Graduate Program in Physics.Kaya, Ali.Özsoy, Ogan.2023-03-162023-03-162011.PHYS 2011 O87https://digitalarchive.library.bogazici.edu.tr/handle/123456789/13716The current model of the early universe is mainly based on the in ationary scenario. Despite its phenomenological success in providing a solid mechanism for the generation of cosmological uctuations in the early universe, this scenario su ers from several conceptual problems. Therefore, it is important to investigate the subject within a fundamental theory. Driven by this motivation, in this thesis, we study the cosmological perturbations in the string gas cosmology formalism. First, we give a review on string thermodynamics. Then we go over a recently proposed scenario of producing scale-invariant cosmological perturbations and examine its problems. Finally, we discuss an alternative model based on the open strings attached on a D-brane and nd out that it is hard to realize a scale-free spectrum for the cosmological perturbations.30cm.Cosmology -- Mathematics.String models -- Mathematics.ix, 40 leaves ;