The dynamics of cosmological scenarios inspired by quantum gravity

Abstract

In this thesis we study the dynamics of cosmological scenarios inspired by quantum gravity. Part I investigates novel features of the semi-classical regime of homogeneous and isotropic loop quantum cosmology. Dynamics in this regime becomes modified by nonperturbative quantum effects, subject to a number of ambiguities. For a flat universe the quantum effects accelerate a scalar field along its self-interaction potential during a period of super-inflation. We study how this behaviour can in principle set the initial conditions for subsequent slow-roll inflation. We also calculate a first approximation for the spectrum of perturbations produced during the super-inflationary phase. For the positively-curved case we investigate how a bounce from a contracting to an expanding phase can occur, and show that this can lead to oscillations of the universe. During the oscillations the inflaton field can roll monotonically up its potential. Once the potential energy becomes sufficiently large, however, the cycles end and inflation commences. For a constant potential the oscillations occur about a centre fixed point allowing the construction of `new emergent universe' scenarios where the universe is past-eternally an Einstein static universe, but subsequently evolves into inflation. Part II considers positively-curved braneworld models in which the dynamical equations become modified in such a way as to permit a bounce. It is conjectured that models of this type can exhibit similar behaviour to the positively-curved LQC scenario. General conditions for this behaviour are determined in braneworld settings and we investigate an explicit example - the baneworld of Shtanov and Sanhi - in detail