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RISS 인기검색어
- 검색키워드
- 저자 : Easson, Damien Alexander (검색결과 1 건)
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M-theory and superstring cosmology: Brane gases in the early universe and nonsingular gravity
Easson, Damien Alexander Brown University 2002 해외박사(DDOD)
This thesis will examine two major themes in modern cosmology. The first part of the thesis is concerned with the interface of superstring theory and M-theory with cosmology. We begin by providing a general background of various superstring cosmological models. In particular, we focus on the “Brane Gas” model of string cosmology (BGC) which was developed, in part, by the author. In this scenario the initial state of the Universe is taken to be small, dense and hot with all fundamental degrees of freedom near thermal equilibrium. Such a starting point is in close analogy with the Standard Big-Bang (SBB) model. The topology of the Universe is assumed to be toroidal in all nine spatial dimensions and the Universe is filled with a gas of p-branes. The dynamics of winding modes allow, at most, three spatial dimensions to become large, thus explaining the origin of our macroscopic 3 + 1-dimensional Universe. Specific solutions that are found within the model exhibit loitering, i.e. the Universe experiences a short phase of contraction during which the Hubble radius grows larger than the physical extent of the Universe. As a result, the brane problem (domain wall problem) in BGC is solved. The initial singularity and horizon problems of the SBB scenario are also solved without relying on an inflationary phase. The second part of the thesis focuses on a particular model of nonsingular gravity. This theory is constructed by adding to the usual string frame, dilaton gravity action, higher derivative terms motivated by the limited curvature construction described below. We examine cosmological, spatially homogeneous and isotropic solutions to the resulting equations of motion. All solutions of the resulting theory of gravity with these symmetries are nonsingular and all curvature invariants are bounded. For initial conditions inspired by the pre-big-bang scenario, solutions exist which correspond to a spatially flat Universe starting in a dilaton-dominated superinflationary phase and making a smooth transition to an expanding Friedmann-Robertson-Walker Universe. Hence, the graceful exit problem of pre-big-bang cosmology is solved in a natural way. Using this modified theory of gravity we discuss the ongoing search for a nonsingular four-dimensional Schwarzschild black hole. We end with comments on the hypothesis that our Universe may have been created on the interior of a black hole and the implications of such a scenario.
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