de jauge, un comportement pathologique commun à d’autres espace-temps quantiques. Un modèle-jouet de causalité est aussi développé sur κ-Minkowski, dans lequel apparaît un analogue de la vitesse de lumière comme vitesse limite. La phénoménologie de la gravité quantique émergeant des espace-temps quantiques est abordée, avec les contraintes qu’elle impose. Finalement, un modèle-jouet de gravité noncommutative, utilisant κ-Minkowski pour décrire l’espace tangent, est traité. Il nécessite le concept de partition de l’unité noncommutative spécialement défini dans ce contexteNoncommutative geometry is a mathematical framework that expresses the structure of space-time in terms of operator algebras. By using the tools of quantum mechanics to describe the geometry, noncommutative space-times are expected to give rise to quantum gravity effects, at least in some regime. This manuscript focuses on the physical aspects of these so-called quantum space-times, in particular through the formalism of field and gauge theories. Scalar field theories are shown to possibly trigger mixed divergences in the infra-red and ultra-violet for the 2-point function at one loop. This phenomenon is generically called UV/IR mixing and stems from a diverging behaviour of the propagator. The analysis of such divergences differs from the commutative case because the momentum space is now also noncommutative. From another perspective, a gauge theory on κ-Minkowski, a quantum deformation of the Minkowski space-time, is derived. A first perturbative computation is shown to break the gauge invariance, a pathological behaviour common to other quantum space-times. A causality toy model is also developed on κ-Minkowski, in which an analogue of the speed-of-light limit emerges. The phenomenology of quantum gravity arising from quantum space-times is discussed, together with the actual constraints it imposes. Finally, a toy model for noncommutative gravity is tackled, using the former κ-Minkowski space-time to describe the tangent space. It necessitates the notion of noncommutative partition of unity specifically defined there
5
Sheppeard, Marni Dee. "Gluon Phenomenology and a Linear Topos." Thesis, University of Canterbury. Physics and Astronomy, 2007. http://hdl.handle.net/10092/1436.
Анотація:
In thinking about quantum causality one would like to approach rigorous QFT from outside the perspective of QFT, which one expects to recover only in a specific physical domain of quantum gravity. This thesis considers issues in causality using Category Theory, and their application to field theoretic observables. It appears that an abstract categorical Machian principle of duality for a ribbon graph calculus has the potential to incorporate the recent calculation of particle rest masses by Brannen, as well as the Bilson-Thompson characterisation of the particles of the Standard Model. This thesis shows how Veneziano n point functions may be recovered in such a framework, using cohomological techniques inspired by twistor theory and recent MHV techniques. This distinct approach fits into a rich framework of higher operads, leaving room for a generalisation to other physical amplitudes. The utility of operads raises the question of a categorical description for the underlying physical logic. We need to consider quantum analogues of a topos. Grothendieck's concept of a topos is a genuine extension of the notion of a space that incorporates a logic internal to itself. Conventional quantum logic has yet to be put into a form of equal utility, although its logic has been formulated in category theoretic terms. Axioms for a quantum topos are given in this thesis, in terms of braided monoidal categories. The associated logic is analysed and, in particular, elements of linear vector space logic are shown to be recovered. The usefulness of doing so for ordinary quantum computation was made apparent recently by Coecke et al. Vector spaces underly every notion of algebra, and a new perspective on it is therefore useful. The concept of state vector is also readdressed in the language of tricategories.
6
Giusti, Andrea. "Planck stars: theory and phenomenology." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2015. http://amslaurea.unibo.it/9315/.
Анотація:
General Relativity (GR) is one of the greatest scientific achievements of the 20th century along with quantum theory.
Despite the elegance and the accordance with experimental tests, these two theories appear to be utterly incompatible at fundamental level.
Black holes provide a perfect stage to point out these difficulties. Indeed, classical GR fails to describe Nature at small radii, because nothing prevents quantum mechanics from affecting the high curvature zone, and because classical GR becomes ill-defined at r = 0 anyway.
Rovelli and Haggard have recently proposed a scenario where a negative quantum pressure at the Planck scales stops and reverts the gravitational collapse, leading to an effective “bounce” and explosion, thus resolving the central singularity. This scenario, called Black Hole Fireworks, has been proposed in a semiclassical framework.
The purpose of this thesis is twofold:
- Compute the bouncing time by means of a pure quantum computation based on Loop Quantum Gravity;
- Extend the known theory to a more realistic scenario, in which the rotation is taken into account by means of the Newman-Janis Algorithm.
7
Brighenti, Francesco <1988>. "Quantum Gravity Phenomenology: Thermal Dimension of Quantum Spacetime, Casuality and Momentum Conservation from Relative Locality." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amsdottorato.unibo.it/7807/1/Brighenti_Francesco_tesi.pdf.
Анотація:
The original results presented in this thesis regard two very common topics of discussion in the quantum gravity debate: the dynamical dimensional reduction of spacetime and locality in quantum gravity regime. The dimensionality of quantum spacetime is often understood in terms of the spectral dimension; here, a different notion of dimensionality, the thermal dimension, is proposed. I discuss its physical properties in relation to those of the spectral dimension through the study of specific models of quantum gravity, including preliminary results obtained in the case of models with relative locality. I show that, in those cases where the spectral dimension has puzzling properties, the thermal dimension gives a different and more meaningful picture. The statistical mechanics developed to define the thermal dimension is applied also to the study of the production of primordial cosmological perturbations assuming a running Newton constant and Rainbow gravity. Concerning locality, I study in particular the theory of Relative Locality, a theoretical framework in which different observers may describe the same event as being local or non-local, depending whether it happens in the origin of their reference frame or far away from it, respectively. I show that requiring that locality is relative is enough to guarantee the objectivity of cause-effect relation in chains of events, the absence of causality-violating loops and processes violating the law of conservation of momentum.I risultati originali presentati in questa tesi riguardano due argomenti di discussione molto comuni nel dibattito sulla gravità quantistica: la riduzione dimensionale dinamica dello spaziotempo e la località nel regime di gravità quantistica. La dimensionalità dello spaziotempo quantistico viene spesso descritta in termini della dimensione spettrale; qui viene proposta una nuova nozione di dimensione, la dimensione termale. Discuto le sue proprietà fisiche in relazione a quelle della dimensione spettrale attraverso lo studio di modelli specifici di gravità quantistica, comprendendo anche risultati preliminari ottenuti nel caso di modelli con località relativa. Mostro che , in quei casi in cui la dimensione spettrale ha proprietà bizzarre, la dimensione termale fornisce un quadro diverso e più significativo. La meccanica statistica costruita per definire la dimensione termale è applicata anche allo studio della produzione delle perturbazioni cosmologiche primordiali, assumendo che costante di Newton dipenda dall'energia e la gravità "Rainbow". Per quanto riguarda la località, studio in particolare la teoria delle Località Relativa, un paradigma teorico in cui diversi osservatori possono descrivere uno stesso evento come locale o non locale, a seconda se questo avviene nell'origine del loro sistema di riferimento o lontano dall'origine, rispettivamente. Mostro che richiedere che la località sia relativa è sufficiente a garantire l'oggettività della relazione di causa-effetto in catene di eventi, l'assenza di processi che violano la causalità e di processi che violano la conservazione del momento.
8
Vernieri, Daniele. "Gravity beyond General Relativity: New Proposals and their Phenomenology." Doctoral thesis, SISSA, 2014. http://hdl.handle.net/20.500.11767/3860.
Анотація:
This Thesis is devoted to the study of phenomenologically viable gravitational theories,
in order to address the most pressing open issues both at very small and very large energy scales. Lovelock’s theorem singles out General Relativity as the only theory with secondorder field equations for the metric tensor. So, two possible ways to circumvent it and modify the gravitational sector are taken into account. The first route consists in giving
up diffeomorphism invariance, which generically leads to extra propagating degrees of
freedom. In this framework Horava gravity is discussed, presenting two restrictions, called
respectively "projectability" and "detailed balance", which are imposed in order to reduce
the number of terms in the full theory. We introduce a new version of the theory assuming
detailed balance but not projectability, and we show that such theory is dynamically
consistent as both the spin-0 and spin-2 gravitons have a well behaved dynamics at low-energy. Moreover three-dimensional rotating black hole solutions are found and fully
studied in the context of Horava gravity, shedding light on its causal structure. A new
concept of black hole horizon, dubbed "universal horizon", arises besides the usual event
horizon one, since in Lorentz-violating gravity theories there can be modes propagating
even at infinite speed. The second route which is considered, consists in adding extra
fields to the gravitational action while diffeomorphism invariance is preserved. In this
respect we consider the less explored option that such fields are auxiliary fields, so they
do not satisfy dynamical equations but can be instead algebraically eliminated. A very
general parametrization for these theories is constructed, rendering also possible to put
on them very tight, theory-independent constraints. Some insight about the cosmological
implications of such theories is also given. Finally in the conclusions we discuss about
the future challenges that the aforementioned gravity theories have to face.
9
Coutant, Antonin. "On the phenomenology of quantum gravity : stability properties of Hawking radiation in the presence of ultraviolet violation of local Lorentz invariance." Thesis, Paris 11, 2012. http://www.theses.fr/2012PA112213/document.
Анотація:
Dans cette thèse, nous étudions plusieurs aspects de la radiation de Hawking en présence de violations de l'invariance locale de Lorentz. Ces violations sont introduites par une modification de la relation de dispersion, devenant non-linéaire aux courtes longueurs d’onde. Les principales motivations de ces travaux ont une double origine. Il y a d’une part le développement en matière condensée de trous noirs analogues, ou l’écoulement d’un fluide est perçu comme une métrique d’espace-temps pour les ondes de perturbations et ou la radiation de Hawking pourrait être détectée expérimentalement. D’autre part, il se pourrait que des effets de gravité quantique puissent être modélisés par une modification de la relation de dispersion. En premier lieu, nous avons obtenu des caractérisations précises des conditions nécessaires au maintien de l’effet Hawking en présence de violation de l’invariance de Lorentz. De plus, nous avons étudié l’apparition d’une onde macroscopique de fréquence nulle, dans des écoulements de type trous blancs et également pour des champs massifs. Une autre partie de ce travail a consisté à analyser une instabilité engendrée par les effets dispersifs, ou la radiation de Hawking est auto-amplifiée, générant ainsi un flux sortant exponentiellement croissant dans le tempsIn this thesis, we study several features of Hawking radiation in the presence of ultraviolet Lorentz violations. These violations are implemented by a modified dispersion relation that becomes nonlinear at short wavelengths. The motivations of this work arise on the one hand from the developing field of analog gravity, where we aim at measuring the Hawking effect in fluid flows that mimic black hole space-times, and on the other hand from the possibility that quantum gravity effects might be approximately modeled by a modified dispersion relation. We develop several studies on various aspects of the prob- lem. First we obtain precise characterizations about the deviations from the Hawking result of black hole radiation, which are induced by dispersion. Second, we study the emergence, both in white hole flows or for massive fields, of a macroscopic standing wave, spontaneously produced from the Hawking effect, and known as ‘undulation’. Third, we describe in detail an instability named black hole laser, which arises in the presence of two horizons, where Hawking radiation is self-amplified and induces an exponentially growing in time emitted flux
10
Belenchia, Alessio. "Exploring spacetime phenomenology: from Lorentz violations to experimental tests of non-locality." Doctoral thesis, SISSA, 2016. http://hdl.handle.net/20.500.11767/4889.
Анотація:
This thesis deals primarily with the phenomenology associated to quantum
aspects of spacetime. In particular, it aims at exploring the phenomenological
consequences of a fundamental discreteness of the spacetime fabric,
as predicted by several quantum gravity models and strongly hinted by
many theoretical insights.
The first part of this work considers a toy-model of emergent spacetime
in the context of analogue gravity. The way in which a relativistic Bose–
Einstein condensate can mimic, under specific configurations, the dynamics
of a scalar theory of gravity will be investigated. This constitutes proof-ofconcept
that a legitimate dynamical Lorentzian spacetime may emerge from
non-gravitational (discrete) degrees of freedom. Remarkably, this model
will emphasize the fact that in general, even when arising from a relativistic
system, any emergent spacetime is prone to show deviations from exact
Lorentz invariance. This will lead us to consider Lorentz Invariance Violations
as first candidate for a discrete spacetime phenomenology.
Having reviewed the current constraints on Lorentz Violations and studied
in depth viable resolutions of their apparent naturalness problem, the
second part of this thesis focusses on models based on Lorentz invariance.
In the context of Casual Set theory, the coexistence of Lorentz invariance
and discreteness leads to an inherently nonlocal scalar field theory over
causal sets well approximating a continuum spacetime. The quantum aspects
of the theory in flat spacetime will be studied and the consequences
of its non-locality will be spelled out. Noticeably, these studies will lend
support to a possible dimensional reduction at small scales and, in a classical
setting, show that the scalar field is characterized by a universal nonminimal
coupling when considered in curved spacetimes.
Finally, the phenomenological possibilities for detecting this non-locality
will be investigated. First, by considering the related spontaneous emission
of particle detectors, then by developing a phenomenological model to test
nonlocal effects using opto-mechanical, non-relativistic systems. In both
cases, one could be able to cast in the near future stringent bounds on the
non-locality scale.
11
Thürigen, Johannes. "Discrete quantum geometries and their effective dimension." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2015. http://dx.doi.org/10.18452/17309.
Анотація:
In einigen Ansätzen zu einer Quantentheorie der Gravitation wie Gruppenfeldtheorie und Schleifenquantengravitation zeigt sich, dass Zustände und Entwicklungen der geometrischen Freiheitsgrade auf einer diskreten Raumzeit basieren. Die dringendste Frage ist dann, wie die glatten Geometrien der Allgemeinen Relativitätstheorie, beschrieben durch geeignete geometrische Beobachtungsgrößen, aus solch diskreten Quantengeometrien im semiklassischen und Kontinuums-Limes hervorgehen. Hier nehme ich die Frage geeigneter Beobachtungsgrößen mit Fokus auf die effektive Dimension der Quantengeometrien in Angriff. Dazu gebe ich eine rein kombinatorische Beschreibung der zugrunde liegenden diskreten Strukturen. Als Nebenthema erlaubt dies eine Erweiterung der Gruppenfeldtheorie, so dass diese den kombinatorisch größeren kinematischen Zustandsraum der Schleifenquantengravitation abdeckt. Zudem führe ich einen diskreten Differentialrechnungskalkül für Felder auf solch fundamental diskreten Geometrien mit einem speziellen Augenmerk auf dem Laplace-Operator ein. Dies ermöglicht die Definition der Dimensionsobservablen für Quantengeometrien. Die Untersuchung verschiedener Klassen von Quantengeometrien zeigt allgemein, dass die spektrale Dimension stärker von der zugrunde liegenden kombinatorischen Struktur als von den Details der zusätzlichen geometrischen Daten darauf abhängt. Semiklassische Zustände in Schleifenquantengravitation approximieren die entsprechenden klassischen Geometrien gut ohne Anzeichen für stärkere Quanteneffekte. Dagegen zeigt sich im Kontext eines allgemeineren, auf analytischen Lösungen basierenden Modells für Zustände, die aus Überlagerungen einer großen Anzahl von Komplexen bestehen, ein Fluss der spektralen Dimension von der topologischen Dimension d bei kleinen Energieskalen hin zu einem reellen Wert zwischen 0 und d bei hohen Energien. Im Spezialfall 1 erlauben diese Resultate, die Quantengeometrie als effektiv fraktal aufzufassen.In several approaches towards a quantum theory of gravity, such as group field theory and loop quantum gravity, quantum states and histories of the geometric degrees of freedom turn out to be based on discrete spacetime. The most pressing issue is then how the smooth geometries of general relativity, expressed in terms of suitable geometric observables, arise from such discrete quantum geometries in some semiclassical and continuum limit. In this thesis I tackle the question of suitable observables focusing on the effective dimension of discrete quantum geometries. For this purpose I give a purely combinatorial description of the discrete structures which these geometries have support on. As a side topic, this allows to present an extension of group field theory to cover the combinatorially larger kinematical state space of loop quantum gravity. Moreover, I introduce a discrete calculus for fields on such fundamentally discrete geometries with a particular focus on the Laplacian. This permits to define the effective-dimension observables for quantum geometries. Analysing various classes of quantum geometries, I find as a general result that the spectral dimension is more sensitive to the underlying combinatorial structure than to the details of the additional geometric data thereon. Semiclassical states in loop quantum gravity approximate the classical geometries they are peaking on rather well and there are no indications for stronger quantum effects. On the other hand, in the context of a more general model of states which are superposition over a large number of complexes, based on analytic solutions, there is a flow of the spectral dimension from the topological dimension d on low energy scales to a real number between 0 and d on high energy scales. In the particular case of 1 these results allow to understand the quantum geometry as effectively fractal.
12
Di, Casola Eolo. "Sieving the Landscape of Gravity Theories. From the Equivalence Principles to the Near-Planck Regime." Doctoral thesis, SISSA, 2014. http://hdl.handle.net/20.500.11767/3911.
Анотація:
This thesis focusses on three main aspects of the foundations of any theory of gravity where the gravitational field admits a geometric interpretation: (a) the principles of equivalence; (b) their role as selection rules in the landscape of extended theories of gravity; and (c) the possible modifications of the spacetime structure at a "mesoscopic" scale, due to underlying, microscopic-level, quantum-gravitational effects.
The first result of the work is the introduction of a formal definition of the Gravitational Weak Equivalence Principle, which expresses the universality of free fall of test objects with non-negligible self-gravity, in a matter-free environment. This principle extends the Galilean universality of free-fall world-lines for test bodies with negligible self-gravity (Weak Equivalence Principle).
Second, we use the Gravitational Weak Equivalence Principle to build a sieve for some classes of extended theories of gravity, to rule out all models yielding non-universal free-fall motion for self-gravitating test bodies. When applied to metric theories of gravity in four spacetime dimensions, the method singles out General Relativity (both with and without the cosmological constant term), whereas in higher-dimensional scenarios the whole class of Lanczos--Lovelock gravity theories also passes the test.
Finally, we focus on the traditional, manifold-based model of spacetime, and on how it could be modified, at a "mesoscopic" (experimentally attainable) level, by the presence of an underlying, sub-Planckian quantum regime. The possible modifications are examined in terms of their consequences on the hypotheses at the basis of von Ignatowski's derivation of the Lorentz transformations. It results that either such modifications affect sectors already tightly constrained (e.g. violations of the principle of relativity and/or of spatial isotropy), or they demand a radical breakdown of the operative interpretation of the coordinates as readings of clocks and rods.
13
Martineau, Killian. "Quelques aspects de cosmologie et de physique des trous noirs en gravitation quantique à boucles Detailed investigation of the duration of inflation in loop quantum cosmology for a Bianchi I universe with different inflaton potentials and initial conditions Some clarifications on the duration of inflation in loop quantum cosmology A first step towards the inflationary trans-Planckian problem treatment in loop quantum cosmology Scalar spectra of primordial perturbations in loop quantum cosmology Phenomenology of quantum reduced loop gravity in the isotropic cosmological sector Primordial Power Spectra from an Emergent Universe: Basic Results and Clarifications Fast radio bursts and the stochastic lifetime of black holes in quantum gravity Quantum fields in the background spacetime of a polymeric loop black hole Quasinormal modes of black holes in a toy-model for cumulative quantum gravity Seeing through the cosmological bounce: Footprints of the contracting phase and luminosity distance in bouncing models Dark matter as Planck relics without too exotic hypotheses A Status Report on the Phenomenology of Black Holes in Loop Quantum Gravity: Evaporation, Tunneling to White Holes, Dark Matter and Gravitational Waves." Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY044.
Анотація:
Cantonnée à la physique mathématique depuis des décennies, la gravitation quantique entre désormais dans le giron de la science expérimentale. Suivant cette mouvance nous considérons dans cette thèse trois cadres d’application de la gravitation quantique à boucles (LQG) : le système Univers, les trous noirs et les astroparticules. Le troisième n’est qu’esquissé tandis que les deux premiers sont présentés plus en détails.Le secteur cosmologique étant l’un des domaines les plus prometteurs pour tester et contraindre des théories de gravité quantique, le développement de différents modèles tentant d’appliquer les idées de la LQG à l’Univers primordial ne s’est pas fait attendre. Les travaux que nous présentons portent sur la phénoménologie associée à ces modèles; tant dans le secteur homogène (où nous nous focalisons notamment sur la durée de la phase d’inflation), que dans le secteur inhomogène (nous étudions ce coup-ci le devenir des spectres de puissance primordiaux). Ces études combinées nous permettent alors de préciser dans quelle mesure des effets de gravité quantique (à boucles) peuvent être observés dans les anisotropies du fond diffus cosmologique.D’autre part les trous noirs, non contents de faire partie des objets les plus étranges et les plus fascinants de l’Univers, constituent également des sondes privilégiées pour tester des théories de gravitation. Nous développons la phénoménologie associée à différents traitements des trous noirs en gravitation quantique à boucles. Celle-ci intervient sur une grande variété de fronts : de l’évaporation de Hawking aux ondes gravitationnelles, en passant par la matière noire. C’est sans nul doute un domaine riche et vaste.Finalement, l’existence d’une échelle de longueur minimale, prédite par la majorité des théories de gravité quantique, suggère une généralisation du principe d’incertitude de Heisenberg. Partant de ce constat nous présentons également dans ce manuscrit une méthodologie permettant de calculer une nouvelle relation de dispersion de la lumière à partir du principe d’incertitude généralisé le plus couramment répanduAfter decades of being confined to mathematical physics, quantum gravity now enters the field of experimental science. Following this trend, we consider throughout this thesis three implementation frameworks of Loop Quantum Gravity (LQG): the Universe as a system, black holes and astroparticles. The last one is only outlined while the first two are presented in more detail.Since the cosmological sector is one of the most promising areas for testing and constraining quantum gravity theories, it was not long before the development of different models attempting to apply the ideas of the LQG to the primordial Universe. The work we present deals with the phenomenology associated with these models; both in the homogeneous sector (where we focus particularly on the duration of the inflation phase), as in the inhomogeneous sector (where this time, we study the fate of the primordial power spectra). These combined studies then allow us to specify to what extent effects of (loop) quantum gravity can be observed in the anisotropies of the cosmic microwave background.On the other hand black holes, not content to be among the strangest and most fascinating objects of the Universe, are also prominent probes to test the theories of gravitation. We develop the phenomenology associated with different treatments of black holes in the loop quantum gravity framework, which intervenes on multiple levels: from the evaporation of Hawking to gravitational waves, including dark matter. This is undoubtedly a rich and vast area.Finally, the existence of a minimal length scale, predicted by the majority of quantum gravity theories, suggests a generalization of the Heisenberg uncertainty principle. On the basis of this observation, we also present in this manuscript a methodology to derive a new relation dispersion of light from the most widely used generalized uncertainty principle
14
TORRI, MARCO DANILO CLAUDIO. "LORENTZ INVARIANCE VIOLATION EFFECTS ON ULTRA HIGH ENERGY COSMIC RAYS PROPAGATION: A GEOMETRICAL APPROACH." Doctoral thesis, Università degli Studi di Milano, 2019. http://hdl.handle.net/2434/625711.
Анотація:
Cosmic rays are highly energetic extraterrestrial particles, mainly originated outside the Solar system, with energy that spans many decades. Since they include the most energetic particles, accessible nowadays, it is very interesting to study this kind of radiation, that constitute a significant source of informations about astrophysical objects. Moreover these highly energetic particles propagate for cosmological distances, so they can furnish a deeper understanding of the physical mechanisms of the universe. Hence it results very important to obtain a deeper understanding of the so called GZK “puzzle”. Universe results opaque to the propagation of the highest energetic particles, because of their interaction with the Cosmic Microwave Background Radiation (CMBR). Therefore the sources of these Ultra High Energy Cosmic Rays (UHECR) must be collocated inside a foreseen opacity sphere (GZK effect). But some recent experimental observations seem to indicate the possibility that certain UHECR correlate with candidate sources collocated farther than expected.
One of the most interesting possible explanations of this GZK suppression “puzzle” consists in introducing a particle kinematics modification, assuming this effect as a relic of the supposed quantum structure of space-time. In this respect, physics is amended by the introduction of small perturbations to the Lorentz symmetry, the so called Lorentz Invariance Violation (LIV) scenario. In this work, to preserve the idea of space-time homogeneity and isotropy, a possible way to introduce a LIV theory, without a preferred class of inertial observers, is explored. The Lorentz symmetry is therefore only modified, as in Doubly Special Relativity theories. Thus the idea of space-time isotropy results restored respect to the new amended Lorentz transformations, here introduced. Hence it results possible to solve the GZK “puzzle” without the necessity of the introduction of a privileged class of inertial observers.
The geometry of space-time is constructed starting from the momentum space modified structure, determined by the amended particle kinematics. The resultant geometry is of Finsler type, with an acquired energy dependance. The Lorentz group is then modified, in order to preserve space-time isotropy. The particle Standard Model results modified, but it still preserve the symmetry structure of the ordinary one, as shown proving the validity of the Coleman-Mandula theorem, with the substitution of the ordinary Lorentz group with the modified one. Finally the model is employed to compute phenomenological predictions on the behavior of UHECR and even of high energy neutrinos.
15
Bianchi, Eugenio. "Loop Quantum Gravity." Doctoral thesis, Scuola Normale Superiore, 2010. http://hdl.handle.net/11384/85828.
16
Mannellli, Lorenzo. "Quantum gravity and cosmology /." Diss., Digital Dissertations Database. Restricted to UC campuses, 2005. http://uclibs.org/PID/11984.
17
Ali, Ahmed Farag. "Aspects of quantum gravity." Thesis, Lethbridge, Alta. : University of Lethbridge, Dept. of Physics and Astronomy, c2012, 2012. http://hdl.handle.net/10133/3119.
Анотація:
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.viii, 154 leaves ; 30 cm
18
Whitt, Brian. "Gravity : a quantum theory?" Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.304522.
19
Mackay, Paul. "Low energy quantum gravity." Thesis, University of Newcastle Upon Tyne, 2012. http://hdl.handle.net/10443/1752.
Анотація:
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.
20
Liu, Hongguang. "Aspects of quantum gravity." Thesis, Aix-Marseille, 2019. http://www.theses.fr/2019AIXM0209.
Анотація:
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 donnéeThis 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
21
Gupta, Arun K. Wise Mark B. Wise Mark B. "Quantum aspects of gravity /." Diss., Pasadena, Calif. : California Institute of Technology, 1990. http://resolver.caltech.edu/CaltechETD:etd-06072007-083631.
22
Gong, Yungui. "Issues in quantum gravity /." Full text (PDF) from UMI/Dissertation Abstracts International, 2001. http://wwwlib.umi.com/cr/utexas/fullcit?p3008342.
23
Flori, Cecilia. "Approaches to quantum gravity." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2011. http://dx.doi.org/10.18452/16344.
Анотація:
In dieser Arbeit beschäftigen wir uns mit zwei Ansätzen zur Quantengravitation (QG), die einander konträr gegenüberstehen: - Erstens mit der Loop Quantum Gravity (LQG), einem eher konservativen Ansatz zur QG, dessen Startpunkt eine Hamiltonsche Formulierung der klassischen Allgemeinen Relativitätstheorie (ART) ist, - zweitens mit der sogenannten Topos-Theorie, angewandt auf die Allgemeine Relativitätstheorie, die die mathematischen Konzepte der Quantentheorie (und möglicherweise auch der ART) radikal umformuliert, was eine immense Redefinition von Konzepten wie Raum, Zeit und Raumzeit zur Folge hätte. Der Grund für die Wahl zweier so verschiedener Ansätzen als Gegenstand dieser Arbeit liegt in der Hoffnung begründet, dass sich diese beiden Ansätze auf einen gemeinsamen Ursprung zurückführen lassen können und somit gegenseitig ergänzen können. Im ersten Teil dieser Arbeit führen wir den allgemeinen Formalismus der LQG ein und gehen dabei insbesondere auf den semiklassischen Sektor der Theorie ein; insbesondere untersuchen wir die semiklassischen Eigenschaften des Volumenoperators. Dieser Operator spielt in der Quantendynamik der LQG eine tragende Rolle, da alle bekannten dynamischen Operatoren auf den Volumenoperator zurückgeführt werden können. Aus diesem Grund ist es auerordentlich wichtig zu überprüfen, dass der klassische Limes des Volumenoperators wirklich mit dem klassischen Volumen übereinstimmt. Anschließend beschäftigen wir uns mit sogenannten Spin Foam Modellen (SFM), welche als ein kovarianter oder Pfadintegralzugang zur kanonischen LQG angesehen werden können. Diese Spin Foam Modelle beruhen auf einer Langrange-Formulierung der LQG mittels einer kovarianten sum-over-histories Beschreibung. Die Entwicklung eines Lagrange-Zuganges zur LQG wurde motiviert durch die Tatsache, dass es in der kanonischen Formulierung der LQG überaus schwierig ist, Übergangsamplituden auszurechnen. Allerdings weichen die Spin Foam Modelle, die wir in dieser Arbeit behandeln in einem entscheidenden Punkt von den bisher in der Literatur diskutierten ab, da wir die Holst-Wirkung Holst [1996] und nicht die Palatini-Wirkung als Ausgangspunkt nehmen. Dies ermöglicht es uns, explizit gewisse Zwangsbedingungen zu lösen, was in den gegenwärtig diskutierten SFM problematisch scheint. Im zweiten Teil dieser Arbeit führen wir in die Topos-Theorie ein und rekapitulieren, wie diese Theorie benutzt werden kann, um die Quantentheorie derart umzuformulieren, dass eine konsistente Quanten-Logik definiert werden kann. Darüber hinaus definieren wir auch eine Topos-Beschreibung der Quantentheorie in der sum-over-histories Formulierung. Unser Ansatz entscheidet sich vom gegenwärtigen consistent-histories Ansatz vor allem dadurch, dass das Konzept der konsistenten Menge (eine Menge von Historien, die nicht mit sich selbst interferieren) keine zentrale Rolle spielt, während es in letzterem grundlegend ist. Diese Tatsache bietet einen interessanten Ausgangspunkt, da eine der Hauptschwierigkeiten im consistent-histories Ansatz darin besteht, die richtige konsistente Menge der Propositionen von Historien zu finden: Im allgemeinen gibt es viele solcher Mengen, und die meisten davon sind nicht miteinander kompatibel. Wir zeigen, dass in unserer Topos-Beschreibung der sum-over-histories Quantentheorie jeder Proposition von Historien Wahrheitswerte zugeteilt werden können; daher ist das Konzept einer konsistenten Menge von Propositionen redundant. Dies bedeutet, dass es im Rahmen einer Quantengravitationstheorie möglich sein könnte, jeder Proposition von vierdimensionalen Metriken (welche als allgemein relativistisches Analogon einer Historie angesehen werden können) einen Wahrheitswert zuzuweisen.One of the main challenges in theoretical physics over the last five decades has been to reconcile quantum mechanics with general relativity into a theory of quantum gravity. However, such a theory has been proved to be hard to attain due to i) conceptual difficulties present in both the component theories (General Relativity (GR) and Quantum Theory); ii) lack of experimental evidence, since the regimes at which quantum gravity is expected to be applicable are far beyond the range of conceivable experiments. Despite these difficulties, various approaches for a theory of Quantum Gravity have been developed. In this thesis we focus on two such approaches: Loop Quantum Gravity and the Topos theoretic approach. The choice fell on these approaches because, although they both reject the Copenhagen interpretation of quantum theory, their underpinning philosophical approach to formulating a quantum theory of gravity are radically different. In particular LQG is a rather conservative scheme, inheriting all the formalism of both GR and Quantum Theory, as it tries to bring to its logical extreme consequences the possibility of combining the two. On the other hand, the Topos approach involves the idea that a radical change of perspective is needed in order to solve the problem of quantum gravity, especially in regard to the fundamental concepts of `space'' and `time''. Given the partial successes of both approaches, the hope is that it might be possible to find a common ground in which each approach can enrich the other.
24
Kerr, Steven. "Topological quantum field theory and quantum gravity." Thesis, University of Nottingham, 2014. http://eprints.nottingham.ac.uk/14094/.
Анотація:
This thesis is broadly split into two parts. In the first part, simple state sum models for minimally coupled fermion and scalar fields are constructed on a 1-manifold. The models are independent of the triangulation and give the same result as the continuum partition functions evaluated using zeta-function regularisation. Some implications for more physical models are discussed. In the second part, the gauge gravity action is written using a particularly simple matrix technique. The coupling to scalar, fermion and Yang-Mills fields is reviewed, with some small additions. A sum over histories quantisation of the gauge gravity theory in 2+1 dimensions is then carried out for a particular class of triangulations of the three-sphere. The preliminary stage of the Hamiltonian analysis for the (3+1)-dimensional gauge gravity theory is undertaken.
25
Vitagliano, Vincenzo. "Gravity beyond general relativity : theory and phenomenology." Doctoral thesis, SISSA, 2011. http://hdl.handle.net/20.500.11767/4681.
Анотація:
Despite the notorious achievements of General Relativity, Einstein's theory is under scrutiny due to the lack of a suitable scheme to quantize gravity as well as for the puzzling features it shows both at strong (early universe, black holes) and weak (Dark Energy problem) regime. The proposal to extend the classical theory of gravity harbours the intriguing goals to cure some of these inconsistencies. A large class of modi cations of General Relativity (GR) has been widely explored in the past; in principle, the main motivation for such early e orts was to solve the problem of non-renormalizability by providing a new framework in which (thanks to higher order corrections in the gravitational action) gravity could be quantized. The analysis of the cosmological implications of such models also showed a number of peculiar features that justi ed further developments. The ultraviolet modi cations that naturally arise at high energy in the context of quantum gravity have been taken into account for their impact on the phenomenology of the very early universe. Furthermore, it was recently argued that alternative infrared extensions of the Einstein-Hilbert (EH) action could be invoked to presumably alleviate the Dark Sector problem.
26
Eriksen, Matin Børstad. "Dark energy from quantum gravity." Thesis, Norwegian University of Science and Technology, Department of Physics, 2009. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-6225.
Анотація:
The thesis consists of two parts, where the last one tries to model the evolution of the universe by using a scalar field. Within this class of models there is a huge amount of freedom and few simple restrictions. Often only examples are studied which are just tested against a subset of the requirements. In a try of being more systematic I developed a simulation for proposing and testing different evolutions of the universe more comprehensively.
A large subset of the proposed models pass the tests, and singling out models against observed requirements is like trying to start from the answer. For this reason I began to look for an underlying reason for the expansion of the universe. All conventional scalar field models are based on classical fields. Could the observed accelerated expansion of the universe emerge as a quantum gravitational phenomenon? The goal was a bit farfetched, but would provide a natural explanation for the expansion and could add insight to the field of quantum gravity.
After a longer period of unsuccessful tries, it was about time to write down the first draft of the thesis. It consisted of a review of dark energy models and quantum gravitational theory. Shortly after finishing the second revision I found a new way of attacking the problem. After a week of calculations I came to the conclusion that the new approach made sense and started over again, discarding the old draft.
The first part, which I regard to be the most important one, contains the new ideas and calculations. Here I studied the effect of uncertainties in the measurement of parallel transported four-vectors. This way of modelling quantum mechanical effects leads to a term which can cancel the vacuum energy contribution. The expansion of space results in an imperfect cancellation leading to a term equal to the observed dark energy. Though the results are not fully rigorous, since they aren't based on a fully developed theory of quantum gravity, they are surprising and give a reasonable basis for further studies.
27
Starodubtsev, Artem. "Topological methods in quantum gravity." Thesis, University of Waterloo, 2005. http://hdl.handle.net/10012/1217.
Анотація:
The main technical problem with background independent approaches to quantum gravity is inapplicability of standard quantum field theory methods. New methods are needed which would be adapted to the basic principles of General Relativity. Topological field theory is a model which provides natural tools for background independent quantum gravity. It is exactly soluble and, at the same time, diffeomorphism invariant. Applications of topological field theory to quantum gravity include description of boundary states of quantum General Relativity, formulation of quantum gravity as a constrained topological field theory, and a new perturbation theory which uses topological field theory as a starting point. The later is the central theme of the thesis. Unlike the traditional perturbation theory it does not require splitting metric into a background and fluctuations, it is exactly diffeomorphism invariant order by order, and the coupling constant of this theory is dimensionless. We describe the basic ideas and techniques of this perturbation theory as well as inclusion of matter particles, boundary states, and other necessary tools for studying scattering problem in background independent quantum gravity.
28
Markopoulou, Fotini G. "Constraints in canonical quantum gravity." Thesis, Imperial College London, 1998. http://hdl.handle.net/10044/1/11817.
29
Lafrance, René. "Two investigations in quantum gravity." Thesis, McGill University, 1996. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=40375.
Анотація:
Two aspects of quantum field theory in curved spacetimes are discussed. First, the limits for applicability of the equivalence principle in the context of low energy effective field theories is considered. In particular, we find three classes of higher-derivative interactions for the gravitational and electromagnetic fields which produce dispersive photon propagation. One of these classes of interactions also produces birefringent propagation. This result is illustrated by calculating the energy-dependent contribution to the bending of light. In the second part, the divergences appearing in statistical black hole entropy are analysed. Using a Pauli-Villars regulator, it is shown that 't Hooft's approach to evaluating black hole entropy through a statistical-mechanical counting of states for a scalar field propagating outside the event horizon yields precisely the one-loop renormalization of the standard Bekenstein-Hawking formula, $S = { cal A}/(4G),$ where $ cal A$ is the black hole area. The calculation also yields a constant contribution to the black hole entropy, which may be associated with the one-loop renormalization of certain higher curvature terms in the gravitational action. The calculation of black hole entropy is done for a Schwarzschild black hole as well as for a Reissner-Nordstrom black hole.
30
Grant, James D. E. "Spacetime distortion and quantum gravity." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.321392.
31
Garcia, Raquel Soledad. "Topology change in quantum gravity." Thesis, Imperial College London, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.322405.
32
NEDAL, RAFAEL KAUFMANN. "SEMICLASSICAL STATES IN QUANTUM GRAVITY." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2006. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=8988@1.
Анотація:
FUNDAÇÃO DE APOIO À PESQUISA DO ESTADO DO RIO DE JANEIROA teoria da gravidade quântica em laços (loop quantum gravity ou LQG) é atualmente uma das mais promissoras abordagens para descrever a relatividade geral em termos quânticos. Um dos problemas-chave é detectar na teoria quântica estados semiclássicos, que apresentem propriedades macroscópicas iguais às de configurações específicas da teoria clássica. Nesta dissertação, começamos apresentando o formalismo da LQG e sua interpretação física. Do ponto de vista matemático, a LQG pode ser pensada como uma quantização canônica de uma teoria de gauge de SU(2) em uma 3-variedade. No entanto, diferentemente da abordagem usual, que gera uma representação apenas por operadores auto-adjuntos, a abordagem polimérica da LQG gera uma representação mista que usa operadores auto-adjuntos e unitários. Tomamos então um modelo polimérico, análogo à LQG, do sistema físico mais simples: o movimento unidimensional de uma partícula pontual. Neste contexto, desenvolvemos um arcabouço que resolve o problema dos estados semiclássicos, que são estudados em detalhe. Finalmente, consideramos a quantização polimérica do campo eletromagnético livre, resultando numa teoria abeliana muito similar à LQG. Neste contexto, o mesmo arcabouço desenvolvido para o caso anterior pode ser aplicado.
Loop quantum gravity (LQG) is currently one of the most promising approaches to describing general relativity in quantum terms. One of its key issues is to detect in the quantum theory semiclassical states whose macroscopic properties are the same as those of specific configurations of the classical theory. In this dissertation, we begin by presenting the LQG formalism and its physical interpretation. From a mathematical point of view, LQG can be thought of as a canonical quantization of a SU(2) gauge theory in a 3-manifold. However, whereas the usual approach generates a representation exclusively by self-adjoint operators, LQG's polymer approach generates a mixed representation using both self- adjoint and unitary operators. We then take a polymer model, analogous to LQG, of the simplest physical system: the one-dimensional movement of a point particle. In this context, we develop a framework that solves the problem of semiclassical states, which are studied in detail. Finally, we consider the polymer quantization of the free electromagnetic field, which results in an abelian theory which is very similar to LQG. In this context, it is possible to apply the same framework that was developed for the previous case.
33
Jubb, Ian. "Causal structure and quantum gravity." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/56610.
Анотація:
In this thesis we will investigate the problem of quantum gravity from a variety of directions. Each avenue we explore begins at the quantum gravity path integral, and throughout our investigations the notion of spacetime causal structure will frequently appear. After a brief introduction to the path integral for quantum gravity we will present several of the concepts behind Causal Set Theory - an approach to quantum gravity in which the continuum spacetime is replaced by a discrete structure. We will then familiarise ourselves with the gravitational action that appears in the path integral, and its necessary boundary terms, in preparation for our discussion of the analogous quantities in Causal Set Theory. In particular, we will focus on the boundary terms in the causal set action and propose causal set expressions for the case of a spacelike boundary. We will then formulate causal set expressions to encode other boundary geometry, and conclude our discussion of the causal set action by investigating what boundary terms, if any, are present in the current proposal for the bulk causal set action. Finally, we will return to the continuum quantum gravity path integral and explore whether the sum over spacetimes should include spacetimes which exhibit spatial topology change. To attempt to answer this question we will focus our attention on the simple case of the trousers spacetime, and use the Sorkin-Johnston formalism to study a scalar quantum field theory living on the spacetime.
34
Lavdas, Ioannis. "AdS₄/CFT₃ and quantum gravity." Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEE041.
Анотація:
Dans le cadre de la dualité holographique entre une vaste famille de vides 1/2-maximalement supersymétriques Anti-de Sitter à quatre dimensions (AdS₄) et des théories des champs superconformes N=4 supersymétriques à trois dimensions (sCFT₃), nous étudions des questions théoriques majeures de gravité quantique et de théories de jauge. Ce travail a deux directions principales : La premiere partie est consacrée aux mécanismes par lesquels le graviton AdS₄ peut acquérir une petite masse, tandis que la seconde partie concerne la cartographie de la variété superconforme des sCFT₃ considérées. En ce qui concerne la question du mecanisme de Higgs pour le graviton d’AdS₄, nous proposons un nouveau mécanisme qui repose sur le couplage ”faible” de deux sCFT₃s, initialement découplées, en jaugent une symmétrie globale commune. Les deux tenseurs de stress initialement conservés se mélangent et le résultat est une combinaison conservée et une combinaison orthogonale, dont la dimension acquiert une petite dimension anormale. Holographiquement, cette configuration correspond à la connexion de deux univers AdS₄ initialement découplés via un AdS₅ × S⁵ fin, autrement appelé une “gorge” de Janus. Le résultat est une théorie AdS₄-bimétrique, avec un graviton sans masse et un graviton massif, dont la petite masse correspond à la dimension anormale de la combinaison duale de tenseurs de stress. Nous calculons la masse du graviton, qui est exprimée en termes de données géométriques de la ”gorge” de Janus et de l’univers AdS₄ vers zéro, résulte en une théorie de gravité massive dans AdS₄. En ce qui concerne la deuxième direction de ce travail, les déformations superconformes des sCFT₃s considérées qui génèrent la variété superconformale sont des déformations préservant N = 2 supersymétrie, générées par des opérateurs exactement marginaux. Nous présentons comment tous ces opérateurs peuvent être systématiquement extraits de l’index superconforme. Les opérateurs de branche de Coulomb et de Higgs sont pris en compte, tandis qu’une attention particuli ère est accordée aux opérateurs mixtes. On montre que les modules de branches mixtes de ces théories sont des opérateurs à double-corde qui se transforment dans la représentation (Adj, Adj) des groupes de saveursélectriques et magnétiques, modulo un surcomptage pour les quivers avec des noeuds de jauge abéliens. Enfin, nous commentons sur l’interprétation holographique des résultats, en affirmant que les supergravités mesurées peuvent capturer l’espace des modules tout entier si, outre les paramètres de la solution d’arri ère-plan, les modules de quantification des conditions aux limites sontégalement pris en compteBased on the holographic duality between a large class of half-maximally supersymmetric four-dimensional Anti-de Sitter (AdS₄) vacua and three-dimensional N = 4 superconformal field theories (sCFT₃), we study quantum gravitational and gauge theoretic questions. This work has two main directions: The first part is devoted to the mechanisms through which the low-lying AdS₄-graviton can acquire a small mass whereas the second part regards the mapping of the superconformal manifold of the considered sCFT₃s. Regarding the question of the graviton Higgsing in AdS₄, we propose a new mechanism which relies on ”weakly” coupling two initially decoupled sCFT₃s, by gauging a common global symmetry. The two initially conserved stress tensors mix and the result of this mixing is a conserved combination and an orthogonal combination, the scaling dimension of which acquires a small anomalous dimension. Holographically, this setup is dual to connecting two initially decoupled AdS₄ universes via a thin AdS₅ × S⁵ or Janus ”throat”. The result is an AdS₄- bimetric theory, with one massless and one massive graviton, the small mass of which corresponds to the anomalous dimension of the dual stress tensor combination. We compute the mass of the graviton, which is expressed in terms of the geometric data of the Janus ”throat” and of the considered AdS₄ universe. A special decoupling limit of this theory, where the effective four-dimensional gravitational coupling of one of the two universes vanishes, results to an AdS₄-Massive gravity theory. Regarding the second direction of this work, superconformal deformations of the considered sCFT3s which generate the superconformal manifold, are N = 2 supersymmetry preserving deformations, generated by exactly marginal operators. We present how all these operators can be consistently extracted from the superconformal index. Coulomb and Higgs branch operators are considered, while particular attention is payed to mixedbranch operators. It is shown that the mixed-branch moduli of these theories are double-string operators transforming in the (Adj,Adj) representation of the electric and magnetic flavour groups, up to overcounting for quivers with abelian gauge nodes. Finally, we comment on the holographic interpretation of the results, arguing that gauged supergravities can capture the entire moduli space if, in addition to the parameters of the background solution, quantization moduli of boundary conditions are also taken into account
35
Pfeffer, Joshua William. "Frontiers of Liouville quantum gravity." Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/126933.
Анотація:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mathematics, May, 2020Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 317-336).
Abstract This thesis studies a universal model of random geometry in two dimensions called Liouville quantum gravity (LQG). LQG was originally described heuristically by physicists, and mathematicians have grappled with the challenge of defining it rigorously and analyzing its properties. We investigate elements of the theory of LQG that are still poorly understood, often even from physicists' heuristic perspective. -- We analyze LQG as a metric space. We prove results necessary for the construction of LQG as a metric space, and prove fundamental estimates for these distances. We prove the most natural formulation of the Knizhnik-Polyakov-Zamolodchikov (KPZ) formula, which relates Hausdorff dimensions of sets with respect to the Euclidean and LQG metric. And we prove upper and lower bounds on the Hausdorff dimension of the LQG metric. --
We propose a model for LQG with matter central charge in (1, 25). We introduce and justify a model for LQG for matter central charge c in the range (1, 25), a regime whose probabilistic and geometric behavior is much less well-understood than the classical regime c < 1, even from a physics perspective. -- We rigorously link the determinant of the Laplacian to the definition of LQG and to the mass of Brownian loops. We give a mathematically precise interpretation of physicists' original definition of LQG in terms of the determinant of the Laplace-Beltrami operator ("Laplacian"). And we rigorously relate the zeta-regularized determinant of the Laplacian to the regularized mass of Brownian loops on the surface. --
We apply the theory of LQG to answer open problems in other areas of probability. We apply tools from LQG to answer open problems about the connectivity of the adjacency graph of complementary connected components of a Schramm-Loewner evolution curve. And we prove a precise asymptotic growth exponent for external diffusion-limited aggregation in the setting of a spanning-tree-weighted random planar map-the first result of its kind on any class of graphs.
by Joshua William Pfeffer.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Mathematics
36
Ridgway, Stuart Alexander Preskill John P. "Wormholes in Euclidean quantum gravity /." Diss., Pasadena, Calif. : California Institute of Technology, 1992. http://resolver.caltech.edu/CaltechETD:etd-12082008-100051.
37
Minassian, Eric Azarian. "Spacetime singularities in quantum gravity /." For electronic version search Digital dissertations database. Restricted to UC campuses. Access is free to UC campus dissertations, 2003. http://uclibs.org/PID/11984.
38
Rodigast, Andreas. "Renormalisation in perturbative quantum gravity." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2012. http://dx.doi.org/10.18452/16571.
Анотація:
In dieser Arbeit berechnen wir die gravitativen Ein-Schleifen-Korrekturen zu den Propagatoren und Wechselwirkungen der Felder des Standardmodells der Elementarteilchenphysik. Wir betrachten hierzu ein höherdimensionales brane-world-Modell: Wärend die Gravitonen, die Austauchteilchen der Gravitationswechselwirkung, in der gesamten D-dimensionalen Raumzeit propagieren können, sind die Materiefelder an eine d-dimensionale Untermanigfaltigkeit (brane) gebunden. Um die divergenten Anteile der Ein-Schleifen-Diagramme zu bestimmen, entwickeln wir ein neues Regularisierungschema welches einerseits die Wardidentitäten der Yang-Mills-Theorie respektiert anderseits sensitiv für potenzartige Divergenzen ist. Wir berechnen die gravitativen Beiträge zu den beta-Funktionen der Yang-Mills-Eichtheorie, der quartischen Selbst-Wechselwirkung skalarer Felder und der Yukawa-Wechselwirkung zwischen Skalaren und Fermionen. Im physikalisch besonders interessanten Fall einer vier-dimensionalen Materie-brane verschwinden die gravitativen Beiträge zum Laufen der Yang-Mills-Kopplungskonstante. Die führenden Beiträge zum Laufen der anderen beiden Kopplungskonstanten sind positiv. Diese Ergebnisse sind unabhängig von der Anzahl der Extradimensionen in denen die Gravitonen propagieren können. Des Weiteren bestimmen wir alle gravitationsinduzierten Ein-Schleifen-Konterterme mit höheren kovarianten Ableitungen für skalare Felder, Dirac-Fermionen und Eichbosonen. Ein Vergleich dieser Konterterme mit den höheren Ableitungsoperatoren des Lee-Wick-Standardmodells zeigt, dass die Gravitationskorrekturen nicht auf letzte beschränkt sind. Eine Beziehung zwischen Quantengravitation und dem Lee-Wick-Standardmodell besteht somit nicht.In this thesis, we derive the gravitational one-loop corrections to the propagators and interactions of the Standard Model field. We consider a higher dimensional brane world scenario: Here, gravitons can propagate in the whole D dimensional space-time whereas the matter fields are confined to a d dimensional sub-manifold (brane). In order to determine the divergent part of the one-loop diagrams, we develop a new regularisation scheme which is both sensitive for polynomial divergences and respects the Ward identities of the Yang-Mills theory. We calculate the gravitational contributions to the beta functions of non-Abelian gauge theories, the quartic scalar self-interaction and the Yukawa coupling between scalars and fermions. In the physically interesting case of a four dimensional matter brane, the gravitational contributions to the running of the Yang-Mills coupling constant vanish. The leading contributions to the other two couplings are positive. These results do not depend on the number of extra dimensions. We further compute the gravitationally induced one-loop counterterms with higher covariant derivatives for scalars, Dirac fermions and gauge bosons. In is shown that these counterterms do not coincide with the higher derivative terms in the Lee-Wick standard model. A possible connection between quantum gravity and the latter cannot be inferred.
39
Schkolnik, Vladimir. "Probing gravity with quantum sensors." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2017. http://dx.doi.org/10.18452/17676.
Анотація:
Quantensensoren, wie Atominterferometer und Atomuhren werden zu hochpräzisen und akkuraten Messungen von Inertialkräften und der Zeit benutzt und sind hervorragend dazu geeignet fundamentale Fragestellungen der Physik anzugehen und die Aussagen der allgemeinen Relativitätstheorie zu testen. Die Empfindlichkeit von Atominterferometern skaliert quadratisch mit der freien Entwicklungszeit und die Verwendung von Quantensensoren im Weltraum ist prädestiniert die Genauigkeit von Tests des Äquivalenzprinzips um mehrere Größenordnungen zu verbessern. Zusätzlich, werden präzise und akkurate Sensoren für Inertialkräfte, im Bereich der Navigation oder Geodäsie benutzt wo mobile auf Atominterferometrie basierende Geräte noch selten sind. Diese Arbeit trägt zur Entwicklung von hochempfindlichen und stabilen mobilen Quantensensoren bei. Im Rahmen dieser Doktorarbeit wurden drei mobile Vergleichsmessungen der Erdbeschleunigung mit dem Atominterferometer GAIN an verschiedenen geographischen Orten durchgeführt. Die demonstrierte Stabilität von 5*10^-11 g nach 10^5 s übertrifft die Stabilität von klassischen Gravimetern. Mit dem Ziel von Weltraumgestützten Atominterferometern wurde ein kompaktes Lasersystem für den Betrieb von Atominterferometrie mit Rubidium Bose-Einstein Kondensaten auf Höhenforschungsraketen entworfen, qualifiziert und in Betrieb genommen. Zusätzlich wurden drei Nutzlasten für dein Einsatz auf Höhenforschungsraketen realisiert um die Reife der notwenigen Subsysteme zu zeigen. Dopplerfreie Laserspektroskopie an Rubidium und Kalium wurde verwendet um eine optische Frequenzreferenz zu realisieren und während der Flüge wurde mit einem Frequenzkamm zu vergleichen. Diese Messung stellt einen ersten Test der Lokalen Lorenz Invarianz im Weltraum dar. Diese Aktivitäten ebnen den Weg für den zukünftigen Einsatz von Quantensensoren im Weltraum die noch nie dagewesene Tests der fundamentalen Physik, Weltraumgeodäsie oder sogar Gravitationswellen ermöglichen.Quantum sensors, such as atom interferometers and atomic clocks are used for high precision and accurate measurements of inertial forces and time and are therefore ideally suited to address fundamental questions in physics and to test the predictions of general relativity. The sensitivity of atom interferometers scales quadratically with the free evolution time and the use of quantum sensors in space is predestined to improve the accuracy of such tests by several orders of magnitude. Additionally, precise and accurate sensors for inertial forces are required in the field of navigation or geodesy where mobile devices based on atom interferometry are still rare. This work contributes to the development of highly sensitive and stable mobile quantum sensors. In the course of this thesis, three measurement comparisons of the gravitational acceleration with the mobile atom interferometer GAIN were performed at different geographic locations. The demonstrated stability of 5*10^-11 g after 10^5 s surpasses the one reached by classical gravimeters. With the goal of space-born atom interferometry, a compact laser system for operation of atom interferometry with Bose-Einstein condensates of rubidium on a sounding rocket was designed, qualified and put in operation. Additionally, three sounding rocket payloads were realized to show the technological maturity of the necessary subsystems. Doppler-free laser spectroscopy of rubidium and potassium was used to realize an optical frequency reference that was compared during the flights to an atomic microwave standard via a frequency comb. This measurement represents the first test of the Local Position Invariance in space. These activities pave the way for future deployment of quantum sensors in space enabling unprecedented tests of fundamental physics, space geodesy or even gravitational wave detection.
40
Conrady, Florian. "Semiclassical analysis of loop quantum gravity." [S.l.] : [s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=982087144.
41
Sahlmann, Hanno. "Coupling matter to loop quantum gravity." Phd thesis, [S.l. : s.n.], 2002. http://pub.ub.uni-potsdam.de/2002/0032/sahlmann.pdf.
42
Markopoulou-Kalamara, Fotini Georgia. "Abelian constraints in canonical quantum gravity." Thesis, Imperial College London, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.695329.
43
Haghnegahdar, Poya. "Observables and dynamics in quantum gravity." Thesis, University of British Columbia, 2009. http://hdl.handle.net/2429/13913.
Анотація:
The treatment of quantum constraint theories in physics is considered. These systems exist in many different branches of physics. One noticeable case is the quantum theory of gravity (which is a completely constraint system).
We contrast two modern methods of solving such quantum constraint systems, namely the method used in the String Theory - BRST Quantization, and that used in Loop Quantum Gravity - the Loop Quantization.
We investigate these methods and the quantum solution space they produce - along with proper observables - for the case of a discrete closed string. This system has the advantage that it lends itself to the investigation of these two methods of quantization easily - thereby highlighting some of their principle features - yet it is simple enough so to side-step the many complications that arise in more general systems.
44
Teo, Edward H. K. "Geometry and topology in quantum gravity." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337207.
45
Giulini, D. "3-manifolds in canonical quantum gravity." Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.333324.
46
De, Medeiros Paul Francis. "Duality in non-pertubative quantum gravity." Thesis, Queen Mary, University of London, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.397234.
47
Kimberly, Dagny Maria. "Testing quantum gravity with observational puzzles." Thesis, Imperial College London, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.423102.
48
Kimpton, Ian. "Classical and quantum modifications of gravity." Thesis, University of Nottingham, 2013. http://eprints.nottingham.ac.uk/13430/.
Анотація:
Einstein’s General Relativity has been our best theory of gravity for nearly a century, yet we know it cannot be the final word. In this thesis, we consider modifications to General Relativity, motivated by both high and low energy physics. In the quantum realm, we focus on Horava gravity, a theory which breaks Lorentz invariance in order to obtain good ultraviolet physics by adding higher spatial derivatives to the action (improving propagator behaviour in loops) but not temporal (avoiding Ostrogradski ghosts). By using the Stückelberg trick, we demonstrate the necessity of introducing a Lorentz violating scale into the theory, far below the Planck scale, to evade strong coupling concerns. Using this formalism we then show explicitly that Horava gravity breaks the Weak Equivalence Principle, for which there are very strict experimental bounds. Moving on to considering matter in such theories, we construct DiffF(M) invariant actions for both scalar and gauge fields at a classical level, before demonstrating that they are only consistent with the Equivalence Principle in the case that they reduce to their covariant form. This motivates us to consider the size of Lorentz violating effects induced by loop corrections of Horava gravity coupled to a Lorentz invariant matter sector. Our analysis reveals potential light cone fine tuning problems, in addition to evidence that troublesome higher order time derivatives may be generated. At low energies, we demonstrate a class of theories which modify gravity to solve the cosmological constant problem. The mechanism involves a composite metric with the square root of its determinant a total derivative or topological invariant, thus ensuring pieces of the action proportional to the volume element do not contribute to the dynamics. After demonstrating general properties of the proposal, we work through a specific example, demonstrating freedom from Ostrogradski ghosts at quadratic order (in the action) on maximally symmetric backgrounds. We go on to demonstrate sufficient conditions for a theory in this class to share a solution space equal to that of Einstein’s equations plus a cosmological constant, before determining the cosmology these extra solutions may have when present.
49
Schröder, Jan. "Aspects of quantum gravity and matter." Thesis, University of Sussex, 2015. http://sro.sussex.ac.uk/id/eprint/54172/.
Анотація:
A quantum theory of gravity remains one of the greatest challenges of contemporary physics. It is well established that a perturbative treatment of gravity as a quantum field theory leads to a non-renormalisable setup. However gravity could still exist as a consistent and predictive quantum field theory on a non-perturbative level. This is explored in the asymptotic safety scenario which was initially proposed by S. Weinberg. In this thesis we investigate the ultraviolet behaviour of gravity within the asymptotic safety conjecture and discuss phenomenological implications. We start out by introducing the concept of the functional renormalisation group and its application to gravity. This non-perturbative tool is the technical basis for our investigation of a template quantum gravity action, namely a function f(R) in the Ricci scalar in four dimensions. We compute exact fixed point solutions to very high polynomial orders via the development of a dedicated high performance code. The picture of an interacting UV fixed point that receives only small quantitative corrections from higher derivative operators is confirmed and extended. The results are then expanded to include minimally coupled matter fields and we investigate the matter effects on the gravitational fixed point. We determine regimes of compatibility in the vicinity of the purely gravitational setup but also find constraints on the number of matter fields. Finally we look at the phenomenological implications of a running Newton's coupling, one of the key features of the asymptotic safety setup, to graviton-mediated eikonal scattering amplitudes. In this kinematic regime we investigate the possibility of a TeV-sized fundamental Planck mass via the introduction of compact extra dimensions. We identify the fingerprints of asymptotic safety in the t-channel scattering amplitude and find crucial differences compared to semi-classical computations.
50
De, Cesare Marco. "Cosmological consequences of Quantum Gravity proposals." Thesis, King's College London (University of London), 2017. https://kclpure.kcl.ac.uk/portal/en/theses/cosmological-consequences-of-quantum-gravity-proposals(fbc51bef-e427-4126-8462-e40014b77318).html.
Анотація:
In this thesis, we study the implications of Quantum Gravity models for the dynamics of spacetime and the ensuing departures from classical General Relativity. The main focus is on cosmological applications, particularly the impact of quantum gravitational effects on the dynamics of a homogenous and isotropic cosmological background. Our interest lies in the consequences for the evolution of the early universe and singularity resolution, as well as in the possibility of providing an alternative explanation for dark matter and dark energy in the late universe. The thesis is divided in two parts, dedicated to alternative (and complementary) ways of tackling the problem of Quantum Gravity. The first part is concerned with cosmological applications of background independent approaches to Quantum Gravity, as well as minisuperspace models in Quantum Cosmology. Particularly relevant in this work is the Group Field Theory approach, which we use to study the effective dynamics of the emergent universe from a full theory of Quantum Gravity (i.e. without symmetry reduction). We consider both approaches based on loop quantisation and on quantum geometrodynamics. In the second part, modified gravity theories are introduced as tools to provide an effective description of quantum gravitational effects, and show how these may lead to the introduction of new degrees of freedom and symmetries. Particularly relevant in this respect is local conformal invariance, which finds a natural realisation in the framework of Weyl geometry. We construct a modified theory of gravity based on such symmetry principle, and argue that new fields in the extended gravitational sector may play the role of dark matter. New degrees of freedom are also natural in models entailing fundamental ‘constants’ that vary over cosmic history. Finally, we discuss prospects for future work and point at directions for the derivation of realistic cosmological models from Quantum Gravity candidates.