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Under certain circumstances, a massive object travelling through a medium may experience a net average deceleration due to the gravitational interaction between it and the medium. This slowing down effect is called dynamical friction. There are a variety of ways in which dynamical friction may be modeled, depending primarily on the nature of the medium, and on what approximations are deemed to be reasonable. This thesis is devoted to reviewing the various models, with an emphasis on the assumptions underlying each, to pinpointing the source of any discrepancies between the models, and to assessing the validity of each. For this purpose the models are grouped into two categories. The first comprises those in which the medium consists of dust or gas. The models in the second describe the interaction between a test object and a medium consisting of other objects of its own mass. These latter are of two general types: those which employ the two-body approximation, and those which describe the interaction in terms of a stochastically fluctuating force arising from the varying distribution of field objects surrounding the test object. The models lead to three different expressions for the dynamical friction experienced by the object: on proportional to TlogX (X is the distance from the object), one proportional to TlogvT (v is the average velocity of the members of the system), and one finite one proportional to TlogD[sub=o], (D[sub=o] is the mean inter-object spacing). It is argued that the TlogD[sub=o] result is based on an analysis which does not take into account the long range nature of gravitational forces. The TlogX models rely on the behaviour of the medium being stationary, but it is demonstrated that it takes an infinite amount of time to establish such behaviour, so these models are not strictly applicable to any real situation. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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Reproduction de : Thèse de doctorat : Sciences Mathématiques : Faculté des sciences de Paris : 1890. / Titre provenant de la page de titre du document numérisé.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

I have presented an evolving wormhole solution in braneworld model. Moreover I have shown that there are observational signatures for using braneworld to solve dark matter problem. Braneworld assumed that four dimensional spacetime embedded into five dimensional bulk. Gravity is a five dimensional interaction and the usual four dimensional description is reproduced by geometric projection. It predicts correction terms to the Einstein equation, for instance, the higher dimensional Weyl curvature projected on the brane. This projected Weyl curvature could make braneworld wormhole satisfies the Null Energy Condition (NEC), which is impossible in general relativity. We considered inflating braneworld wormhole that enlarge with the Universe, the wormhole is supported by the Weyl curvature so that it satisfies NEC. We demonstrated how this wormhole embedded into a five dimensional bulk and studied how it evolve with our Universe. The result is that inflating wormhole satisfying general initial condition will collapse into black hole when the scalar field oscillates. In addition, the projected Weyl curvature introduces a new source of gravity. This Weyl fluid of geometrical origin (reducing in the spherically symmetric, static configuration to a dark radiation and dark pressure) modifies space-time geometry around galaxies and has been used to explain the flatness of galactic rotation curves. Independent observations for discerning between the Weyl fluid and other dark matter models are necessary. Gravitational lensing could provide such a test. Therefore we study null geodesics and weak gravitational lensing in the dark radiation dominated region of galaxies in a class of spherically symmetric braneworld metrics. We find that the lensing profile in the braneworld scenario is distinguishable from dark matter lensing, despite both the braneworld scenario and dark matter models can fit the rotation curve data. In particular, in the asymptotic regions light deflection is 18% enhanced as compared to dark matter halo predictions. For a linear equation of state of the Weyl fluid we further find a critical radius, below which braneworld effects reduce, while above it they amplify light deflection. This is in contrast to any dark matter model, which always increases the deflection angle. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

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published_or_final_version / Physics / Master / Master of Philosophy

Made available in DSpace on 2016-05-17T16:50:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-07-25. Added 1 bitstream(s) on 2016-05-17T16:54:24Z : No. of bitstreams: 1 000855795.pdf: 568325 bytes, checksum: e744ad1774f7a18e0bb9480c6bdab2fb (MD5) / No presente trabalho, faremos um estudo de algumas propriedades da versão teleparalela da relatividade geral. No primeiro capítulo, estudaremos diferentes maneiras de obter sua Lagrangiana. No segundo, faremos uma introdução à dinâmica de sistemas vinculados, e no terceiro estudaremos a formulação ADM do teleparalelismo, obtendo a álgebra dos vínculos. Por fim, estudaremos o equivalente às variáveis de Ashtekar teleparalelas / In the present work we study some properties of the teleparallel equivalent of general relativity. In the first chapter, we analyse different ways to obtain the teleparallel Lagrangian. In the second, we make an introduction to the dynamics of constrained systems, and in the third we study the ADM formulation of teleparallel gravity, obtaining its constraint algebra. Finally, we make an analysis of the teleparallel equivalent to the Ashtekar variables