Brane cosmology in string/M-theory and cosmological parameters estimation.

In this dissertation, I mainly focus on two subjects: (I) highly effective and efficient parameter estimation algorithms and their applications to cosmology; and (II) the late cosmic acceleration of the universe in string/M theory. In Part I, after developing two highly successful numerical codes, I apply them to study the holographical dark energy model and LCMD model with curvature. By fitting these models with the most recent observations, I find various tight constraints on the parameters involved in the models. In part II, I develop the general formulas to describe orbifold branes in both string and M theories, and then systematical study the two most important issues: (1) the radion stability and radion mass; and (2) the localization of gravity, the effective 4D Newtonian potential. I find that the radion is stable and its mass is in the order of GeV, which is well above the current observational constraints. The gravity is localized on the TeV brane, and the spectra of the gravitational Kluza-Klein towers are discrete and have a mass gap of TeV. The contributions of high order Yukawa corrections to the Newtonian potential are negligible. Using the large extra dimensions, I also show that the cosmological constant can be lowered to its current observational value. Applying the formulas to cosmology, I study several models in the two theories, and find that a late transient acceleration of the universe is a generic feature of our setups.