Browsing by Subject "Cosmology."
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Item Classification of cosmological models in Einstein's general theory of gravity.(2009-08-24T20:25:14Z) Ha, Te, 1985-; Wang, Anzhong.; Physics.; Baylor University. Dept. of Physics.In this thesis, I first review the fundamentals of Hot Big Bang cosmology, the observational cosmology and the late accelerating universe. Then I systematically study the evolution of the Friedmann-Robertson-Walker (FRW) universe with a cosmological constant Λ and a perfect fluid that has the equation of state p=wρ, and classify all the solutions into various cases, where p and ρ are the pressure and energy density of the fluid, and w is a constant. In each case the main properties of the evolution of the universe are studied in detail, including the periods of deceleration or acceleration, and the existence of big bang, big crunch, and big rip singularities. Finally, I mention some future work along the direction laid down in this thesis.Item Colliding branes and formation of spacetime singularities in superstring theory.(2009-06-10T17:14:59Z) Tziolas, Andreas Constantine.; Wang, Anzhong.; Physics.; Baylor University. Dept. of Physics.A systematic study of space-times containing two timelike colliding branes is made, in the framework of 10-dimensional string theory. After developing the general formulas to describe such events, we study several classes of exact solutions and space-time singularities in both the (D+d)-dimensional string theory and its D-dimensional effective theory, obtained by Kaluza-Klein compactification. It is found that space-time singularities in the low dimensional effective theory may or may not remain after lifted to the (D+d)-dimensional string theory, depending on the specific solutions. In some cases, solutions of the low dimensional effective theory are free of singularities, but after they are lifted to string theory, the higher dimensional space-times become singular. Therefore, simply lifting low dimensional effective theories to high dimensions seemingly does not solve the singularity problem, and additional physical mechanisms are needed. In general however, the space-time is singular, due to the mutual focus of the two colliding branes. Non-singular cases also exist, but with the price that both of the colliding branes violate all the three energy conditions, weak, dominant, and strong.Item Hořava-Lifshitz theory of gravity and its applications to cosmology.(2013-09-16) Huang, Yongqing, 1984-; Wang, Anzhong.; Physics.; Baylor University. Dept. of Physics.In this dissertation, I studied Hořava-Lifshitz gravity and its applications to inflationary cosmology. After introducing the original proposal with the projectability and detailed balance conditions, I discussed its attracting features as well as the problems it faces. An extended model without the detailed balance condition was then studied and found to be stable in de Sitter spacetime, but still possess an extra scalar mode in the gravity sector. I then studied a model with an extra U(1) symmetry dubbed as the projectable general covariant HL gravity and showed that it has the same degree of freedom as general relativity, and is free of the stability problem. It was found that the FLRW universe is necessarily flat, given that the coupling with matter takes a specific form. I also studied the scalar perturbations around the FLRW metric and presented all the possible gauge choices. Applications of the general covariant model in inflationary cosmology were studied in the second part. After deriving the slow-roll conditions in this model, I showed that in the super-horizon regions the scalar perturbations become adiabatic, and the comoving curvature perturbation is constant, because of this slow-roll condition. By using the uniform approximation technique, power spectra and indices of primordial scalar and tensor perturbations under the slow-roll approximations were expressed explicitly in terms of the slow-roll parameters and the various coupling constants. I found that they are in general different from, but reducible to, the values in the class of simplest inflation models. Next I studied the non-Gaussianities of these perturbations. For scalar perturbations, by properly choosing the coupling constants, a large nonlinearity parameter fNL is possible. I also found that the bispectrum favors the equilateral shape as a result of the higher order spatial derivatives, and that folded shape is enhanced when the vacuum is from the Bunch-Davis vacuum. Both the squeezed and the equilateral shapes appear in the bispectrum for primordial gravitational waves. In addition, the polarization tensors of the tensor fields have strong effects on the shapes of the bispectrum.Item Orbifold branes in string theory and their applications to cosmology.(2011-09-14) Devin, Michael J.; Wang, Anzhong.; Physics.; Baylor University. Dept. of Physics.This dissertation contains two distinct compactification schemes of 10-dimensional string theory, as well as some of the implications of one of these schemes for string cosmology. The first half of this work begins with a brief overview of cosmology and goes through constructing and then analyzing the first model, inspired by the work of Santos and Wang. The second part consists of an attempt to construct similar models using the the popular warped conifold compactification scheme, as well as an appendix with a variant of the first model and its derivation. The work concludes with the observation that the latter attempt does not admit solutions of the same form, and that the variant model in the appendix is degenerate to previously studied KK-type models.