CALIFANO, MATTEO (2024) Gravitational WavesPhenomenology: from late to early Universe, from present to future observations. [Tesi di dottorato]

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Abstract

This Thesis explores the importance of the detection of the gravitational waves for cosmology, focusing on how current and next-generation detectors can be used to constrain cosmological parameters, test the fundamental structure of the universe, and probe modifications to General Relativity. The first part of the Thesis investigates the use of gravitational waves as standard sirens for estimating the Hubble constant ($H_0$). In particular, we examine two different types of gravitational wave events: bright sirens, which are accompanied by an electromagnetic counterpart, and dark sirens, which lack such a counterpart. Through simulations based on future detectors such as the Einstein Telescope and Cosmic Explorer, we demonstrate that bright sirens can achieve sub-percent level accuracy on $H_0$, potentially resolving the longstanding tension between early and late-time measurements. Dark sirens, though more challenging to analyze, are also shown to provide meaningful constraints on cosmological parameters using two statistical methods. The first one is based on the hierarchical Bayesian method in order to jointly fit both astrophysical and cosmological parameters from the analysis of a set of GW events. The second one sample the cosmological parameters directly in the gravitational wave waveform. The second part of the Thesis focuses on testing fundamental physics through gravitational waves. We explore parity-violating gravity theories, which predict different behaviors for the left- and right-handed polarization modes of gravitational waves. Using the parametric post-Einsteinian formalism, we forecast how next-generation detectors can place constraints on these theories, providing new opportunities to test modifications to General Relativity. Finally, the last part is devoted to the stochastic gravitational wave background as a tool for probing the early universe. Recent detections by Pulsar Timing Arrays, such as NANOGrav, are analyzed to explore the implications for inflationary models and the large-scale geometry of the universe. The results show that the gravitational wave background offers a unique avenue to study the universe’s expansion history and early dynamics. Overall, this Thesis demonstrates the power of gravitational waves as a cosmological and fundamental physics tool, with applications ranging from measuring $H_0$ to testing alternative theories of gravity and exploring the primordial universe. The research presented here highlights the potential of future gravitational wave detectors to revolutionize our understanding of the cosmos and provide critical tests of theoretical models.

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