Abstract

It is known from the modern accurate observations, surprising us greatly, that the current universe is mainly about 90 percents fulfilled with the unknown dark components: dark energy and dark matter. The most recent WMAP observations are consistent with the universe made up of 74% dark energy, 22% dark matter, and 4% ordinary matter such as baryon. The modern universe not only needs the dark components but also inflaton leading to early accelerating expansion. These unknown components of the universe may be not independent of the unified theory of all forces existing in nature. For the recent studies of the unified theory, one of most promising approaches is a superstring theory, or M-theory, considered as a quantization of fields including gravitational interaction. The cosmological implications of string theory are currently receiving considerable attention, the so-called string cosmology. This interest has been inspired in part by the recent advances that have been made towards a non– perturbative formulation of the theory. The goal of superstring cosmology is to examine the dynamical evolution in these theories and re-examine cosmological questions in the light of our new understanding of string theory such as dark energy and dark matter. String theory has a much richer set of fundamental degree of freedom, consisting of D-branes. This fundamental objects, D-branes denote non–perturbative effects of string theory as “soliton” of strings, while string theory has been only described in perturbative form. Inspired by such speculation, recently a new paradigm on the early universe has been proposed, the so-called brane-world. The existence of models with more than one brane suggests that branes may collide. Colliding branes would be a fundamental phenomena in the string cosmology. We have studied several applications of colliding branes to string cosmology.

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