On Observational Signatures Of String Theory In The Cosmic Microwave Background
In this dissertation, we discuss the possibility of detecting evidence for string theory in the sky. Inflation, the leading paradigm for describing the very early universe, is ultraviolet (UV) sensitive. In order to understand the microphysics of inflation, it is necessary to study inflation models in a UV-complete theory, such as string theory. Recent developments in flux compactification lead to several mechanisms for inflation in which quantum corrections are under control. Here we present the observational consequences of two notable examples: warped D-brane inflation and axion monodromy inflation. After the first explicit models of warped D-brane inflation were constructed, many analyses considered the single-field approximation of these models in detail. We take a statistical approach to study the full six-field dynamics. By evolving milllions of realizations of the model numerically, we find that the probability of inflation follows a power law, which is independent of the initial conditions, of the distributions from which we draw the coefficients, and of the physical parameters. Without making slow-roll approximations, we look for multifield contributions to the power spectrum from bending of the trajectories; we find that the contributions of more than two fields are non-negligible for a sizable fraction of the samples we use. However, after imposing the observational constraints on the tilt, the imprints of these multifield effects on the power spectrum and bis- pectrum are too small to be detected. The other example we study is axion monodromy inflation. It has an approximately linear potential and produces a detectable tensor signature. This linear potential is corrected sinusoidally by instantons. We study the effects on the scalar power spectrum and bispectrum from these sinusoidal corrections. We find that there are models that satisfy all the microphysical constraints from string theory and the constraints from the current cosmological data. Moreover, a non-negligible fraction of these models also produce experimentally interesting signatures: detectable undulations of the power spectrum and/or enhanced resonant non-Gaussianity.
string cosmology; CMB signatures; D-brane inflation
Tye, Sze-Hoi Henry; Alexander, James Paul
Ph.D. of Physics
Doctor of Philosophy
dissertation or thesis