Ph.D. Theses
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Browsing Ph.D. Theses by Subject "Black holes (Astronomy)"
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Item Black holes and cosmic censorship(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2013., 2014.) Düztaş, Koray.; Semiz, İbrahim.This thesis starts with a brief review of causality conditions, gravitational collapse and singularity theorems. Cosmic censorship conjecture is introduced and the need for such a conjecture to preserve the deterministic nature of physics is explained. Examples of thought experiments which are designed to test the validity of cosmic censorship conjecture, are given. These examples involving cases of both particles and fields, are considered to have conceptual importance. Spinor algebra and analysis is reviewed. Newman-Penrose formalism is introduced. The derivations of Teukolsky’s solutions for test fields in Kerr space-time are given. Using these solutions an original thought experiment involving an extremal Kerr black hole and a test electromagnetic is constructed. It is shown that cosmic censorship conjecture is not violated in this case. The examples of thought experiments with particles that start with nearly extremal black holes instead of extremal ones are given. Another original thought experiment is constructed using integer-spin test fields and a nearly extremal Kerr black hole. It is found that cosmic censorship conjecture is violated in this case as long as radiative and self-force effects are ignored. Finally the case of neutrino fields are considered as an example of half integer-spin. It is shown that neutrino fields lead to a generic violation of the conjecture which is unique among all the attempts since the conjecture was introduced.Item Black holes in an expanding universe(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2016., 2016.) Şenikoğlu, Yorgo.; Turgut, Teoman.; Arık, Metin.By performing transformations on the isotropic forms of di erent metrics, new cosmological black hole solutions were obtained by generalizing the static solution, the Schwarzchild solution, the McVittie and the Kerr solutions. We present cosmological black holes solutions that satisfy the known behaviours of the scale factor through the history of our Universe, the Radiation, Matter and Dark Energy dominated eras. In the static case we presented solutions satisfying all of the energy conditions, then a time generalization and analysis of the previous isotropic solution gave us the behaviour in time of the Schwarzschild black hole solution. By a choice of parameter, we obtained a non-singular equation of state parameter even though the energy density and pressure tend to in nity at the horizon. In that case the solution suggests that in the expanding universe, the equation of state parameter is negative and may eventually be satis ed by a dark energy dominated representation of the Universe. For the classical McVittie and Kerr solutions, an inductive reasoning led us to obtain all the solutions, Radiation, Matter and Dark energy dominated eras, by introducing two conformal factors and generalizing the metrics. Finally, the Weyl curvature and the volume expansion are calculated for the cosmological black holes and the e ects, increases or decreases, of the volume expansions for all the cases are discussed. It is shown at the end that a multitude of black holes can impact the Universe as a whole by increasing or decreasing its volume.Item Black holes, cosmology and Brans-Dicke theories in 4+1 dimensional universe(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2007., 2007.) Çiftci, Dilek.; Arık, Metin.We consider the embedding of 3+1 dimensional cosmology in 4+1 dimensional Jordan-Brans-Dicke theory. We show that exponentially growing and power law scale factors are implied. We solve the Brans-Dicke equations and we realize that only a linear warp factor satisfies these equations. In both exponentially expanding and power law expanding cases we find out that the scalar field is a function of the warp factor and the scale factor. Whereas the 4 + 1 dimensional scalar field is approximately constant for each case, the effective 3 + 1 dimensional scalar field is constant for exponentially growing scale factor and time dependent for power law scale factor. We calculate the effective gravitational constant and realize that same results are valid. We construct a static solution for 4 + 1 dimensional bulk such that the 3 + 1 dimensional world has a linear warp factor and describes the Schwarzschild-dS4 black hole. The 4 + 1 dimensional space-time is taken to be flat. For zero mass this four dimensional universe and Friedmann-Robertson-Walker universe are related with an explicit coordinate transformation. Also we realize that if there is a contribution from the mass term, both energy momentum and cosmological constant vanish in the bulk. We explain the four dimensional cosmological constant originates from the hidden brane and its effects cause the localized mass densities in our visible brane. Using hierarchy between the Planck mass and electroweak energy scale, we obtain the size of the extra dimension as close to Hubble length but smaller than it. We emphasize that for linear warp factors the effect of bulk on the brane world shows up as the dS4 background which is favored by the Big Bang cosmology.