The Early Universe as a Probe of New Physics

The Standard Model of Particle Physics has been verified to unprecedented precision in the last few decades. However there are still phenomena in nature which cannot be explained, and as such new theories will be required. Since terrestrial experiments are limited in both the energy and precision that can be probed, new methods are required to search for signs of physics beyond the Standard Model. In this dissertation, I demonstrate how these theories can be probed by searching for remnants of their effects in the early Universe. In particular I focus on three possible extensions of the Standard Model: the addition of massive neutral particles as dark matter (through the use of several minimal models of dark matter), the addition of charged massive particles (through catalyzed Big Bang Nucleosynthesis), and the existence of higher dimensions (through its effects on BBN and galactic positron production). For each new model, I review the existing experimental bounds and the potential for discovering new physics in the next generation of experiments.