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Title: Cosmology with scalar fields
Author: Ashcroft, P. R.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2004
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In this thesis we investigate a number of roles that scalar fields can play in cosmology. In particular it is generally believed that the early Universe underwent a period of very rapid expansion. We call this epoch inflation. Initially we investigate the evolution of two slow-rolling scalar fields with potentials of the form V = Voφ-a exp(-m). By considering different values of the parameters a, b and m, we drive several new inflationary solutions in which one field just evolves in the background and is not important for the inflationary dynamics. In addition, we find new solutions where both fields are dynamically important during inflation. Moreover, we discuss the evolution of perturbations in both the scalar fields and the spacetime metric, concentrating on the production of entropy perturbations. We find that for a large region in parameter space and initial conditions, tensor modes are negligible, and that adiabatic and isocurvature perturbations are essentially uncorrelated. We move on to consider perturbations in the Randall-Sundrum braneworld scenario. At energies higher than the brane tension, the dynamics of a scalar field rolling down a potential are modified compared with the predictions of General Relativity. These modifications imply, among other things, that steeper potentials can be used to drive an epoch of slow-roll inflation. The evolution of entropy and adiabatic modes during inflation driven by two scalar fields confined to the brane is studied. We show that the amount of entropy perturbation produced during inflation is suppressed compared to the predictions made by General Relativity. As a consequence, the initial conditions do not matter in multiple field inflation in braneworlds if inflation is driven at energies much higher than the brane tension. Following this, we study the evolution of slow-roll inflation in a five-dimensional braneworld model with two boundary branes and a bulk scalar field. Assuming that the inflationary scale is below the brane tension, we can employ the moduli space approximation to study the dynamics of the system. Detuning the brane tension results in a potential for the moduli fields which we show will not support a period of slow-roll inflation. We then study an inflation field confined to the positive tension brane, to which the moduli fields are non-minimally coupled. We discuss in detail the two cases of V (χ) = ½m2 χ 2 and V (χ) = l χ 4 and demonstrate that increasing the coupling results in spectra which are further away from scale-invariance. Furthermore, there is an increase in the tensor mode production, while entropy perturbations are subdominant. Finally, we point out that the five-dimensional spacetime is unstable during inflation because the negative tension brane collapses.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available