Дисертації з теми "Foundations of gravity theories"
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1
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.
Повний текст джерелаАнотація:
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.
2
Matas, Andrew. "Foundations of Massive Gravity." Case Western Reserve University School of Graduate Studies / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=case1464275510.
Повний текст джерела
3
Sbisa, Fulvio. "Modified theories of gravity." Thesis, University of Portsmouth, 2013. https://researchportal.port.ac.uk/portal/en/theses/modified-theories-of-gravity(3b9310e3-5d97-4e48-aa05-0444d1e89363).html.
Повний текст джерелаАнотація:
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.
4
Clifton, Timothy. "Alternative theories of gravity." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612712.
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5
Sbisa', F. "MODIFIED THEORIES OF GRAVITY." Doctoral thesis, Università degli Studi di Milano, 2013. http://hdl.handle.net/2434/214951.
Повний текст джерелаАнотація:
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. A necessary condition for the viability of a theory of ``modified gravity'' is that it has to reproduce to high precision the results of General Relativity in experimental setups where the latter is well tested. Quite in general, modifying General Relativity introduces new degrees of freedom, which are responsible for the different large distance behavior. 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 known as the Vainshtein mechanism, which involves derivative self-interaction terms for these degrees of freedom.
In this thesis, we consider a class of nonlinear massive gravity theories known as dGRT Massive Gravity. These theories are candidates as viable models to modify gravity at very large distances, and, apart from the mass, they contain two free parameters. We investigate the effectiveness of the Vainshtein screening mechanism in this class of theories. There are two branches of static and spherically symmetric solutions, and we consider only the branch in which the Vainshtein mechanism can occur. We truncate the analysis to scales below the gravitational Compton wavelength, and consider the weak f\mbox{}ield 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. We analyze in detail in which cases the solutions match in an intermediate region. 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 where global solutions do not exist, and characterise precisely in which regions of the phase space the Vainshtein mechanism takes place.
6
Ranchin, Andre. "Alternative theories in quantum foundations." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/52462.
Повний текст джерелаАнотація:
Abstraction is an important driving force in theoretical physics. New insights often accompany the creation of physical frameworks which are both comprehensive and parsimonious. In particular, the analysis of alternative sets of theories which exhibit similar structural features as quantum theory has yielded important new results and physical understanding. An important task is to undertake a thorough analysis and classification of quantum-like theories. In this thesis, we take a step in this direction, moving towards a synthetic description of alternative theories in quantum foundations. After a brief philosophical introduction, we give a presentation of the mathematical concepts underpinning the foundations of physics, followed by an introduction to the foundations of quantum mechanics. The core of the thesis consists of three results chapters based on the articles in the author’s publications page. Chapter 4 analyses the logic of stabilizer quantum mechanics and provides a complete set of circuit equations for this sub-theory of quantum mechanics. Chapter 5 describes how quantum-like theories can be classified in a periodic table of theories. A pictorial calculus for alternative physical theories, called the ZX calculus for qudits, is then introduced and used as a tool to depict particular examples of quantum-like theories, including qudit stabilizer quantum mechanics and the SpekkensSchreiber toy theory. Chapter 6 presents an alternative set of quantum-like theories, called quantum collapse models. A novel quantum collapse model, where the rate of collapse depends on the Quantum Integrated Information of a physical system, is introduced and discussed in some detail. We then conclude with a brief summary of the main results.
7
Gullu, Ibrahim. "Massive Higher Derivative Gravity Theories." Phd thesis, METU, 2011. http://etd.lib.metu.edu.tr/upload/12613975/index.pdf.
Повний текст джерелаАнотація:
In this thesis massive higher derivative gravity theories are analyzed in some detail.
One-particle scattering amplitude between two covariantly conserved sources mediated by a graviton exchange
is found at tree-level in D dimensional (Anti)-de Sitter and flat spacetimes for the most
general quadratic curvature theory augmented with the Pauli-Fierz mass term. From the amplitude expression, the
Newtonian potential energies are calculated for various cases.
Also, from this amplitude and the propagator structure, a three dimensional unitary theory is identified. In the second part of the thesis,
the found three dimensional unitary theory is studied in more detail from a canonical point of view. The general higher order action is written
in terms of gauge-invariant functions both in flat and de Sitter backgrounds. The analysis is extended by
adding static sources, spinning masses and the gravitational Chern-Simons term separately to the theory in
the case of flat spacetime. For all cases the microscopic spectrum and the masses are found. In the discussion of curved
spacetime, the masses are found in the relativistic and non-relativistic limits. In the Appendix,
some useful calculations that are frequently used in the bulk of the thesis are given.
8
Bahamonde, Sebastian. "Modified teleparallel theories of gravity." Thesis, University College London (University of London), 2018. http://discovery.ucl.ac.uk/10055604/.
Повний текст джерелаАнотація:
Teleparallel gravity is an alternative formulation of gravity which has the same field equations as General Relativity (GR), therefore, it is also known as the Teleparallel equivalent of General Relativity (TEGR). This theory is a gauge theory of the translations with the torsion tensor being non-zero but with a vanishing curvature tensor, hence, the manifold is globally flat. An interesting approach for understanding the late-time accelerating behaviour of the Universe is called modified gravity where GR is extended or modified. In the same spirit, since TEGR is equivalent to GR, one can consider its modifications and study if they can describe the current cosmological observations. This thesis is devoted to studying several modified Teleparallel theories of gravity with emphasis on late-time cosmology. Those Teleparallel theories are in general different to the modified theories based on GR, but one can relate and classify them accordingly. Various Teleparallel theories are presented and studied such as Teleparallel scalar-tensor theories, quintom models, Teleparallel non-local gravity, and f(T,B) gravity and its extensions (coupled with matter, extensions of new GR and Gauss-Bonnet) where T is the scalar torsion and B is the boundary term which is related with the Ricci scalar via R=-T+B.
9
Schreckenberg, Stephan Reinhold. "Structural foundations of quantum history theories." Thesis, Imperial College London, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.309236.
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10
Michele, Oliosi. "New viable theories of modified gravity : Minimal Theories and Quasidilaton." Kyoto University, 2019. http://hdl.handle.net/2433/244509.
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11
German, Velarde G. "Aspects of gravity and supergravity theories." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380006.
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12
Uddin, Kotub. "The viability of modified gravity theories." Thesis, Queen Mary, University of London, 2009. http://qmro.qmul.ac.uk/xmlui/handle/123456789/386.
Повний текст джерелаАнотація:
This thesis studies the viability of classes of modified gravity (MG) theories based on generalisations of the Einstein-Hilbert action. Particular emphasis is given to f(R) theories in both the metric and Palatini formalisms, scalar-tensor theories and generalised Gauss-Bonnet theories. An urgent task at present is to devise stringent tests in order to reduce the range of candidate models based on these theories. In this thesis a detailed study is made of the viability of these models using constraints from requirement of stability, background cosmological dynamics, local gravity constraints (LGC) and matter density perturbations. In each case the conditions required for stability and viability of the background dynamics are presented. In the case of generalised Gauss-Bonnet theories the circumstances leading to the existence and stability of cosmological scaling solutions are established. In the scalar-tensor theories considered here, which includes metric-f(R) theories as a special case, there is a strong coupling of the scalar field to matter in the Einstein frame which violates all LGC. It is shown that using a chameleon mechanism, models that are compatible with LGC may be constructed. It is found that such models, which are also consistent with background dynamics, are constrained to be close to the CDMmodel during the radiation/matter epochs and can lead to the divergence of the equation of state of dark energy. In contrast, such constraints only impose mild restrictions on Palatini-f(R) models. Still more stringent constraints are provided by studying matter density perturbations. In particular, it is shown that the unconventional evolution of perturbations in the Palatini formalism leads to f(R) models in this case to be practically identical to the CDM model. For each case it is also shown that (for viable models) matter perturbation equations derived under a sub-horizon approximation are reliable even for super-Hubble scales provided the oscillating mode does not dominate over the matter-induced mode. Such approximate equations are especially reliable in the Palatini formalism, where the oscillating mode is absent. In summary, the analyses carried out in this thesis suggest that subjectingMG theories to observational constraints confines the viable range of models to be very close to (and in some cases indistinguishable from) the CDM model.
13
Oreta, Timothy. "Vector-Galileon-Tensor theories of gravity." Master's thesis, University of Cape Town, 2016. http://hdl.handle.net/11427/20925.
Повний текст джерелаАнотація:
A detailed study of the cosmological evolution in a particular vector-tensor theory of gravity with a potential and a Galileon-motivated interaction terms is presented. The evolution of vector field self interactions that are relatively related to Galileon fields throughout the expansion history of the universe is considered and a classification of the parameters M (mass term) and H (Hubble parameter) according to the behaviour of the field in each cosmological epoch is carried out. In particular, we obtain conditions for the parameters so that the field grows exponentially or oscillates with decreasing amplitude. We also obtain an autonomous system for the inflationary case. The general features of the phasemaps are given and the critical point is appropriately characterised. It is not possible to obtain an autonomous system for radiation and matter dominated epochs hence, we consider other analytical methods. We obtain eigenvalues and hence, phasemaps. The general features of the phasemaps are given and the point to which the trajectories on the phasemaps converge is appropriately characterised. Therefore, we show that it is possible to obtain a wide variety of behaviours or interesting phenomenologies for the cosmological evolution of vector field self-interactions that are relatively related to Galileon fields by choosing suitable values for the parameters M and H of given conditions.
14
Hackebill, Aric. "Thermodynamics of Modified Theories of Gravity." VCU Scholars Compass, 2010. http://scholarscompass.vcu.edu/etd/2143.
Повний текст джерелаАнотація:
Einstein’s equations are derived by following Jacobson’s thermodynamic method. It is seen that a family of possible field equations exist which satisfy the thermodynamic argument. Modified theories of gravity are addressed as possible candidates for replacing dark matter as an explanation for anomalous cosmological phenomena. Many of the proposed modified theories are not powerful enough to explain the currently observed phenomena and are rejected as viable theories of gravity. A surviving candidate, TeVeS, is further analyzed under the aforementioned thermodynamic argument to check for its consistency with thermodynamics.
15
Cremonesi, Stefano. "Cascading theories in gauge/gravity duality." Doctoral thesis, SISSA, 2008. http://hdl.handle.net/20.500.11767/4096.
Повний текст джерела
16
Lundmark, Kristofer. "Gravity approach to strongly coupled gauge theories." Thesis, Uppsala universitet, Teoretisk fysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-155121.
Повний текст джерелаАнотація:
A written report of a paper titled Holographic dual of collimated radiation by Veronika E. Hubeny where a new and easier method is proposed to estimate the “radiation due to an accelerated quark in a strongly coupled medium”. The method is able to reproduce the results from an earlier paper without the need of solving the linearized Einstein equations but by way of calculating geodesics in AdS using the AdS/CFT correspondence and the gravitational dual of the quark being a string. A quick introduction to synchrotron radiation and general relativity is given after which the AdS/CFT correspondence is introduced along with the results and method of V. Hubeny.A bachelor thesis in theoretical physics.
17
Hovdebo, Jordan. "Instabilities in Higher-Dimensional Theories of Gravity." Thesis, University of Waterloo, 2006. http://hdl.handle.net/10012/2967.
Повний текст джерелаАнотація:
A number of models of nature incorporate dimensions beyond our observed four. In this thesis we examine some examples and consequences of classical instabilities that emerge in the higher-dimensional theories of gravity which can describe their low energy phenomenology. We first investigate a gravitational instability for black strings carrying momentum along an internal direction. We argue that this implies a new type of solution that is nonuniform along the extra dimension and find that there is a boost dependent critical dimension for which they are stable. Our analysis implies the existence of an analogous instability for the five-dimensional black ring. We construct a simple mode of the black ring to aid in applying these results and argue that such rings should exist in any number of space-time dimensions.
Next we consider a recently constructed class of nonsupersummetric solutions of type IIB supergravity which are everywhere smooth and have no horizon. We demonstrate that these solutions are all classically unstable. The instability is a generic feature of horizonless geometries with an ergoregion. We consider the endpoint of this instability and argue that the solutions decay to supersymmetric configurations. We also comment on the implications of the ergoregion instability for Mathur's 'fuzzball' proposal.
Finally, we consider an interesting braneworld cosmology in the Randall-Sundrum scenario constructed using a bulk space-time which corresponds to a charged AdS black hole. In particular, these solutions appear to 'bounce', making a smooth transition from a contracting to an expanding phase. By considering the space-time geometry more carefully, we demonstrate that generically in these solutions the brane will encounter a singularity in the transition region.
18
Zlosnik, Thomas. "cosmological consequences of modified theories of gravity." Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.504343.
Повний текст джерела
19
Lima, Nelson Daniel de Aguiar. "Dark energy and modified theories of gravity." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/23558.
Повний текст джерелаАнотація:
It is now a consolidated fact that our Universe is undergoing an accelerated expansion. According to Einstein's General Relativity, if the main constituents of our Universe were ordinary and cold dark matter, then we would expect it to be contracting and collapsing due to matter's attractive nature. The simplest explanation we have for this acceleration is in the form of a component with a negative ratio of pressure to density equal to -1 known as cosmological constant, Λ , presently dominating over baryonic and cold dark matter. However, the Λ Cold Dark Matter (Λ CDM) model suffers from a well known fine tuning problem. This led to the formulation of dark energy and modified gravity theories as alternatives to the problem of cosmic acceleration. These theories either include additional degrees of freedom, higher-order equations of motion, extra dimensionalities or imply non-locality. In this thesis we focus on single field scalar tensor theories embedded within Horndeski gravity. Even though there is currently doubt on their ability to explain cosmic acceleration without having a bare cosmological constant on their action, the degree of freedom they introduce mediates an additional fifth force. And while this force has to suppressed on Solar system scales, it can have interesting and observable effects on cosmological scales. Over the next decade there is a surge of surveys that will improve the understanding of our Universe on the largest scales. Hence, in this work, we take several different modified gravity theories and study their impact on cosmological observables. We will analyze the dynamics of linear perturbations on these theories and clearly highlight how they deviate from Λ CDM, allowing to break the degeneracy at the background level. We will also study the evolution of the gravitational potentials on sub horizon scales and provide simplified expressions at this regime and, for some models, obtain constraints using the latest data.
20
Mifsud, Jurgen. "Cosmological consequences of theories of modified gravity." Thesis, University of Sheffield, 2018. http://etheses.whiterose.ac.uk/21618/.
Повний текст джерелаАнотація:
Our theoretical understanding of the dynamical evolution of the Universe has certainly improved during the recently established era of precision cosmology. However, the nature of the dark sector remains the greatest puzzle in cosmology. Although we re–establish that the concordance model of cosmology is in agreement with current cosmological observations, this simplistic model is unequivocally theoretically unappealing. Thence, we investigate a number of alternative cosmological models and illustrate their distinctive cosmological consequences. For instance, we consider a scalar–tensor theory of gravitation, such that the minimally coupled scalar field is explicitly coupled to multiple fluid components. The assumed coupling functions are specified by the theoretically well–motivated conformal and disformal coupling functions. We perform a dynamical systems analysis, in which we establish the existence and stability conditions for every fixed point, and illustrate that disformally coupled systems have a dissimilar cosmological evolution with respect to the conformally coupled and uncoupled systems. We further show that a disformal coupling between the matter and radiation sectors is characterised by a varying fine–structure constant. Moreover, a direct coupling between dark energy and dark matter is not theoretically forbidden and might be incorporated in extensions of the standard model of particle physics. We consider a coupled quintessence model, in which the dark energy scalar field only couples to dark matter via the conformal and disformal coupling functions, and is decoupled from the conventional baryonic matter sector. We scrutinise the distinctive features of this cosmological model, where we particularly show that when the dark sector constituents are disformally coupled, intermediate–scales and time–dependent damped oscillations appear in the matter growth rate function. We confront this coupled quintessence model with current cosmological data sets, and illustrate that Nature is consistent with a null coupling within the dark sector of the Universe.
21
Colombo, Mattia. "Aspects of Lorentz violating theories of gravity." Thesis, University of Nottingham, 2016. http://eprints.nottingham.ac.uk/38419/.
Повний текст джерелаАнотація:
Lorentz symmetry is arguably the most fundamental symmetry of physics, at least in its modern conception. On the other hand, some of the issues that plague the currently accepted theory of gravitation could be solved by breaking such symmetry. The theory proposed by Petr Horava in 2009 brings forward exactly this aspect. The theory, dubbed Horava gravity, is a UV complete theory of gravity that is also renormalisable. It represents therefore a good candidate for a quantum theory of gravity. There are some issues though, which typically arise in any theory which explicitely violates Lorentz symmetry. In this thesis we will be concerned with two of these issues, in particular the matter problem and the existence of black holes. The first issue mentioned arises every time we try to couple matter to a Lorentz violating theory of gravity. Indeed, in the matter sector Lorentz symmetry is extremely well constrained, and therefore we need to find a way to avoid the percolation of Lorentz violations to the matter sector through higher order operators. One possible solution based on the separation of scales was proposed in the last few years (Pospelov et al.,2010). While studying the proposed mechanism though, the authors uncovered a naturalness problem in the vector sector of the theory. The solutions they proposed relies on the use of some higher derivative terms that are not normally present in the ``traditional'' Horava theory. It is unclear then what impact this type of terms can have on the whole theory. In our work we precisely addressed this question. We analysed the perturbations around Minkowski of the most generic theory extended to these type of terms, both from the point of view of the stability of the theory and of the renormalisability. What we found is that the theory retains its renormalisability, but some instabilities occur in the scalar sector. More work is hence required in order to understand whether such instabilities could be tamed, or if the mixed derivatives should be abandoned in favour of some alternative solution, not presently available. The second theme we concentrated on is that of the existence of black holes. The definition of black hole in general relativity rely strongly on the causal structure dictated by Lorentz symmetry. As soon as Lorentz symmetry is broken it is therefore unclear whether black holes will still exist. Surprisingly enough black holes have been shown to exist in Lorentz breaking theories, but a rigorous definition was still to be found. In our work we developed the mathematically rigorous definitions for the causal structure of foliated spacetimes and we defined for the first time black holes in such spacetimes. We also uncovered a number of interesting properties of this objects and we developed a local characterisation that allows one to locate horizons without the knowledge of the whole structure of the complete spacetime. Finally we developed the Initial Value Problem for these types of theory in the hope that new simulations of gravitational collapse will be performed using our analysis as a starting point. The thesis is organised as follows. In the first Chapter we give an introduction on gravity and the problems with its renormalization. We also introduce some of the theories that have been proposed to solve this difficulties. In the second Chapter we start discussing Lorentz violations and we provide a proof of the power-counting renormalizability of a toy model of a Lorentz violationg scalar field theory. We also introduce the theories that we will be studying throughout the thesis. In the third Chapter we discuss the mixed derivative extension to Horava gravity and we discuss the consequences of the new terms that occur in the theory. In the fourth and fifth Chapters we introduce the causal structure of spacetimes which violate Lorentz symmetry by means of a preferred foliation, we discuss the notion of black holes and horizons and we formalise some results present in the literature adapting them to our framework. In the sixth Chapter we then discuss the Initial Value Problem for such spacetimes, with some attention to the process of gravitational collapse leading to the formation of black holes. Finally in the last Chapter we draw some conclusions and discuss some ideas for future work.
22
Cerioni, Alessandro <1982>. "Cosmological perturbations in generalized theories of gravity." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2011. http://amsdottorato.unibo.it/3562/1/Cerioni_Alessandro_tesi.pdf.
Повний текст джерелаАнотація:
The first part of the thesis concerns the study of inflation in the context of a theory of gravity called "Induced Gravity" in which the gravitational coupling varies in time according to the dynamics of the very same scalar field (the "inflaton") driving inflation, while taking on the value measured today since the end of inflation. Through the analytical and numerical analysis of scalar and tensor cosmological perturbations we show that the model leads to consistent predictions for a broad variety of symmetry-breaking inflaton's potentials, once that a dimensionless parameter entering into the action is properly constrained. We also discuss the average expansion of the Universe after inflation (when the inflaton undergoes coherent oscillations about the minimum of its potential) and determine the effective equation of state. Finally, we analyze the resonant and perturbative decay of the inflaton during (p)reheating.
The second part is devoted to the study of a proposal for a quantum theory of gravity dubbed "Horava-Lifshitz (HL) Gravity" which relies on power-counting renormalizability while explicitly breaking Lorentz invariance. We test a pair of variants of the theory ("projectable" and "non-projectable") on a cosmological background and with the inclusion of scalar field matter. By inspecting the quadratic action for the linear scalar cosmological perturbations we determine the actual number of propagating degrees of freedom and realize that the theory, being endowed with less symmetries than General Relativity, does admit an extra gravitational degree of freedom which is potentially unstable. More specifically, we conclude that in the case of projectable HL Gravity the extra mode is either a ghost or a tachyon, whereas in the case of non-projectable HL Gravity the extra mode can be made well-behaved for suitable choices of a pair of free dimensionless parameters and, moreover, turns out to decouple from the low-energy Physics.
23
Cerioni, Alessandro <1982>. "Cosmological perturbations in generalized theories of gravity." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2011. http://amsdottorato.unibo.it/3562/.
Повний текст джерелаАнотація:
The first part of the thesis concerns the study of inflation in the context of a theory of gravity called "Induced Gravity" in which the gravitational coupling varies in time according to the dynamics of the very same scalar field (the "inflaton") driving inflation, while taking on the value measured today since the end of inflation. Through the analytical and numerical analysis of scalar and tensor cosmological perturbations we show that the model leads to consistent predictions for a broad variety of symmetry-breaking inflaton's potentials, once that a dimensionless parameter entering into the action is properly constrained. We also discuss the average expansion of the Universe after inflation (when the inflaton undergoes coherent oscillations about the minimum of its potential) and determine the effective equation of state. Finally, we analyze the resonant and perturbative decay of the inflaton during (p)reheating.
The second part is devoted to the study of a proposal for a quantum theory of gravity dubbed "Horava-Lifshitz (HL) Gravity" which relies on power-counting renormalizability while explicitly breaking Lorentz invariance. We test a pair of variants of the theory ("projectable" and "non-projectable") on a cosmological background and with the inclusion of scalar field matter. By inspecting the quadratic action for the linear scalar cosmological perturbations we determine the actual number of propagating degrees of freedom and realize that the theory, being endowed with less symmetries than General Relativity, does admit an extra gravitational degree of freedom which is potentially unstable. More specifically, we conclude that in the case of projectable HL Gravity the extra mode is either a ghost or a tachyon, whereas in the case of non-projectable HL Gravity the extra mode can be made well-behaved for suitable choices of a pair of free dimensionless parameters and, moreover, turns out to decouple from the low-energy Physics.
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Sebastiani, Lorenzo. "General Aspects of Modified Theories of Gravity." Doctoral thesis, Università degli studi di Trento, 2011. https://hdl.handle.net/11572/367870.
Повний текст джерелаАнотація:
The aim of this work is to investigate the both, some mathematical and physical general aspect of modified gravity, and, more specifically, the proprieties of viable, realistic models of modified gravity which can be used to reproduce the inflation and the dark energy epoch of universe today.
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Sebastiani, Lorenzo. "General Aspects of Modified Theories of Gravity." Doctoral thesis, University of Trento, 2011. http://eprints-phd.biblio.unitn.it/672/1/TESI.pdf.
Повний текст джерелаАнотація:
The aim of this work is to investigate the both, some mathematical and physical general aspect of modified gravity, and, more specifically, the proprieties of viable, realistic models of modified gravity which can be used to reproduce the inflation and the dark energy epoch of universe today.
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Yadin, Benjamin. "Resource theories of quantum coherence : foundations and applications." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:facfa689-d474-4bdf-9ef6-a43d1b1746e6.
Повний текст джерелаАнотація:
One of the fundamental features that separates quantum physics from classical physics is the idea of quantum superposition, also known as coherence. This thesis concentrates on understanding quantum coherence in the mathematical framework of resource theories, viewing it both as a resource to be harnessed and as a way to quantitatively characterise quantum states in contrast to classical states. We first examine the type of coherence resource theory which has emerged recently to cope with general settings where the physical nature of the medium encoding information is not crucial, such as computation. We identify the set of quantum processes in which coherence is neither created nor used, and use these to provide a physically motivated resource theory pictured in terms of interferometry. Using the same concepts, we then find connections between coherence and discord, a type of quantum correlation. In particular, we show how coherence can be used to generate discord, and explore basis-dependent discord as an intermediate quantity. The second part of the thesis applies the resource theory framework to quantify quantum macroscopicity, taken here to mean the extent to which coherence exists in a system on a macroscopic scale. We find the appropriate type of resource theory for this purpose, giving criteria for good measures of macroscopic coherence. We use these criteria to evaluate some previously proposed measures and highlight the role of the quantum Fisher information. Next, we build up measures based on the concept of macroscopic distinguishability and use them to show that macroscopic quantum states are fragile to noise induced by interaction with an environment. Finally, we apply measures based on the Fisher information to a range of experiments involving mechanical degrees of freedom, in order to compare their macroscopicity.
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Guttmann, Walter. "Algebraic foundations of the Unifying Theories of Programming." [S.l. : s.n.], 2007. http://nbn-resolving.de/urn:nbn:de:bsz:289-vts-60992.
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Muench, Uwe. "Studies in the physical foundations of gravitational theories /." free to MU campus, to others for purchase, 2002. http://wwwlib.umi.com/cr/mo/fullcit?p3060127.
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Högås, Marcus. "On the consistency of multigravity theories." Thesis, Stockholms universitet, Fysikum, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-136315.
Повний текст джерелаАнотація:
In this thesis a set of recently proposed multigravity theories is analysed. In the special case of bimetric gravity, the theory has been conclusively shown to be ghost-free. On the other hand, for multigravity theories in general, the ghost-issue has not been settled conclusively. Motivated by this fact, the main object of this thesis is to clarify what has been proven so far and what issues that still needs to be addressed. We also provide new calculations and results pointing in the direction that the multigravity theories must be restricted to a set of bimetric Hassan-Rosen couplings in a tree-type structure in order to be consistent. In particular, we prove that for a multivielbein theory of
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Hernando, Cabrero Álvaro. "Expansion of Existing Gravity-Based Offshore Wind Turbine Foundations." Thesis, KTH, Betongbyggnad, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-289105.
Повний текст джерелаАнотація:
Wind energy is one of the most promising sources of renewable energy worldwide. Its utilization has substantially increased for the last decades, both onshore and offshore. Offshore wind energy will have a lot to offer within the following decades, thus their foundations need to be prepared. Some of the current wind farms and wind turbines are now reaching their lifespan and, the turbines’ market is developing itself so rapidly that current turbines are getting behind the times with tremendous ease. It is here where the scope of this Master Thesis comes: what shall we do? Should we dismantle wind farms when they reach their lifespan, or should we maybe try to give them a further use? Accommodating for a new a larger wind turbine will need to account for new and higher climate actions and loads, namely winds, waves, ocean currents, the water level variation and the always difficult to predict ice actions. What is aimed in this Master Thesis is to set the basis for a procedure to dimension and define feasible solutions for the offshore wind turbines Gravity-Based Foundations to be expanded, avoiding the necessity of replacing them completely, with the environmental and economic benefits this would have. As this could turn to be an unmanageable problem to be solved, a Case Study where details can be set is performed at the Lillgrund Wind Farm site, in the south-west coast of Sweden, in the Öresund that separates Copenhagen and Malmö. A thorough description of the climatic actions and surrounding aspects is performed, while always dealing with uncertainties. With all that information, an analytical stability analysis is performed to account for three failure modes, namely: sliding, tilting andground failure. Additionally, a numerical FE-model is carried out in ANSYS in the aim of assessing the stresses and deformations that this kind of structure will suffer. Four alternatives are evaluated, and their behaviour is assessed based on the new external design actions. Analytical results show stability difficulties in two of the geometries inspected, while assure it in the other two. The FE-analyses show high concentrations of stresses on the GBS shaft, while model affordable deformations under the load combinations inspected. These results are also compared and contrasted in between them, and sensitivity analyses for the FE-models are performed in order to assure their good behaviour and development, and the trustworthiness ofthe results found. Based on these results, some conclusions are drawn from the developed processes. The main finding is the width and weight dependence of the solution, as well as the shape and dimensions. Future research needs such as scouring effects are finally accounted for necessary inspection to be made as continuation of the work here presented.
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Tavouktsoglou, N. S. "Scour and scour protection around offshore gravity based foundations." Thesis, University College London (University of London), 2018. http://discovery.ucl.ac.uk/10044922/.
Повний текст джерелаАнотація:
The prediction of seabed scour around offshore gravity based foundations with complex geometries is currently a significant barrier to optimising and providing cost effective foundation designs. A significant aspect that has the potential to reduce the uncertainty and costs related to the design of these foundations is the understanding of the effect the structural geometry of the foundation has on scour. This thesis focuses on an experimental investigation of the scour and scour protection around complex structure geometries. The first part of this research considers scour under clear water conditions. During this study different foundation geometries were subjected to a range of different hydrodynamic forcings which enabled a better understanding of the scour process for these foundations. The second part of the research encompasses the design and execution of a series of experiments which investigated stability of the scour protection around such structures. The structures were tested against different combinations of wave and current conditions to determine the bed shear stress required to initiate sediment motion around each structure. This research has led to a number of novel results. The experimental investigation on scour around complex geometries showed that the scour depth around cylindrical structures (with both uniform and complex cross-sections) is linked to the depth averaged pressure gradient. Following a dimensional analysis, the controlling parameters were found to be the depth averaged Euler number, pile Reynolds number, Froude number, sediment mobility number and the non-dimensional flow depth. Based on this finding a new scour prediction equation was developed which shows good agreement with experimental and prototype scour measurements. The scour protection tests indicated that under wave dominated conditions the amplification of the bed shear stress around these structures does not exceed the value of 2. In the case of current dominated flow conditions the amplification of the bed shear stress is a function of the structure type and the Keulegan–Carpenter number. The results of these experiments were used to develop a “Shields type” diagram that can guide designers to select the appropriate rock armour size that will be stable for a certain set of flow conditions. The study also revealed that the long term persistence of flow conditions that just lead to incipient motion of the scour protection material can eventually lead to complete failure of the scour protection. The study provides a set of new design techniques that can allow designers to predict the scour depth around cylindrical and complex foundation geometries and also select the appropriate stone size for their scour protection system. Together, these techniques may allow for the reduction of costs associated with the scour protection of offshore and coastal structures.
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Rajamanoharan, Senthooran. "Cosmic superstrings and bimetric gravity." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610853.
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Meyer, Frank. "Gauge-Field Theories and Gravity on Noncommutative Spaces." Diss., lmu, 2006. http://nbn-resolving.de/urn:nbn:de:bvb:19-53357.
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Banahene-Sabulsky, Dylan. "Constraining theories of modified gravity with atom interferometry." Thesis, Imperial College London, 2018. http://hdl.handle.net/10044/1/62326.
Повний текст джерелаАнотація:
Matter-wave interferometry is ideal for detecting small forces, being able to sense changes of acceleration as small as 1 nm s^-2 as a result of quantum interference. In this thesis, I prepare a cloud of ultracold Rb-87 atoms and measure the force between an atom and a cm-sized source mass using atom interferometry. The interferometer uses a sequence of optical Raman pulses to split, reflect, and recombine the atomic wavefunction. The force that is measured is consistent with standard Newtonian gravity. Some theories that have been advanced to explain the accelerating expansion of the universe - otherwise known as dark energy - predict a departure from the Newtonian force in my experiment. I use my result to constrain the parameters of these theories. The sensitivity of the experiment is sufficient to probe physics at energies approaching the Planck scale.
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Li, Baojiu. "Physical and cosmological implications of modified gravity theories." Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611653.
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Lovis, Kenneth John. "Strings, branes, and gravity duals of gauge theories." Thesis, Durham University, 2002. http://etheses.dur.ac.uk/4128/.
Повний текст джерелаАнотація:
We study the correspondence between certain supersymmetric gauge theories and their dual supergravity descriptions. Using low-energy brane probes of the super-gravity geometries we find moduli spaces of vacua, as expected from considering the dual gauge theories. The metrics on these spaces can be put into a form consistent with field theory expectations. This provides a non-trivial check on the supergravity solutions, in addition to strong-coupling predictions for the gauge theories. In the case of N = 2 supersymmetric gauge theory, proposed supergravity duals have previously been shown, using brane probe techniques, to display the 'enhangon mechanism'. In particular, the supergravity geometries correctly reproduce the per-turbative behaviour of the gauge theory. We calculate exact non-perturbative results at low-energies using the method of Seiberg & Witten. These correctly reproduce the perturbative results in the supergravity limit, but also make predictions for when the supergravity approximation is not valid. Finally, we study the Penrose limit of a geometry that is dual to a known N= 1 superconformal gauge theory. The resulting spacetime is a new plane-wave solution with constant three-form fluxes. We quantize type IIB superstrings on this background using the Green-Schwarz formalism. We find the spectrum of string excitations and discover that it is particularly simple, due to the specific form of the plane-wave background. Using the gauge theory/gravity duality, we make predictions (beyond the supergravity approximation) for gauge theory quantities in the corresponding limit.
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Li, Wenliang. "Aspects of Gravitational Theories : holography and modified gravity." Sorbonne Paris Cité, 2015. http://www.theses.fr/2015USPCC288.
Повний текст джерелаАнотація:
Dans cette thèse, nous étudions deux aspects de la théorie de la gravitation : la correspondance holographique et les théories de la gravité modifiée. La correspondance holographique est une conjecture remarquable qui établit l'équivalence entre certaines théories de la gravitation et certaines théories quantiques des champs. Les recherches dans le domaine de la gravité modifiée portent sur le développement des théories cohérentes de la gravité qui diffèrent de la relativité générale d'Einstein. La première partie de la thèse est dédiée à la correspondance holographique, ou la dualité gauge/gravité. Nous présentons un nouveau formalisme pour étudier les théories d'Einstein- scalaires du point de vue de l'holographie. Nous appliquons ce formalisme à la théorie holographique duale à une théorie de Yang- Mills à quatre dimensions. Nous calculons holographiquement l'action efficace pour le condensat de gluons, ainsi que pour la version de cet operateur qui est invariant sous le groupe de renormalisation. La deuxième partie de cette thèse traite les théories de la gravité modifiée. Nous nous concentrons sur une limite intéressante de la gravité massive autour de l'espace de Sitter. La théorie est connue comme gravité partiellement massless. Nous abordons la question s'il existe une extension non-linéaire de la gravité partiellement masslessIn this thesis, we will investigate two aspects of gravitational theories: holographic correspondence and modified gravity theories. Holographic correspondence is a remarkable conjecture which establishes the equivalence between certain gravitational theories and certain quantum field theories. The research in the domain of modified gravity concerns the development of consistent theories of gravity that are different from the standard general relativity. The first part of this thesis is dedicated to the holographic correspondence or the gauge/gravity duality. We will present a novel formalism to study the Einstein-scalar theories from the perspective of holography. We will apply this novel formalism to holographic Yang-Mills theory. We will compute the effective action for the gluon condensate and its relative that is renormalization-roup invariant. The second part of this thesis is about modified theories of gravity. We will focus on an interesting limit of massive gravity around de Sitter space. The theory is known as partially massless gravity. We will investigate whether a non-linear extension for partially massless gravity exists
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Darmé, Luc. "Unconventional particle behaviours in supersymmetric theories and gravity." Thesis, Paris 6, 2016. http://www.theses.fr/2016PA066161/document.
Повний текст джерелаАнотація:
Nous étudions dans un premier temps deux théories supersymétriques basées sur la présence de Gauginos de Dirac à travers deux scénarios à la phénoménologie bien distincte. La première, dite de "Fake Split SUSY", se caractérise par un spectre de particules scindé entre une partie à l'échelle électrofaible et l'autre plus lourde. Ces modèles prédisent avec une grande précision la masse du boson de Higgs et sont compatibles avec de nombreux résultats de cosmologie, au prix d'un spectre très peu naturel. La seconde présente un scénario supersymétrique dont l'un des bosons scalaire pourrait être identifié avec la résonance à 750 GeV observée au LHC. Dans un second temps, nous analysons deux comportements non-conventionnels du graviton et de son partenaire supersymétrique, le gravitino. Lorsque la symétrie de Lorentz est brisée par la présence d'un fluide, nous montrons que la pseudo-particule générée par cette brisure, le phonino, devient la composante longitudinale du gravitino et se propage suivant une relation de dispersion non relativiste que nous étudions en détails. Finalement, nous explorons des théories de gravité étendue dans lesquelles, en sus du Lagrangien d'Hilbert-Einstein, nous ajoutons des opérateurs construits à partir de produits de tenseurs de Riemann. En présence d'un fluide, nous prouvons qu'un graviton peut se propager "prestement", une notion reliée celle de vitesse superluminaleWe will first focus on supersymmetric theories with Dirac Gaugino masses. We investigate two advantages of such models. First, the possibility to reconcile the measured Higgs mass with an arbitrary large scale of supersymmetry breaking. Second, we show how the scalar singlet present in such models is a sound candidate for a resonance explaining the 750 GeV diphoton excess observed by the LHC experiments. In a second part, we start by discussing the propagation of a massive spin 3/2 state in a fluid (for instance the gravitino when supergravity is coupled to a background fluid). We show that the degrees of freedom corresponding to different helicities travel with different velocities. We then discuss the separate issue of graviton speed in extended gravity theories where the usual Einstein-Hilbert Lagrangian is supplemented by various higher order terms constructed from Riemann tensors
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von, Braun-Bates F. "Non-linear gravitational collapse in extended gravity theories." Thesis, University of Oxford, 2017. http://ora.ox.ac.uk/objects/uuid:910fd25d-38e0-4bd4-84cf-bf5c196c8f99.
Повний текст джерелаАнотація:
General Relativity (GR) is one theory amongst a wider range of plausible descriptions of the Universe. The aim of this thesis is to examine the behaviour of so-called screened theories, which are designed to avoid local tests of modified gravity (MG). We establish that these theories may be treated in a unified manner in the context of halo formation. A prerequisite for this is the clarification that the quasi-static approximation can be applied in cosmologically-plausible scenarios. Amongst the plethora of MG theories, we select three, each of which exhibit a different form of screening. This describes a self-concealing property whereby each theory behaves like GR in the conditions of the local Universe. Only at regions of high energy density (chameleon), large coupling to matter (symmetron) or large derivatives of the scalar field (Vainshtein) does their modified behaviour emerge. We examine f(R), symmetron and DGP gravity in the context of non-linear gravitational collapse for the remainder of the thesis. Relativistic scalar fields are ubiquitous in our modern understanding of structure formation. They arise as candidates for dark energy and are at the heart of many modified theories of gravity. While there has been tremendous progress in calculating their effects on large scales there are still open questions on how to best quantify their effects on smaller scales where non-linear collapse becomes important. In these regimes, it has become the norm to use the quasi-static approximation in which the time evolution of perturbations in the scalar fields are discarded, akin to what is done in the context of non-relativistic fields in cosmology and the corresponding Newtonian limit. We show that considerable care must be taken in this regime by studying linearly perturbed scalar field cosmologies and quantifying the error that arise from taking the quasi-static limit. We focus on f(R) and chameleon models to assess the impact of the quasi-static approximation and discuss how it might affect studying the non-linear growth of structure in N-body numerical simulations. The halo mass function (HMF) n(M) dM is the number of haloes with mass in the range [ M, M+dM ] per unit volume. It has two remarkable properties which render it a useful probe of extensions to general relativity (GR). On the one hand, it is (nearly-)universal, in the sense that it can be written in a form (f(v) which is (practically) insensitive to changes in redshift and cosmological parameters and redshift. We develop a method to generalise fitting functions derived in GR to a variety of screened MG theories, in order to examine whether they are universal in the sense of being insensitive to MG. On the other hand, the HMF is sensitive to both the expansion history of the universe and the non-linear behaviour of spherical collapse via the critical density parameter and the matter power spectrum via the halo resolution. This greatly complicates the theoretical framework required to calculate the HMF, particularly given the sensitivity of chameleon MG to the surrounding environment. We explore a variety of new and existing methods to do so. Finally we re-calibrate the MG halo mass functions with the same rigour as has been done in GR. An important indicator of modified gravity is the effect of the local environment on halo properties. This paper examines the influence of the local tidal structure on the halo mass function, the halo orientation, spin and the concentration-mass relation. We generalise the excursion set formalism to produce a halo mass function conditional on large-scale structure. Our model agrees well with simulations on large scales at which the density field is linear or weakly non-linear. Beyond this, our principal result is that f(R does affect halo abundances, the halo spin parameter and the concentration-mass relationship in an environment-independent way, whereas we find no appreciable deviation from LCDM for the mass function with fixed environment density, nor the alignment of the orientation and spin vectors of the halo to the eigenvectors of the local cosmic web. There is a general trend for greater deviation from LCDM in under-dense environments and for high-mass haloes, as expected from chameleon screening. Given the broad spectrum of MG theories, it is important to design new probes of MG. Despite the fact that we examine only three theories of MG, the techniques and methodology developed in this thesis can be applied to a wide variety of theories and can be extended to improve the results in this work.
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Cofano, M. C. "Chiral deformations of Yang-Mills and Gravity theories." Thesis, University of Nottingham, 2018. http://eprints.nottingham.ac.uk/50369/.
Повний текст джерелаАнотація:
In this thesis we present a modification of Yang-Mills and Gravity theories both written in terms of gauge connections. In particular gravity is presented in its pure connection formulation realizing a Diffeomorphism Invariant Gauge Theory. The modifications do not change the kinematical content of the theories, but they add an infinite set of new vertices retaining the symmetries of the original Lagrangians without introducing higher derivatives in the equations of motion. These new interactions are chiral, leaving the original Yang Mills and Gravity theories as the only parity-invariant theories in the set. In the Yang-Mills case we proved that at tree level the theory is still constructible via the so-called BCFW recursion relations. Both theories are claimed to be closed under the renormalization group. The Yang-Mills Deformations are indeed proved to be one-loop renormalizable and an explicit diagrammatic computation of the simplest deformation beta-function is given. For gravity a new formulation is introduced that could lead to a similar result at one-loop.
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Yagi, Kent. "Probing Alternative Theories of Gravity with Binary Gravitational Waves." 京都大学 (Kyoto University), 2012. http://hdl.handle.net/2433/157775.
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Burton, Howard Steven. "On the Palatini variation and connection theories of gravity." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape9/PQDD_0009/NQ38225.pdf.
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Middleton, Jonathan Ian. "The cosmology of higher-order Lagrangian theories of gravity." Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609455.
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Mussa, A. "Spherical symmetry and hydrostatic equilibrium in theories of gravity." Thesis, University College London (University of London), 2014. http://discovery.ucl.ac.uk/1419850/.
Повний текст джерелаАнотація:
Static, spherically symmetric solutions of the Einstein-Maxwell equations in the presence of a cosmological constant are studied, and new classes of solutions are derived. Namely the charged Einstein static universe and the interior and exterior charged Nariai spacetimes, these solutions form a subclass of the RNdS solution with distinct properties. The charged Nariai solutions are then matched at a common boundary. When constructing solutions to gravitational theories it is important that these matter distributions remain in hydrostatic equilibrium. If this equilibrium is lost, with internal gravitational forces dominating internal stresses, the solution will collapse under its gravitational field. An upper bound on the mass-radius ratio Mg/R for charged solutions in de Sitter space is derived, this bound implies hydrostatic equilibrium. The result is achieved by assuming the radial pressure p≥0 and energy density ρ≥0, plus p+2p⊥≤ρ where the tangential pressure p⊥≠ p. The bound provides a generalisation of Buchdahl's inequality, 2M/R ≤8/9, valid for Schwarzschild's solution. In the limit Q→0, Λ→0, the bound reduces to Buchdahl's inequality. Solutions in hydrostatic equilibrium are also considered in modified f(T) gravity. It is shown that the tetrads eⁱμ impact the structure of the field equations, and certain tetrads impose unnecessary constraints. Two particular tetrads are studied in more detail, solutions are then found for both tetrads, and a conservation equation is obtained using an analogous method to obtaining the Tolman-Oppenheimer-Volkoff equation. Although both tetrad fields locally give rise to the spherically symmetric metric, the tetrad fields are not globally well-defined and hence cannot be described as spherically symmetric. We then derive an upper bound on M/R which also implies hydrostatic equilibrium, this yields some constraints on the form of f(T) given a particular tetrad that locally gives rise to the line element ds²=exp(a)dt²-exp(b)dr²-r²dΩ².
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Leach, Jannie A. "Alternative theories of gravity and their application to cosmology." Doctoral thesis, University of Cape Town, 2008. http://hdl.handle.net/11427/4901.
Повний текст джерелаАнотація:
Includes abstract.Includes bibliographical references (p. 131-140).
In this thesis we study extended theories of gravity in the context of cosmology. The first part is dedicated to the application of the theory of dynamical systems, which allow us to investigate the global dynamics of some cosmological models resulting from scalar-tensor and higher-order theories of gravity. We use the dynamical systems approach with non–compact expansion normalised variables to study the isotropisation of Bianchi type I models in Rn–gravity. We find that these type of models can isotropise faster or slower than their general relativity counterparts. We extend this analysis to the full class of orthogonal spatially homogeneous Bianchi models to study the effect of spatial curvature on the isotropisation of these models. A compact state space is constructed by dividing the state space into different sectors, that allows us to also investigate static solutions and bouncing or recollapsing behaviours which is not possible when using non-compact expansion normalised variables. We find no Einstein static solutions, but there do exist cosmologies with bounce behaviours. We also find that all isotropic points are flat Friedmann like. We discuss the advantages and disadvantages of compactifying the state space, and illustrate this using two examples. We next study the phase-space of Friedmann models derived from scalar-tensor gravity where the non-minimal coupling is F(φ) = ξφ2 and the self-interaction potential is V (φ) = λφn. Transient almost-Friedmann phases evolving towards accelerated expansion and unstable inflationary phases evolving towards stable ones are found. In the last part of this work, we set out a framework to analyse tensor anisotropies in the cosmic microwave background of scalar-tensor cosmologies. As an example, we consider one of the exact solutions found for the class of scalar-tensor theories considered above.
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FEOLA, PASQUALE. "Astrophysical and Cosmological applications of Extended Theories of Gravity." Doctoral thesis, Università degli studi di Genova, 2020. http://hdl.handle.net/11567/1006198.
Повний текст джерелаАнотація:
The aim of this thesis is to study the possible astrophysical and cosmological applications of Extended Theories of Gravity.
In particular, Neutron Stars are studied, both on astrophysical and cosmological scale where, at cosmological level,
they can assume a macroscopic configuration, i.e. a cosmological probe, which can be represented like a Fermionic condensate.
The goal is to provide answers consistent with observational evidences that are not justified by General Relativity.
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Barausse, Enrico. "Exploring gravity theories with gravitational waves and compact objects." Doctoral thesis, SISSA, 2008. http://hdl.handle.net/20.500.11767/4125.
Повний текст джерелаАнотація:
This thesis is devoted to the study of tests of General Relativitywhich could be performed using astrophysical observations of stars or compact objects. The thesis consists of two parts. In the first one, I have investigated how the future gravitational wave observations by the space-based detector LISA will permit mapping the spacetime of the supermassive black holes which are thought to reside in galactic centres. In particular, I have analysed the dynamics of a stellar black hole orbiting around a supermassive black hole and have investigated under which conditions the gravitational wave signal emitted by such a system can allow one to detect the presence of an accretion torus around the supermassive black hole. I have also studied the motion of a stellar black hole in the very strong field region of a nearly extreme supermassive black hole: contrary to our expectations and to suggestions present in the literature, we have found that although the motion presents peculiar characteristics, the emitted gravitational waves do not retain an observable imprint of the almost maximal rotation of the supermassive black hole. Also, I considered black hole binaries with arbitrary masses and spins. Although the coalescence of such systems can be studied only with numerical simulations, I have derived a compact analytic formula for the spin of the final remnant. This formula is in agreement with all the numerical simulations available to date.
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Narain, Gaurav. "Renormalization Group Studies of Scalar-Tensor theories of Gravity." Doctoral thesis, SISSA, 2010. http://hdl.handle.net/20.500.11767/4144.
Повний текст джерелаАнотація:
The unification of Quantum Mechanics (QM) and General Relativity (GR) is one of the biggest challenges of contemporary physics. In spite of the fact that the two theories were discovered a century ago and a lot of effort has been made to combine the principles of the two theory, still we have not been a able to find a consistent quantum theory of gravitation, which describes the nature in a satisfactory manner.
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Lavdas, Ioannis. "AdS₄/CFT₃ and quantum gravity." Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEE041.
Повний текст джерелаАнотація:
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
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Hodson, Alistair. "A non-Newtonian perspective of gravity : testing modified gravity theories in galaxies and galaxy clusters." Thesis, University of St Andrews, 2017. http://hdl.handle.net/10023/12016.
Повний текст джерелаАнотація:
This thesis attempts to test several frameworks of non-Newtonian gravity in the context of galaxies and galaxy clusters. The theory most extensively discussed was that of Modified Newtonian Dynamics (MOND) with Galileon gravity, Emergent Gravity (EG) and Modified Gravity (MOG) mentioned to a lesser extent. Specifically, the main focus of this thesis was to determine whether MOND and MOND-like theories were compatible with galaxy cluster data, without the need to include cold dark matter. To do this, the paradigms of Extended MOND (EMOND), Generalised MOND (GMOND) and superfluid dark matter were investigated. The theories were outlined and applied to galaxy cluster data. The main findings of this were that EMOND and GMOND had some success with explaining galaxy cluster mass profiles, without requiring an additional dark matter component. The superfluid paradigm also enjoyed some success in galaxy clusters, which was expected as it behaves in a similar manner to the standard cold dark matter paradigm in cluster environments. However, the superfluid paradigm may have issues in the very centre of galaxy clusters due to the theory predicting constant density cores, whereas the cold dark matter paradigm predicts density cores which are cuspier. The EMOND paradigm was also tested against ultra-diffuse galaxy (UDGs) data as they appear in cluster environments, where EMOND becomes important. It was found that EMOND can reproduce the inferred mass of the UDGs, assuming they lie on the fundamental manifold (FM). The validity of the assumptions used to model the UDGs are discussed in the text. A two-body problem was also conducted in the Galileon gravity framework. The amount of additional gravitational force, compared to Newtonian was determined for a small galaxy at the edge of a galaxy cluster.