UC San Diego

This dissertation is a collection of several works by the author on neutrino astrophysics and cosmology during the course of his doctoral degree. Topics presented in this dissertation include: collective neutrino flavor and spin transformations in compact object environments (Chapters 2--4), a model of "sterile" (i.e., right-handed) neutrino dark matter involving out-of-equilibrium entropy production in the early universe (Chapter 5), and a cosmological model wherein a phase transition in the vacuum energy can engineer overdense regions, potentially seeding early supermassive black hole formation (Chapter 6). Natural units, i.e., $\hbar = c = k_B = 1$, have been used throughout, unless otherwise specified.