Cosmic Frontiers: From the Origins to Gravitational Wave Astronomy

Abstract

In this thesis, we present different aspects of cosmology, focusing on the early universe, large scale structure formation, and gravitational wave astrophysics. We begin by introducing cosmological perturbation theory as a framework to describe the origin of the universe, emphasizing its observational implications. We discuss early universe models and, motivated by the avoidance of singularities, we propose bouncing cosmologies. In particular, we discuss the Cuscuton model; which modifies gravity to generate a bounce without introducing new degrees of freedom, provides a smooth transition between contraction and expansion, and generates scale-invariant entropy perturbations. We analyze scalar perturbations and demonstrate that the model remains stable and weakly coupled. Additionally, we compute three-point functions, showing negligible non Gaussianities on observable scales, with potential signals arising from the conversion of isocurvature to curvature perturbations.

Building on this foundation, we investigate the formation of large scale structures and study bias parameters that describe the statistical distribution of dark matter halos and galaxies. We extend this analysis to gravitational wave (GW) sources, introducing the GW bias parameter. We propose a methodology to link binary black hole (BBH) mergers to their host galaxies using phenomenological GW host-galaxy probability functions. These functions are constructed based on observed astrophysical properties of galaxies, including stellar mass, star formation rate, and metallicity. We calculate the GW bias using the angular power spectrum for photometric surveys and the 3D power spectrum for spectroscopic surveys. We find that the GW bias depends on the parameters of the host-galaxy probability functions, which are linked to the formation channels of BBHs. Future measurements of the GW bias could provide insights into the astrophysical processes governing GW sources and their relation to large-scale structure.

Lastly, we emphasize that cosmology may be entering a golden era, driven by a wealth of upcoming experiments, including the next generation of Cosmic Microwave Background (CMB) telescopes, galaxy surveys, and GW detectors. These experiments have the potential to provide hints for modifications to ΛCDM cosmology, the nature of dark matter and dark energy, black hole physics, and possible deviations from general relativity, paving the way for new theoretical and observational breakthroughs.