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Perturbations in Lemaître-Tolman-Bondi and Assisted Coupled Quintessence cosmologiesLeithes, Alexander January 2017 (has links)
In this thesis we present research into linear perturbations in Lemaître-Tolman-Bondi (LTB) and Assisted Coupled Quintessence (ACQ) Cosmologies. First we give a brief overview of the standard model of cosmology. We then introduce Cosmological Perturbation Theory (CPT) at linear order for a at Friedmann-Robertson-Walker (FRW) cosmology. Next we study linear perturbations to a Lemaître-Tolman-Bondi (LTB) background spacetime. Studying the transformation behaviour of the perturbations under gauge transformations, we construct gauge invariant quantities in LTB. We show, using the perturbed energy conservation equation, that there is a conserved quantitiy in LTB which is conserved on all scales. We then briefly extend our discussion to the Lemaître spacetime, and construct gauge-invariant perturbations in this extension of LTB spacetime. We also study the behaviour of linear perturbations in assisted coupled quintessence models in a FRW background. We provide the full set of governing equations for this class of models, and solve the system numerically. The code written for this purpose is then used to evolve growth functions for various models and parameter values, and we compare these both to the standard CDM model and to current and future observational bounds. We also examine the applicability of the "small scale approximation", often used to calculate growth functions in quintessence models, in light of upcoming experiments such as SKA and Euclid. We nd the results of the full equations deviates from the approximation by more than the experimental uncertainty for these future surveys. The construction of the numerical code, Pyessence, written in Python to solve the system of background and perturbed evolution equations for assisted coupled quintessence, is also discussed.
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Linear perturbations of a Schwarzschild black holeKubeka, Amos Soweto 17 February 2015 (has links)
We firstly numerically recalculate the Ricci tensor of non-stationary axisymmetric
space-times (originally calculated by Chandrasekhar) and we find some discrepancies
both in the linear and non-linear terms. However, these discrepancies do not affect
the results concerning linear perturbations of a Schwarzschild black hole. Secondly,
we use these Ricci tensors to derive the Zerilli and Regge-Wheeler equations and use
the Newman-Penrose formalism to derive the Bardeen-Press equation. We show the
relation between these equations because they describe the same linear perturbations
of a Schwarzschild black hole. Thirdly, we illustrate heuristically (when the angular
momentum (l) is 2) the relation between the linearized solution of the Einstein vacuum
equations obtained from the Bondi-Sachs metric and the Zerilli equation, because
they describe the same linear perturbations of a Schwarzschild black hole. Lastly, by
means of a coordinate transformation, we extend Chandrasekhar's results on linear
perturbations of a Schwarzschild black hole to the Bondi-Sachs framework. / Mathematical Sciences / M. Sc. (Applied Mathematics)
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Linear perturbations of a Schwarzschild black holeKubeka, Amos Soweto 17 February 2015 (has links)
We firstly numerically recalculate the Ricci tensor of non-stationary axisymmetric
space-times (originally calculated by Chandrasekhar) and we find some discrepancies
both in the linear and non-linear terms. However, these discrepancies do not affect
the results concerning linear perturbations of a Schwarzschild black hole. Secondly,
we use these Ricci tensors to derive the Zerilli and Regge-Wheeler equations and use
the Newman-Penrose formalism to derive the Bardeen-Press equation. We show the
relation between these equations because they describe the same linear perturbations
of a Schwarzschild black hole. Thirdly, we illustrate heuristically (when the angular
momentum (l) is 2) the relation between the linearized solution of the Einstein vacuum
equations obtained from the Bondi-Sachs metric and the Zerilli equation, because
they describe the same linear perturbations of a Schwarzschild black hole. Lastly, by
means of a coordinate transformation, we extend Chandrasekhar's results on linear
perturbations of a Schwarzschild black hole to the Bondi-Sachs framework. / Mathematical Sciences / M. Sc. (Applied Mathematics)
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