Abstract: Cosmology is firmly at an inflection point. On one hand, we have a simple six parameter model that neatly captures cosmic evolution history and fits a host of precision data. On the other hand, this model is descriptive rather than explanatory: several key ingredients in this model, including dark energy, dark matter, and inflation, lack a fundamental physics explanation. These mysteries motivate us to explore new ideas in both the theoretical models and the observational probes of new physics. 

In this thesis, I focus this exploration on two main topics: the role and consequences of cosmological vector fields, and new ideas for constraining fundamental physics with state-of-the-art experiments. These topics are disparate in content and technique but unified in their attempt to leverage novel approaches to better understand longstanding questions in cosmology. These questions, such as ``What is causing the universe to accelerate today?'' and ``What are the neutrino masses?'', underpin the modern cosmological paradigm. They play a key role in our understanding of cosmic history, the formation of structure, and the fate of our universe. Answers to or hints about these questions would have wide-reaching consequences for cosmology, astronomy, particle physics, and gravitational physics. 

I present three projects that aim to shed light on these questions. First, I investigate the consequences of relic cosmological vector fields which may be left over from the early universe. I demonstrate that these relics can significantly transform the shape, amplitude, and net circular polarization of a stochastic gravitational wave background (SGWB) and I point out the consequences for detecting such a background both directly and via cosmic microwave background (CMB) polarization. Second, I introduce a novel model of late-time cosmic acceleration which relies on a coupling between a scalar and gauge fields. This model achieves slow roll dynamically and can drive dark energy without fine tunings. I show the generic imprints this scenario leaves on a SGWB, with implications for the experimental search for GWs and for distinguishing this model from standard scenarios. Lastly, I demonstrate the power of combining measurements of the CMB and large-scale structure to measure the sum of the neutrino masses. I show how utilizing the reconstruction of velocities from tomographic measurements improves our ability to constrain this important physical quantity, and I discuss prospects for future advancement. 

Graduate Advisor: Professor Robert Caldwell

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