Probing the Universe by using Gravitational Waves

This thesis presents a comprehensive analysis of Gravitational Wave (GW) events in a cosmological context, based on the MillenniumTNG (MTNG) simulations and GW event catalogues obtained by post-processing these hydrodynamical runs with the binary population-synthesis code SEVN. SEVN self-consistently models binary stellar systems, i.e. pairs of stars, that evolve into compact objects (black holes or neutron stars) and eventually merge, thereby producing GW events. This work aims to characterise the intrinsic properties of binary merging systems — formation and merger times, metallicities, and remnant masses — and map their sky distribution to test whether GW sources trace the underlying matter distribution. By combining MTNG outputs with GW catalogues, this study links the small-scale astrophysics of binary evolution to the hierarchical structure of the Universe. The cross-match between datasets shows that binary black hole progenitors predominantly form at early cosmic epochs within low-metallicity environments, whereas binary neutron star progenitors arise from younger, more metal-enriched populations. Black hole–neutron star mergers remain subdominant at all cosmic times. The generated full-sky lightcone maps of GW mergers, stellar mass and total matter reveal that GW events cluster in regions of enhanced stellar density, acting as biased tracers of stellar structures. The corresponding angular power spectra, correlations and bias functions quantify these connections: GW and stellar mass maps exhibit similar scale-dependent shapes but differ in amplitude, while correlations with total matter are positive yet moderate, indicating that GW sources trace the large-scale structure indirectly through their stellar hosts. The results establish a predictive framework linking compact-binary formation environments to their cosmological distribution, providing theoretical guidance for future GW surveys and paving the way for GW sources to serve as cosmological probes.