LEE Fizik Mühendisliği Lisansüstü Programı
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Sustainable Development Goal "Goal 8: Decent Work and Economic Growth" ile LEE Fizik Mühendisliği Lisansüstü Programı'a göz atma
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ÖgeQuantum corrected correlation functions and power spectra of spectator scalar and inflaton fields during inflation(Graduate School, 20220531) Karakaya, Gülay ; Önemli, Vakıf Kemal ; 509132104 ; Physics EngineeringThe inflationary cosmology not only explains apparently mysterious features of the universe like the flatness, horizon, and relic particle abundance problems, but also provides a natural mechanism to produce primordial density fluctuations that eventually lead to the cosmic structure formation. The primordial density perturbations are generated spontaneously from the quantum fluctuations of a scalar field, called inflaton, that drives the inflation. The physical origin of quantum fluctuations is the uncertainty principle that manifests itself as the virtual particle pair production out of vacuum. The observed anisotropies in the cosmic microwave background (CMB) are believed to be the amplified imprints of the fluctuations of an inflaton which later grow, due to gravitational instability, to become galaxies and clusters of galaxies. Variation of the amplitudes of fields with length scales is measured by the spectral index. Predictions of the inflation models for the spectral index and its variation with scale, quantified the by running of the spectral index, can be compared with observations to discriminate between the alternative inflation models. It is, therefore, important to study the correlation function and power spectrum of the quantum fluctuations to understand the distribution and the origin of large scale structure in the universe.

ÖgeTheoretical and observational aspects of inflationary cosmology(Graduate School, 20220622) Akın, Kemal ; Arapoğlu, Savaş ; 509181112 ; Physics EngineeringA testable theory of the universe has come up with Einstein's theory of general relativity. Combination of the theory with fundamental physics has provided significant understanding of the universe in the light of modern cosmological observations. On the other hand, the success of the hot Big Bang and ΛCDM relies on the existence of dark energy and dark matter which are beyond the standard model of particle physics. Another required extension is inflationary mechanism which was suggested as a resolution to shortcomings Big Bang such as horizon and flatness problems. However, the biggest success of the inflationary paradigm is to explain the generation of initial perturbations that are responsible for the structure formation in the universe. A scalar field, called inflaton, leads to a exponential expansion in the early stage of the universe. Although inflation is a very strong theory for the early universe, direct test of the theory is not possible due to extremelyhigh energy scales. Instead, inflationary models are tested against observations come from imprints of the primordial density perturbations. An important pair of parameters that comes from the observations are spectral index n_s and tensortoscalar ratio r. Many inflationary models rely on slowroll mechanism in which the inflaton slowly rolls through its potential minima so that equation of state parameter satisfies the acceleration condition ω < 1/3. Slowroll parameters are used to dictate such behaviour to the inflaton field. Besides, perturbations and therefore inflationary observables can be expressed in terms of slowroll parameters. Despite the fact that various minimally coupled single field models are consistent with current observations, quantum field theory in curved space anticipates a nonminimal coupling of scalar field to curvature scalar R. In this study, inflationary dynamics within the context of general relativity and scalartensor theories of gravitation is investigated. In the minimally coupled case, inflaton with a potential of the form φ^n is studied. In the nonminimally coupled case, same model with a coupling F(φ) = 1 + ξ φ^2 to curvature scalar is examined. In order to study inflation in scalartensor theories, usually conformal transformations are used, and for convenience the analysis is performed in Einstein Frame. In addition to standard Einstein frame analysis, we also perform the analysis in the Jordan frame. The predictions of the models are compared with PLANCK dataset using CosmoMC.