Loverde, MarilenaNascimento, Caio B. de S.2025-10-022025-10-022025-10-022025Nascimento_washington_0250E_28825.pdfhttps://hdl.handle.net/1773/54091Thesis (Ph.D.)--University of Washington, 2025In the past few decades the field of cosmology has entered a new era, where specific models for the dynamics of our universe on the largest scales can be tested against a diverse set of precision astronomical surveys, that map out the distribution of matter and energy in our Universe at different stages in its evolution. This revolution in the field was mostly driven by very precise observations of the Cosmic Microwave Background (CMB) radiation, a relic of the early universe from the time when free electrons and protons first combined to form neutral hydrogen. At this point the universe was very homogeneous and isotropic, with deviations from homogeneity of the order of one part in a hundred thousand. Over the course of the subsequent billions of years, the anisotropies slowly increased due to the instability associated to gravitational attraction, resulting in an intricate pattern for the large-scale distribution of matter known as the cosmic web, where galaxies eventually formed. The next frontier in observational cosmology consists in mapping out the distribution of galaxies in our observable Universe, an effort which is already underway. Such a comprehesive map of the cosmos carries a lot of information about the initial conditions of our Universe, its matter and energy contents, and the nature of gravitational interactions. The prospect of extracting fundamental physics from large-scale structure probes will only be fully realized if this rich experimental program is matched with both accurate theoretical models and efficient computational tools. This dissertation develops novel methods in both directions, which help to maximize the scientific return from ongoing and upcoming surveys, and to ensure that the unprecedented amounts of data to be collected in the near future can be efficiently analyzed. Most of these new developments focus on the goal of using cosmology to extract the neutrino mass scale, which in many ways is the best motivated science target of upcoming astronomical surveys. At least two of the three neutrino species are known to be massive due to terrestrial neutrino oscillation experiments, but the mechanism underlying their mass generation is yet unknown, and is likely to be a key piece of the puzzle to reveal new physics beyond the standard model. We examine the implementation of massive neutrinos in Newtonian simulations of non-linear structure formation, particularly emphasizing the systematic bias introduced by neglecting special relativistic effects. We show that such simulations necessarily overestimatethe total distance traversed by neutrino particles, and one must carefully choose the initial conditions to mitigate this effect. We propose a novel numerical implementation of massive neutrinos in state-of-the-art Boltzmann surveys, the Generalized Boltzmann Hierarchy (GBH), suitable for the linear regime of structure formation. The GBH integrates out the momentum dependence of the neutrino distribution function from the outset, producing a system of ordinary differential equations which are much simpler and faster to solve than the traditional Boltzmann hierarchy. We also introduce a novel fluid approximation for the dynamics of massive neutrino perturbations, which is much more accurate than previously introduced fluid approximation schemes and nicely complements the GBH in solving for the dynamics across all relevant physical scales. We make significant advancements in the theoretical modeling of neutrino wakes, a new signature of neutrino masses in the large-scale structure, which refers to the preferential accumulation of neutrino particles downstream of moving cold dark matter structures, and results in the emergence of a dipole distortion to the neutrino density field. We derive the effect from first principles, allowing for a better characterization of its observational signatures, and to forecast the capability of future surveys in detecting this effect. We finally explore perturbative methods for the nonlinear evolution of cosmological perturbations in the late Universe. We develop a phase-space approach which circumvents the need to assume a perfect fluid from the outset, but show how to recover the results of Standard Perturbation Theory (SPT). We further explain how a key ingredient of the Effective Field Theory approach to Large Scale Structure (EFTofLSS), the sound speed counterterm, naturally emerges within our framework. However, we argue for the necessity of the full EFTofLSS framework to self-consistently model the departures from an ideal fluid. We also provide a novel analytic calculation to estimate the numerical value of the sound speed counterterm, based on separate universe techniques. This calculation is in good agreement with the measurements from full N-body simulations, and provides valuable information about the cosmology dependence of the sound speed and what types of nonlinear structures shape its value.application/pdfen-USCC BYPhysicsAstrophysicsPhysicsThesis