Quantum effects in the early universe
The subject of this dissertation lies in the region where modern particle physics and cosmology intersect. Broadly speaking, its context is provided by the shortcomings of the 'standard cosmology', and the more recent inflationary scenario. The particular aspects which are tackled - the origin of density fluctuations and the boundary conditions of the universe - are outlined below. A. The Cosmic String Scenario: Phase transitions in the early universe may have produced topological 'knots' or defects, such as monopoles cosmic strings and domain walls. Cosmic strings, in particular, have attracted much interest recently because of the alluring model of galaxy formation they have the potential to produce. The viability of this new scenario, however, rests on untested assumptions about string interaction properties - whether or not they intercommute. A detailed study of global U(1)-strings has demonstrated that under most circumstances they will intercommute, a result expected for other varieties of strings (Part A). Given this firmer foundation for the cosmic string scenario, some of their astrophysical implications are explored. In particular, the large scale peculiar velocities predicted in this model are examined. B. The domain wall problem of the Axion: The axion has attracted considerable interest as a cold dark matter candidate. In part B, we concentrate on what potentially is a cosmological flaw of the axion: The non-trivial vacuum topology of axion models gives rise to cosmic vortex strings and domain walls. The latter are catastrophic unless removed by some mechanism soon after formation. In the dissertation we demonstrate the efficacy of their removal through string/domain wall intercommuting and annihilation. Causality constraints purporting to restrict the rate of this mechanism are found to be circumvented. C. Quantum Cosmology and Recollapse: Regardless of whether all the physical laws of system are known, its boundary conditions must still be given. In a quantum cosmological context, Hartle and Hawking have proposed that these be specified by a path-integral over compact four geometries. In Part C detailed studies of the implications of this proposal are made for two restricted models, the Freidmann-Robertson-Walker universe with a massive scalar field and the anisotropic Kantowski-Sachs cosmology. We evaluate the respective wavefunctions and the trajectories to which they correspond in the classical limit. Attention is focussed on the fact that most classical trajectories will recollapse to a singularity - the difficulty that this presents for the H-H proposal is discussed. The Kantowski-Sachs universes generically evolve from isotropy during expansion to increasing anisotropy during recollapse, ending as a black hole interior. Some comment is made on the arrow of time thereby induced by the Hartle-Hawking proposal.