UC Irvine

The Λ-Cold Dark Matter (ΛCDM) model successfully describes the evolution and largescale structure of the Universe, supported by high-precision cosmological observations. Yet, it leaves key questions unresolved—most notably the nature of dark matter, dark energy, and neutrinos—and faces persistent “cosmological tensions,” such as discrepancies in the local Hubble expansion rate and the amplitude of matter clustering. These challenges suggest that extensions to ΛCDM or new physics may be required to fully capture cosmic dynamics.This dissertation investigates these issues through studies of extended cosmological models and their implications for neutrino physics and the early Universe. We examine a broad class of ΛCDM extensions—including phantom and early dark energy, spatial curvature, primordial magnetic fields, and additional or self-interacting neutrino species—in the context of the Hubble and structure growth tensions. We then explore sterile neutrinos as probes of the pre–Big-Bang-nucleosynthesis epoch, focusing on scenarios involving decays or new interactions and their relevance to current and upcoming laboratory searches. We then perform comprehensive analyses using cosmic microwave background, baryon acoustic oscillation, and supernova datasets to study the effects of neutrino mass hierarchies and extra relativistic species. We further test extra-radiation cosmologies with the latest DESI observations and develop a sound-horizon-independent framework for inferring the Hubble constant and neutrino mass from large-scale structure and lensing data.Overall, this work draws connections between new physics in the neutrino sector and cosmological tensions, constraining Standard Model and beyond-Standard-Model neutrino properties, and advancing our understanding of the open challenges faced by ΛCDM in light of forthcoming precision data.