Aspects of String Gas Cosmology: Cyclic Models and Decompactification
Abstract
String gas cosmology is a natural approach to cosmology within string theory. Two of its novelties are that it provides means to replace the singular evolution of the early universe with a smooth cyclic phase and that it suggests a dynamical mechanism for selecting at most three spatial dimensions that can grow large cosmologically. This thesis presents progress in these two directions. By implementing corrections to gravitational dynamics, we show how string gas cosmology gives rise to cyclic and bouncing models in the early universe. In these models entropy production naturally leads to an exit from the cyclic phase and drives an eventual transition to a radiation dominated, expanding universe. Further, between the cyclic and radiation phases, the scale factors can have long loitering phases that can address the horizon problem of standard big-bang cosmology. We then address the decompactification mechanism, as originally suggested by Brandenberger and Vafa. This mechanism relies on the argument that winding string states generically intersect, and therefore interact appreciably, in at most three spatial dimensions. When these strings are not energetically favored they can decay to unwound strings and if not, they oppose the expansion of the dimensions they wrap. Since they interact efficiently in at most three dimensions this is the maximum number of dimensions that is allowed to expand. We show that a semiclassical treatment of fundamental string interactions makes this dimensional dependence manifest. This allows us to demonstrate in a cosmological setting that decompactification of d = 3 dimensions is largely favored over d > 3. We conclude with a study of the mechanism on anisotropic backgrounds.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2010
- Bibcode:
- 2010PhDT.......207M
- Keywords:
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- Physics, Theory;Physics, Elementary Particles and High Energy