Building Beyond the Standard Model: Tools from Cosmology to Particle Theory

Abstract

The current landscape of high-energy theory and cosmology suffers from an abundance of theoretical models coupled with a lack of immediate new data to constrain them. This thesis addresses several open questions in early-universe cosmology and beyond-the-Standard-Model (BSM) particle physics, specifically focusing on inflation, dark matter, axions and axion strings, and baryogenesis. Rather than relying solely on forthcoming experimental results, I argue for refining the BSM model space through theoretical consistency checks and cross-examination against multiple existing datasets. For instance, inflationary models often encounter severe naturalness (or $\eta$) problems; applying effective field theory techniques and symmetry considerations helps exclude models that fail these theoretical standards. Similarly, axion models naturally predict distinctive topological defects, such as axion strings, allowing their observational signatures (or lack thereof) to impose valuable constraints. On the observational side, I emphasize that jointly analyzing the Hubble and large-scale structure (LSS) tensions can significantly constrain dark-sector theories. Additionally, I introduce a novel observational probe, analyzing the effect of early universe inhomogeneities generated prior to Big Bang Nucleosynthesis (BBN) on predicted deuterium abundances. This probe rules out baryogenesis scenarios that produce excessive inhomogeneities which are not fully erased by diffusion, and it can potentially constrain regions of parameter space in prominent high energy scale models such as electroweak baryogenesis (EWBG). Together, these theoretical and observational approaches offer robust methods for refining the BSM landscape and demonstrate the critical role particle theorists can play in interpreting cosmological data, even in the absence of new experimental results.