A Comparison of two Cosmological Models: Inflationary Cosmology and the Cyclic Universe

To explain current observations the early universe must have been incredibly flat and homogeneous. Tiny density fluctuations must have been present to seed large scale structure. Any viable model of the universe has to explain these cosmological puzzles and be able to generate a nearly flat spectrum of density perturbations, as required by microwave background data. Inflation is a period of accelerated expansion in the very early universe. Combined with the big bang model it forms the consensus model of cosmology. The cyclic model, inspired by M-theory, describes a universe undergoing an infinite sequence of cycles that start with a big bang and end in a big crunch. Each cycle includes a slowly contracting ”ekpyrotic” phase. Both inflation and an ekpyrotic phase can resolve the cosmological problems and lead to the required perturbation spectrum. In this paper inflationary cosmology and the cyclic model are compared. Their dynamics are described using scalar fields and their solutions to the cosmological problems are presented. Strengths and weaknesses of the models are compared; open issues and conceptual problems are identified. The observational signatures are contrasted and confronted with experiments. Future possibilities to distinguish between the models due to differing predictions for non-gaussianity and gravitational waves are discussed. Submitted in partial fulfilment of the requirements for the degree of Master of Science of Imperial College London