Quintessence Dark Energy and Cosmic String Gravitational Radiation

Abstract

I discuss two main topics, quintessence dark energy and cosmic string gravitational radiation. I first apply observational and theoretical constraints to thawing quintessence dark energy models to determine the viable parameter spaces for λ=|V'/V| and K=√(1-(4V''(ϕ_i))/(3V(ϕ_i))), which was determined to be λ≈(0.1,1) and K∈( √(7/3),5 ), respectively. This region was found to be within observational limits without being finely tuned and allowed by the refined de Sitter conjecture, which is contrary to previous results which used different priors. Therefore, I conclude that viability of quintessence models is highly dependent on chosen priors. I also produced observational constraints on inflection point quintessence potentials of the form V(ϕ)=V_0+V_3 ϕ^3 (similar results are expected for other inflection point models). It was found that, aside from typical asymptotic de Sitter evolution, there is a permitted parameter space that would result in a transient period of acceleration. Regarding gravitational waves, the initial undecayed power spectrum P_n for Garfinkle-Vachaspati (GV) loops was calculated to be on the order of n^(-2) (1+ln⁡(n)) for large values of n. This logarithmic correction differs from the previously-assumed power law of n^(-2) for kink-kink collisions. Decayed GV loops, to a first-order approximation, with small opening angles have even slightly more power at larger modes, which will have a small but nonzero effect on predictions for the stochastic background from cosmic strings. Finally, I also studied the waveforms of the GV loop gravitational radiation, and discovered that, although the period decreases, the waveform remains similar to the undecayed loop up to the applicability of the approximation used for the decay for larger opening angles ( α>π/4 ), but for smaller opening angles, the waveform becomes distorted and therefore unique, which should be noted in periodic searches for GV loops.