Signals of Early-Universe Physics in Cosmology

Abstract

This is a thesis on theoretical cosmology. The first and largest part is a study of cosmic strings, in particular their dynamics and signals in higher dimensional spacetimes. The second part is a study of black holes in a quintessence background.

Cosmic strings are predicted by models of the early universe. They were thought to arise, originally, from Grand Unified Theories, and more recently from brane inflationary models based in string theory.

In Chapter 3 we find exact solutions for cosmic string loop trajectories in higher dimensions, and find the regions of parameter space for which cusps exist. We find that winding the internal dimensions slows the average velocity of string loops, and conjecture that the periodicity of internal space may contribute to self-intersections.

In Chapter 4, we calculate the gravitational wave signal from cosmic string cusps in higher dimensions, and find it is much reduced relative to the 4D case. The main reason for this is the large reduction in the probability of cusps occurring on loops in higher dimensions, as well as a slight reduction in signal from individual cusps.

In Chapter 5, we study cosmic string trajectories in warped spacetimes, such as may be found in realistic brane inflation models. We find that contrary to claims in the literature, the warping of the internal space does not prevent the internal motion of strings. The energy associated with the warping of spacetime means that the energy of a loop appears to change over time from our 4D perspective.

Finally, in Chapter 6, we find an analytic, general-relativistic solution describing a black hole in a quintessence universe. Quintessence is a model of late-time cosmic acceleration in which expansion is sourced by a scalar field. Our solution shows the interaction between this scalar field and a black hole. The scalar field is shown to continue its cosmological "rolling" behaviour everywhere, including on the black hole event horizon, and the black hole is shown slowly to accrete scalar field. This is a perturbative solution valid throughout all of space but only over a finite period of time.