Titolo della tesi: Morphology of gravitational waves in macroscopic and modified gravity
In this thesis we address the theoretical description of gravitational waves when extended or modified theories of gravity are considered. In particular, we are interested in highlighting peculiar phenomenological features which are absent in the gravitational radiation propagating in vacuum as predicted by General Relativity. Specifically, we refer to both the emergence of extra polarizations and the possibility of dispersion and attenuation. We search for these markers of non-standard propagation in two main contexts, namely by considering gravitational waves from General Relativity travelling in a medium of massive particles, in the theoretical framework of Macroscopic Gravity, and analyzing the morphology of the gravitational radiation predicted by modified theories of gravity, referring in particular to f(R) Gravity and Horndeski theories. In general, we find that anomalous polarizations are typically excited and that dispersion of specific modes can arise. Moreover, we establish the possibility of Landau damping for the scalar massive mode, common to many theories of modified gravity, when the propagation in a medium of collisionless particles is addressed. Our findings constitute a set of reliable tools when the analysis of the gravitational radiation detected by current experiments is performed in the spirit of searching for evidences of departure from General Relativity.