UC Irvine

This dissertation investigates three of the most elusive phenomena in modern astrophysics: black holes, dark matter, and neutrinos, studied through theoretical modeling and large-scale numerical simulation to confront a new generation of gravitational-wave, electromagnetic, and neutrino observations.The first part addresses the astrophysical population of merging binary black holes (BBHs) detected by LIGO-Virgo-KAGRA. Using the population-synthesis code SEVN combined with a galaxy-evolution framework spanning cosmic star formation, metallicity, and galaxy stellar mass, I construct a model for the volumetric BBH merger rate density and its dependence on primary mass, secondary mass, and mass ratio, finding that the observed primary mass distribution requires either a top-heavy initial mass function in low-metallicity dwarf galaxies or a substantial dynamical-capture contribution above ∼ 30 M⊙. I then examine mass ratio reversal (MRR), binaries in which the initially less massive star forms the more massive compact object, across the SEVN and COMPAS codes. Both codes show that MRR leaves a distinct imprint on the merger-rate landscape, though the size and character of that imprint depends on the treatment of mass transfer and supernova physics. This makes clear that the primary-mass distribution inferred by LVK is not a direct tracer of the initially more massive stars, but a superposition of physically distinct evolutionary populations, an effect that must be accounted for to robustly connect gravitational-wave observations to massive binary evolution.The second part turns to the nature of dark matter. Motivated by JWST’s discovery of unexpectedly massive, rapidly assembled galaxies in the early Universe, I use cosmological N-body simulations to test whether self-interacting dark matter (SIDM) can enhance early star formation relative to cold dark matter (CDM). I find that SIDM leaves the halo mass function unchanged but produces systematically rounder halos and suppressed sub halo survival by z ≈ 6, with the total accretion rate onto hosts remaining comparable to CDM — indicating that self-interactions primarily enhance post-infill sub halo disruption rather than large-scale structure growth. Using TNG50-1, I estimate that this disruption liberates gas that could plausibly boost star-formation efficiency. I further show that gravothermal collapse, though analytically expected for a sizable portion of the simulated population, is not realized in these simulations, an absence attributable to the merger-dominated environment of early halo assembly.The third part develops a model-independent framework for extracting the total emitted energy and net deleptonization of the next Galactic core-collapse supernova from its neutrino signal, using B-spline spectral unfolding applied jointly across Super-Kamiokande, DUNE, and JUNO. I show that this approach recovers the total emitted energy with percent-level precision without assuming a parametric spectral shape, that a nonzero net deleptonization can be robustly established despite its larger reconstruction uncertainty, that DUNE’s dedicated νe channel is indispensable to the reconstruction, and that the resulting energy measurement is precise enough to serve as a direct, model-independent probe of the photoneutron star’s mass.Together, these studies demonstrate how upcoming and current observational facilities, spanning gravitational waves, deep imaging, and neutrino detection, can be used to test and constrain fundamental physics across some of the most extreme environments in the Universe