Tests of early universe physics from observational astronomy

Abstract

Early universe physics has, in recent years, become a vibrant, interdisciplinary, observationally driven field. In this thesis, I present a range of studies examining how modern observational techniques can inform and help build (or tear down) theories of the early universe. In the first half, I present discussions of two new approaches to the search for big bang relics, the first pertaining to cosmic strings and the second to primordial black holes. In both cases, I show how long-baseline radio interferometry, which is not typically employed in early universe studies, has the potential to bring unprecedented depth and precision to the search for big bang relic populations. In the case of cosmic strings, this is through searches for strong gravitational lensing by cosmic string loops. Primordial black holes may be observable as their Hawking radiation ionizes and heats the intergalactic medium at high redshifts; this effect could be seen by future low-frequency radio observatories. The second half of the thesis is devoted to two studies of the role of the QCD axion in cosmology. First, I discuss the multitude of axion-like fields potentially produced in string-theoretic axion models, and show how they can lead to an extreme fine-tuning problem in the context of inflationary cosmology when current observational constraints are taken into account. Then, I describe how the QCD axion itself suffers from a tuning problem if it is produced prior to inflation. I conclude that a QCD axion arising from string theory is too problematic to be considered an appealing solution without some modification of generic models of string theory or inflation.