Coupled Scalar Tachyon Bounce (CSTB)Cosmos:Model Building, Systematic Analysis And Experimental Exploration

In this Ph.D. thesis, a string-inspired bounce universe model utilizing scalar field and tachyon field coupling (Coupled Scalar Tachyon Bounce Cosmos, CSTB Cosmos for short) is proposed and system-atically studied. Utilizing the scalar-tachyon coupling as well as contribution from curvature in a closed Friedmann-Lemaitre-Robertson-Walker (FRLW) background, the universe undergoes a bouncing/cyclic evolution up to the zeroth order of the background. The minimum size of the universe is nonzero for generic initial values. It indicates CSTB Cosmos is free from the Initial Singularity Problem(namely the Big Bang Singularity).According to CSTB model, the universe undergoes a phase of locked inflation, an era of tachyon mat-ter dominated expansion, turnaround and contraction, followed by the subsequent deflation and bounce in each cycle of the cosmological evolution. Extensive analytic and numerical studies of the above evolu-tion processes have been performed and presented in detail in this thesis. No ghosts are ever generated at any point in the entire evolution of the universe, with the Null, Weak, and Dominant Energy Conditions preserved even at the bounce points, contrary to many bounce models previously proposed. The Strong Energy Condition is satisfied in periods of tachyon matter domination.Furthermore, to make contact with the observations of the Cosmic Microwave Background Radiation, the primordial power spectrum of the cosmological perturbations in CSTB Cosmos are investigated in detail. Explicit computation of its primordial spectral index shows that the power spectrum of curvature perturbations, generated during the tachyon matter dominated contraction phase, is nearly scale invariant--consistent with the current array of observations.After an in-depth study of various inflationary/bouncing universe models, especially for CSTB Cos-mos as well as Slow-roll Inflation, a unified space of parameters suitable for systematic studies of infla-tionary and bounce universe models emerges. The blue/red-shift term of the cosmological perturbations is generalized to be mHχ. Combining it with the power-law index of the cosmological background, v, a parameter space,(v, m), to classify the perturbation spectra of the inflationary and bounce universe models, is constructed. In this unified parameter space, there exist two groups of stable solutions with the scale-invariant power spectra. One group is generated as usual in the accelerating expanding phase of the inflationary models; while the other group is generated in the accelerating contracting phase of the bouncing/cyclic universe model. Moreover, a complete duality transformation is introduced to connect these two groups of stable and scale-invariant solutions.Encouragingly, CSTB cosmos is dual to the Slow-roll Inflation in this complete transformation--both possess stable and scale invariant spectra with the latter generated in an exponential expansion while the former in a tachyon matter dominated contraction. Guaranteed by the dynamical attractor behavior of CSTB Cosmos, this scale invariance is free of the fine-tuning problem, in contrast to the slow-roll inflation model.All in all the string-inspired CSTB model can produce a stable and scale-invariant spectrum of per-turbations in the pre-bounce contraction, which then becomes the primordial density perturbation of our observed universe. Unlike Inflation it also avoids the Big Bang Singularity problem.In the rest of this thesis we investigate the observational implications of the bounce cosmology. The possibility of using dark matter particle’s mass and its cross section as a smoking gun signal of the exis-tence of a Big Bounce at the early stage in the evolution of our currently observed universe is reported.A model independent study of dark matter production in the pre-bounce contraction and the post-bounce expansion epochs of the bounce universe reveals a new venue for achieving the observed relic abundance of our present universe, in which a significantly smaller amount of dark matter with a smaller cross section-as compared to the prediction of Standard Cosmology--is produced and carries the information about the bounce universe evolution in the relic abundance to present time.Furthermore, by explicit computation of the entire process of dark matter production and evolution, as well as subsequent freeze-out in this new venue, a relation between the cross section and dark matter mass for satisfying current observation is obtained. Once the values of dark matter mass and interaction cross section are obtained by direct detections in laboratories, this alternative route may become a useful test of the bounce universe scenario.