| Several issues at the nexus of particle physics, gravitation and cosmology are studied. One set of questions involves observational consequences of alternative theories of gravity. Two related theories are examined, featuring pseudoscalar gravitational fields; one has a unique status as a theory satisfying the Weak Equivalence Principle but not the Einstein Equivalence Principle, while the other is a simple model with propagating torsion. Experimental constraints on each theory are discussed. Another question centers around the creation of closed timelike curves in general relativity. Explicit obstacles to the creation of such time machines are demonstrated in an exactly solvable (2 + 1)-dimensional model. The final chapter studies the appearance of textures and topological defects in theories with spontaneously broken symmetries. By computing the homotopy groups of the vacuum manifolds in the theories under consideration, it is revealed which theories predict textures and defects. |
