Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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2007 - 200912010 - 20163

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Now showing 1 - 4 of 4
  • Master Thesis
    Affine Dynamics With Torsion
    (Izmir Institute of Technology, 2016) Gültekin, Kemal; Demir, Durmuş Ali
    In this study, we give a thorough analysis of a general affine gravity with torsion. After a brief exposition of the affine gravities considered by Eddington and Schr¨odinger, we construct and analyze different affine gravities based on determinants of the Ricci tensor, the torsion tensor, the Riemann tensor and their combinations. In each case we reduce equations of motion to their simplest forms and give a detailed analysis of their solutions. Our analyses lead to the construction of the affine connection in terms of the curvature and torsion tensors. Our solutions of the dynamical equations show that the curvature tensors at different points are correlated via non-local, exponential rescaling factors determined by the torsion tensor.
  • Master Thesis
    Higher Curvature Gravity in Large Extra Dimension: Phenomenological Implications
    (Izmir Institute of Technology, 2007) Tanyıldızı, Şükrü Hanif; Demir, Durmuş Ali
    This thesis is devoted to a detailed study of the higher curvature gravity and its phenomenological implications in large extra dimensions. This work is intended as a discussion of effective interactions among brane matter induced by modifications of higher dimensional Einstein gravity via the replacement of Einstein-Hilbert term with a generic function f(R) of the curvature scalar R.In this work, following the introductory chapters on extra dimensions and higher curvature gravity in large extra dimensions, we derive the graviton propagator and then we analyze impact of virtual graviton exchange on interactions among brane matter. We find that f(R) gravity effects are best probed by high-energy processes involving massivegauge bosons, heavy fermions or the Higgs boson. We perform a comparative analysis of the predictions of f(R) gravity and of Arkani.Hamed, Dimopoulos and Dvali (ADD) scenario, and find that the former competes with the latter when f00(0) is positive and comparable to the fundamental scale of gravity in higher dimensions (Demir and Tanyıldızı 2006). In addition, we briefly discuss graviton emission from the brane as well as itsdecays into brane-localized matter and we find that they hardly compete with the ADD expectations.Consequently, we discussed that possible existence of higher-curvature gravitational interactions in large extra spatial dimensions opens up various signatures to be confronted with existing and future collider experiments.
  • Master Thesis
    Abelian-Non Abellian Mixing and Cosmic Inflation in Born-Infeld Type Gravity
    (Izmir Institute of Technology, 2013) Altaş, Aslı; Demir, Durmuş Ali
    General Relativity (GR), which forms the basic framework for understanding cosmological and astrophysical phenomena, is based solely on general covariance. Therefore, the theory admits extensions regarding various phenomena related to inflation, massive gravity, dark energy etc. In this thesis work we study Born-Infeld type extensions of the GR. There are mainly two parts in the thesis: The extension based on Ricci tensor (already known in literature) and a novel extension based on Riemann tensor. We call them respectively Born-Infeld-Einstein (BIE) and Born-Infeld-Riemann (BIR) extensions. The BIR formalism is being proposed and studied in this thesis work. In a comparative fashion, we study these two extensions for determining their implications for 1. Mixing between Abelian and Non-Abelian gauge fields, and 2. Inflationary phase of cosmic evolution. As we prove explicitly, the two approaches yield distinct predictions for these phenomena. We emphasize that a slow-roll inflationary dynamics is naturally realized in BIR. The mixing between Abelian and Non-Abelian sectors enables cosmic photon production in inflationary phase.
  • Master Thesis
    Conformal Transformationsin Metric-Affine Gravity
    (Izmir Institute of Technology, 2010) Düztürk, Canan Nurhan; Demir, Durmuş Ali
    Conformal transformations play a widespread role in gravitation in regard to their cosmological and the other implications. In this thesis, the effects of conformal transformations on General Relativity comparatively in metric formulation and in metric-affine formulation are analyzed. In the metric formulation of General Relativity ( pure metric theory of gravity ), conformal transformations, like gauge transformations, add a new degree of freedom to the system - the conformal factor. In this sense, they change a frame to a new one involving an additional degree of freedom. However, this new degree of freedom turns out to be a ghost field in pure metrical formulation i.e. Einstein-Hilbert action. This possesses a serious problem since ghosts are manifestly unphysical. To overcome this problem, we explore conformal transformations in metric-affine formulation of General Relativity ( metric-affine theory of gravity ) in which the metric and connection are treated as independent variables from the scratch. In metric-affine formulation, there is no a priori relation between metric and connection, and thus, their transformations under conformal transformations do not need to exhibit the correlation present in pure metrical formulation. We thus exploit this fact by assigning different transformation rules for connection to have ghost-free Lagrangians. Firstly, we use the conformally invariant connection, while the metric changes as in metric formulation. After these transformations, there is no ghost field generated by conformal factor. Indeed, there appears no kinetic term of the scalar field (auxiliary field-nondynamical field). This result is not sufficient sufficient for us. Because the main goal of our study is the obtaining a conformally invariant theory for gravity with a dynamical scalar field. Then, we use the multiplicatively transforming connection. This transformation does not give the result corresponding to our aim. Finally, we find that if connection transforms additively yet differently than in metrical formulation, the ghost generated by the conformal factor disappears. Additionally, we discuss the physical implications of these transformation rules.