Nucleosynthesis in alternative theories of gravity ; Alternatif kütle çekim teorilerinde nükleosentez

Thesis (M.Sc.) -- İstanbul Technical University, Graduate School, 2022 ; Big Bang nucleosynthesis (BBN) is one of the most reliable tools for testing standard model cosmology, as well as alternative models, well-known models are Brans-Dicke's theory of gravity, quintessence models, and higher-dimensional models. Standard BBN employs general relativity and the standard model of particle physics, thus, relying solely on one adjustable parameter; the baryon number density. Predicted primordial abundances based on SBBN are calculated with the help of BBN codes that contain well-established thermonuclear reactions network involved during the early evolution of the universe and presented as a function of the baryon number density. Observations from CMB and large-scale structure distributions indicate that the baryon number density can be restricted to a small range, allowing us to derive the basic relationship between predicted primordial abundances and new parameters emerging from alternative models of cosmology. All modifications to SBBN enforce the expansion rate of the early evolution of the universe to change, resulting in new relic abundances that differ from element abundances predicted by SBBN. Hence, we can parameterize the deviations from SBBN by introducing the $S$ parameter as $S\equiv H'/H $ where $H'$ is the modified Hubble parameter, $H$ is the Hubble parameter in the first Friedmann equation derived from the Einstein equation inserting the FRW metric. $S$ is constrained with the range of $0.85 \leq S \leq 1.15$ to obtain the simple relations between relic abundances and free parameters of the alternative models. Therefore, with this range of $S$, we can bound for free parameters of non-standard cosmological models. This thesis focuses on two models; Brans-Dicke's theory of gravity and its extensions with self-coupling potentials, and five-dimensional pure gravity which has an extra curled and compact dimension. Both theories have two free parameters. For the five-dimensional pure gravity, the ...