Early Seeds and Cosmic Skies: Quantum Origins of the Universe and Insights from Large-Scale Structure

Abstract

Dark matter, dark energy and inflation form the backbone of the current cosmological model. Collectively, they paint a picture of a universe's structure sourced by quantum fluctuations and then billions of years of gravitational clustering in an expanding universe. These concepts are illustrated in Part I, which serves as an introduction. This thesis then breaks new ground on all three topics: we simplify inflationary calculations, showcase the power of current and future large-scale structure analyses, and advance our understanding of galaxy formation and the nature of dark energy. In Part II, we establish a technique to better understand cosmological correlators, the primary observable in the inflationary universe. Traditionally, cosmological correlators are computed with the standard but complicated Schwinger-Keldysh in-in formalism. Here, we show that for non-dissipative systems, we can calculate correlators using the more widely used in-out formalism. In de Sitter space, we achieve this by extending the expanding Poincaré patch with a contracting one. This leads to simplified calculations with fewer Feynman diagrams and only one propagator. Part III jumps forward a couple of billion years, and we study the clustering of dark matter and galaxies in the mildly nonlinear regime using its Effective Field Theory. Specifically, we develop the one-loop bispectrum of galaxies in redshift space, addressing key subtleties about its renormalisation. We then analyse BOSS data and demonstrate that the inclusion of the bispectrum significantly reduces error bars in key cosmological parameters compared to using only the power spectrum, showcasing the importance of including the one-loop bispectrum in future data analyses. Finally, we forecast the predictive power of two next-generation galaxy surveys, DESI and MegaMapper, again using the loop power spectrum and bispectrum. Our analysis focuses on new physics, such as neutrino masses, and, more ambitiously, on primordial Non-Gaussianity - a key probe of the nature of inflation. In Part IV, we discuss two topics in the late universe. We develop a method to obtain direct signals of the formation time of galaxies and show that the large-scale distribution of galaxies is sensitive to their formation time. This is due to additional parameters appearing in the bias expansion when assuming galaxies form over a prolonged period of time. Lastly, we turn to dark energy, where we model the influence of clustering quintessence on the distribution of galaxies. Again, we analyse BOSS data, this time with particular emphasis on the dark energy equation of state parameter.