Targeted Energy Transfer is a resonance phenomenon in coupled anharmonic oscillators. In this thesis we investigate if the concept of Targeted Energy Transfer is applicable to Bose-Einsteain condensates in optical lattices. The model used to describe Bose-Einstein condensates in optical lattices is based on the Gross-Pitaevskii equation. Targeted Energy Transfer in these systems would correspond to energy being transferred from one lattice site to another. We also try to expand the concept of Targeted Energy Transfer to a system consisting of three sites, where one of the sites are considered a perturbation to the system. We have concluded that it is possible to achieve Targeted Energy Transfer in a three-site system. The set-up of the system will in some of the cases studied lead to interesting properties, such as more energy being transferred to the acceptor site than what was initially localized on the donor site.

This thesis reviews the use of 2-dimensional conformal field theory applied to gravity, specifically calculating Bekenstein-Hawking entropy of black holes in (2+1) dimensions. A brief review of general relativity, Conformal Field Theory, energy extraction from black holes, and black hole thermodynamics will be given. The Cardy formula, which calculates the entropy of a black hole from the AdS/CFT duality, will be shown to calculate the correct Bekenstein-Hawking entropy of the static and rotating BTZ black holes. The first law of black hole thermodynamics of the static, rotating, and charged-rotating BTZ black holes will be verified. / vii, 119 leaves : ill. ; 29 cm

This thesis examines the role of shear in inhomogeneous spherically symmetric spacetimes in the field of general relativity. The Einstein field equations are derived for a perfect fluid source in comoving coordinates. By assuming a barotropic equation of state, two classes of nonaccelerating solutions are obtained for the Einstein field equations. The first class has equation of state p = ⅓µ and the second class, with equation of state p = µ, generalises the models of Van den Bergh and Wils (1985). For a particular choice of a metric potential a new class of solutions is found which is expressible in terms of elliptic functions of the first and third kind in general. A class of nonexpanding cosmological models is briefly studied. The method of Lie symmetries of differential equations generates a self-similar variable which reduces the field and conservation equations to a system of ordinary differential equations. The behaviour of the gravitational field in this case is governed by a Riccati equation which is solved in general. Another class of solutions is obtained by making an ad hoc choice for one of the gravitational potentials. It is demonstrated that for a stiff fluid a particular case of the generalised Emden-Fowler equation arises. / Thesis (Ph.D.)-University of Natal, Durban, 1996.

In this thesis, we seek exact solutions for the interior of a radiating relativistic star undergoing gravitational collapse. The spherically symmetric interior spacetime, when matched with the exterior radiating Vaidya spacetime, at the boundary of the star, yields the governing equation describing the gravitational behaviour of the collapsing star. The investigation of the model hinges on the solution of the governing equation at the boundary. We first examine shear-free models which are conformally flat. The boundary condition is transformed to an Abel equation and several new solutions are generated. We then study collapse with shear in geodesic motion. Two classes of solutions are generated which are regular at the stellar centre. Our treatment extends the results of Naidu et al (2006) which had the undesirable feature of a singularity at the centre of the star. In an attempt to find more general models, we transform the fundamental equation to a Riccati equation. Two general classes of solution are found and are used to study the thermal evolution in the causal theory of thermodynamics. These solutions are shown to reduce to the Friedmann dust solution in the absence of heat flow. Furthermore, we obtain new categories of solutions for the case of gravitational collapse with expansion, shear and acceleration of the stellar fluid. This is achieved by transforming the boundary condition into a Riccati equation. In special cases the Bernoulli equation is regained. The solutions are given in terms of elementary functions and they permit the investigation of the physical features of radiative stellar collapse. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2007.

The study of exact solutions to the Einstein and Einstein-Maxwell field equations, by imposing a symmetry requirement on the manifold, has been the subject of much recent research. In this thesis we consider specifically conformal symmetries in static and nonstatic spherically symmetric spacetimes. We find conformally invariant solutions, for spherically symmetric vectors, to the Einstein-Maxwell field equations for static spacetimes. These solutions generalise results found previously and have the advantage of being regular in the interior of the sphere. The general solution to the conformal Killing vector equation for static spherically symmetric spacetimes is found. This solution is subject to integrability conditions that place restrictions on the metric functions. From the general solution we regain the special cases of Killing vectors, homothetic vectors and spherically symmetric vectors with a static conformal factor. Inheriting conformal vectors in static spacetimes are also identified. We find a new class of accelerating, expanding and shearing cosmological solutions in nonstatic spherically symmetric spacetimes. These solutions satisfy an equation of state which is a generalisation of the stiff equation of state. We also show that this solution admits a conformal Killing vector which is explicitly obtained. / Thesis (Ph.D.)-University of Natal, Durban, 1993.

In this thesis we study spherically symmetric spacetimes related to the Einstein field equations. We consider only neutral matter and apply the Einstein field equations with isotropic pressures. Our object is to model relativistic stellar systems. We express the Einstein field equations and the condition of pressure isotropy in terms of Schwarzschild coordinates and isotropic coordinates. For Schwarzschild coordinates we consider the transformations due to Buchdahl (1959), Durgapal and Bannerji (1983), Fodor (2000) and Tewari and Pant (2010). The condition of pressure isotropy is integrated and new exact solutions of the field equations are obtained utilizing the transformations of Buchdahl (1959) and Tewari and Pant (2010). These exact solutions are given in terms of elementary functions. For isotropic coordinates we can express the condition of pressure isotropy as a Riccati equation or a linear equation. An algorithm is developed that produces a new solution if a particular solution is known. The transformations reduce to a nonlinear Bernoulli equation in most instances. There are fundamentally three new classes of solutions to the condition of pressure isotropy. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2011.

In this thesis we consider spherically symmetric cosmological models when the shear is nonzero and also cases when the shear is vanishing. We investigate the role of the Emden-Fowler equation which governs the behaviour of the gravitational field. The Einstein field equations are derived in comoving coordinates for a spherically symmetric line element and a perfect fluid source for charged and uncharged matter. It is possible to reduce the system of field equations under different assumptions to the solution of a particular Emden-Fowler equation. The situations in which the Emden-Fowler equation arises are identified and studied. We analyse the Emden-Fowler equation via the method of Lie point symmetries. The conditions under which this equation is reduced to quadratures are obtained. The Lie analysis is applied to the particular models of Herlt (1996), Govender (1996) and Maharaj et al (1996) and the role of the Emden-Fowler equation is highlighted. We establish the uniqueness of the solutions of Maharaj et al (1996). Some physical features of the Einstein-Maxwell system are noted which distinguishes charged solutions. A charged analogue of the Maharaj et al (1993) spherically symmetric solution is obtained. The Gutman-Bespal'ko (1967) solution is recovered as a special case within this class of solutions by fixing the parameters and setting the charge to zero. It is also demonstrated that, under the assumptions of vanishing acceleration and proper charge density, the Emden-Fowler equation arises as a governing equation in charged spherically symmetric models. / Thesis (M.Sc.)-University of Natal, Durban, 1998.

<p>We examine the Einstein equation coupled to the Klein-Gordon equation for a complex-valued scalar field. These two equations together are known as the Einstein-Klein-Gordon system. In the low-field, non-relativistic limit, the Einstein-Klein-Gordon system reduces to the Poisson-Schrödinger system. We describe the simplest solutions of these systems in spherical symmetry, the spherically symmetric static states, and some scaling properties they obey. We also describe some approximate analytic solutions for these states.</p><p>The EKG system underlies a theory of wave dark matter, also known as scalar field dark matter (SFDM), boson star dark matter, and Bose-Einstein condensate (BEC) dark matter. We discuss a possible connection between the theory of wave dark matter and the baryonic Tully-Fisher relation, which is a scaling relation observed to hold for disk galaxies in the universe across many decades in mass. We show how fixing boundary conditions at the edge of the spherically symmetric static states implies Tully-Fisher-like relations for the states. We also catalog other ``scaling conditions'' one can impose on the static states and show that they do not lead to Tully-Fisher-like relations--barring one exception which is already known and which has nothing to do with the specifics of wave dark matter.</p> / Dissertation

Ce manuscrit présente mes travaux de recherche depuis la fin de ma thèse début 2005. J'essaye en particulier de replacer mes recherches dans leur contexte, et d'expliquer mes choix scientifiques de façon chronologique. Les principaux résultats ont donne lieu a des publications. Celles-ci ont été écrites avec soin et c'est pourquoi, j'ai choisi de ne décrire les résultats principaux que de fa con succincte. Pour avoir plus de détails et pour retrouver les gures expérimentales, mes articles sont reproduits a la n de chaque chapitre et cites à l'endroit adéquat. J'ajoute parfois des parties plus techniques qui ne sont pas explicitées dans les publications. Le Chapitre 2 est une courte introduction au domaine des atomes ultrafroids, s'adressant a un lecteur non spécialiste. Cela permet de présenter mon domaine de recherche, c'est a dire l'utilisation des gaz d'atomes ultra-froids en tant que systèmes modèles pour l'étude des propriétés quantiques des systèmes a N-corps. Le chapitre 3 est consacre a mes recherches durant mon séjour postdoctoral de deux ans dans le groupe de T. Esslinger a l'ETH Zurich. J'ai participé a une série d'expériences utilisant une cavité de haute nesse pour détecter individuellement les atomes issus d'un gaz ultra-froids. J'explique la théorie de ce système quantique ouvert et sa résolution numérique qui permet de comprendre quantitativement le processus de mesure de la présence d'un atome dans la cavit e. Experimentalement, nous avons mis en œuvre une technique consistant a extraire deux faisceaux d'atomes en deux points distincts du nuage d'atomes pour avoir accès a la fonction de corrélation g1 en perturbant peu le système. Nous avons ainsi étudie la dynamique de la transition de Bose-Einstein de fa con beaucoup plus ne que cela n'avait et e 15 fait auparavant. Nous avons pu comparer la dynamique de croissance de la densite a la dynamique d'apparition de la cohérence. Nous avons ensuite observe le comportement de la fonction g1 dans le régime critique tr es proche de la condensation et extrait une valeur expérimentale de l'exposant critique associe a la longueur de cohérence. En n, nous avons mis en place un ascenseur a atomes constitue de deux lasers contra-propageants contrôles en phase. Le condensat de Bose-Einstein a ainsi et e transport e vers la cavite de haute nesse pour créer un système quantique couple atome-rayonnement dans un régime de couplage extrême et nouveau. Le chapitre 4 est d edi e a mes trois premières années (2007-2009) au laboratoire Charles Fabry de l'Institut d'Optique dans le groupe d'optique atomique. J'explique d'abord le d em enagement du syst eme exp erimental a Palaiseau ainsi que les differentes ameliorations apportees au piege magn etooptique 2D. Nous avons realise un condensat de Bose-Einstein de rubidium par une methode entierement optique. Notre laser de piegeage est un laser a bre dop ee erbium de puissance a 1565 nm. Cette longueur d'onde n'avait jamais et e utilisee auparavant dans des experiences avec des atomes ultrafroids. Nous avons utilise les specifcites de ce laser, et notamment le fort decalage lumineux de la transition optique pour demontrer une technique de tomographie du champ lumineux vu par les atomes. Cette compr ehension des d ecalages lumineux nous a guide vers une melasse tres fortement decalee (environ 200 MHz) pour charger les atomes le plus e cacement possible dans le piege optique. Nous avons montre qu'une nouvelle geometrie permet de contrôler independamment la profondeur du piege et sa raideur, et ainsi d'optimiser l' evaporation dans le piege optique. En n, nous avons utilise notre dispositif pour une premiere application qui consiste a faire rebondir les atomes sur une onde stationnaire dans le but d'allonger le temps d'interrogation dans les interferometres o u les atomes sont en chute libre. Le trampoline a atomes peut fonctionner dans un r egime quantique o u les interferences entre chemins quantiques permettent d'allonger le temps de levitation. Le chapitre 5 est consacre a mes recherches sur les gaz 2D et desordonnes qui se poursuivent encore aujourd'hui. Lorsque les interactions entre atomes sont negligeables, la physique est a un corps. Le phenomene de diffusion a et e mis en evidence et caracteris e pour la premi ere fois avec des atomes ultra-froids dans un potentiel conservatif. En allant vers le regime quantique ou la longueur d'onde de DeBroglie des atomes devient de l'ordre de la taille caracteristique des grains de desordre, on s'attend alors a voir des effets de localisation d'Anderson, li es aux interferences entre ondes de mati ere. Les conditions necessaires pour observer la localisation d'Anderson dans un gaz en expansion sont detaillees. Pour des gaz pieges, les interactions jouent un r^ole predominant a 2D et la transition de Bose-Einstein est alors rem- 16 placee par une transition super uide de type Berezinskii-Kosterliz-Thouless. Exp erimentalement, nous avons etudie cette transition via la distribution en impulsion qui permet de caract eriser les propri et es de coherence du gaz. Ensuite, nous avons observe quantitativement l'in uence du d esordre sur la transition super uide. Nous formons alors un systeme quantique complexe pour lequel il n'y a pas de pr ediction th eorique pr ecise et dont la physique est liee a celle de certains materiaux de matiere condensee. J'insiste sur le rôle de la longueur de correlation du desordre en la comparant aux longueurs caracteristiques du gaz. Dans le regime d'un desordre correle a longue portee, l'approximation de densite locale est valable dans le desordre et alors des predictions quantitatives sur le diagramme de phase du systeme sont possibles. En n, je propose des directions pour nos recherches futures. Le refroidissement du potassium par un m ethode enti erement optique est un d e experimental mais permettra d'avoir acces a des resonances de Feshbach larges et ainsi de controler la force des interactions. De plus, un champ magnetique effectif donnera un parametre de controle suppl ementaire sur le syst eme. Ces deux outils seront utiles non seulement pour l' etude de la physique a un corps mais aussi pour une etude plus pr ecise du gaz de Bose 2D en pr esence de d esordre et d'interactions. On cherchera notamment a mettre en evidence le diagramme de phase et a observer une phase isolante exotique, le verre de Bose.

In this thesis we study classes of static spherically symmetric spacetimes admitting a perfect fluid source, electromagnetic fields and anisotropic pressures. Our intention is to generate exact solutions that model the interior of dense, relativistic stars. We find a sufficient condition for the existence of series solutions to the condition of pressure isotropy for neutral isolated spheres. The existence of a series solution is demonstrated by the method of Frobenius. With the help of MATHEMATICA (Wolfram 1991) we recovered the Tolman VII model for a quadratic gravitational potential, but failed to obtain other known classes of solution. This establishes the weakness, in certain instances, of symbolic manipulation software to extract series solutions from differential equations. For a cubic potential, we obtained a new series solution to the Einstein field equations describing neutral stars. The gravitational and thermodynamic variables are non-singular and continuous. This model also satisfies the important barotropic equation of state p = p(p). Two new exact solutions to the Einstein-Maxwell system, that generalise previous results for uncharged stars, were also found. The first of these generalises the solution of Maharaj and Mkhwanazi (1996), and has well-behaved matter and curvature variables. The second solution reduces to the Durgapal and Bannerji (1983) model in the uncharged limit; this new result may only serve as a toy model for quark stars because of negative energy densities. In both examples we observe that the solutions may be expressed in terms of hypergeometric and elementary functions; this indicates the possibility of unifying isolated solutions under the hypergeometric equation. We also briefly study compact stars with spheroidal geometry, that may be charged or admit anisotropic pressure distributions. The adapted forms of the pressure isotropy condition can be written as a harmonic oscillator equation. Two simple examples are presented. / Thesis (M.Sc.)-University of Natal, Durban, 2002.