The observational consequences of primordial fields

Thesis

The research presented in this thesis concerns three main topics. First, we investigate the presence of large scale magnetic fields in the universe. We focus on the possibility that they have originated in the primordial phases of the evolution of the universe, and derive a strong constraint on the intensity of causally generated fields. We also analyse the imprint of a helical magnetic field in the cosmic microwave background radiation. Second, we study the observational consequences of a hypothetical electric charge asymmetry in the universe, and we deduce upper limits on the charge of some fundamental particles, improving the current limits obtained from laboratory experiments or astrophysical observations. Third, we consider the theory of inflation, and in the context of the slow roll approximation we derive a constraint on the fourth derivative of the inflaton potential. The common methodology at the basis of these three analyses is the use of the observational properties of the cosmic microwave background radiation to constrain the presence of different components in the universe, which can act as a source of perturbation in the background metric. Part of the thesis is dedicated to the review of the theoretical framework at the basis of the topics considered. This is mainly done in the first three chapters. We start with a summary of the standard cosmological model, introducing the covariant formalism which is used in the analysis of the model of a charged universe. The details and results of this analysis are presented in the fifth chapter. The second chapter contains a review of the basic properties of cosmic magnetic fields, and then focuses on the derivation of strong upper limits on their intensity. The third chapter is an overview on cosmological linear perturbation theory, and on the formalism of the microwave background temperature anisotropies and polarisation. It introduces the technical tools used in the fourth chapter, where we analyse the temperature and polarisation signal induced in the microwave background by a primordial, helical magnetic field. We conclude by presenting, in the sixth chapter, the derivation of an upper bound on the fourth derivative of the inflaton potential, obtained using the inflationary flow equations.

Authors: Caprini, C

• DOI: 10.5287/ora-jbvnverkg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford