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In linear perturbation theory, a static perturber in the vicinity of a Schwarzschild black hole (BH) enhances [suppresses] the Gauss-Bonnet (GB) curvature invariant, $\mathcal{R}_{\rm GB}$, in the high [low] tide regions. By analysing exact solutions of the vacuum Einstein field equations describing one or two BHs immersed in a multipolar gravitational field, which is locally free of pathologies, including conical singularities, we study the corresponding non-linear tides on a fiducial BH, in full General Relativity (GR). We show that the tidal field due to a far away, or close by, static BH creates high/low tides that can deviate not only quantitatively but also qualitatively from the weak field/Newtonian pattern. Remarkably, the suppression in low tide regions never makes $\mathcal{R}_{\rm GB}$ negative on the BH, even though the horizon Gaussian curvature may become negative; but $\mathcal{R}_{\rm GB}$ can vanish in a measure zero set, a feature qualitatively recovered in a Newtonian analogue model. Thus, purely gravitational, static, tidal interactions in GR, no matter how strong, cannot induce GB$^-$ scalarization. We also show that a close by BH produces noticeable asymmetric tides on another (fiducial) BH., Comment: 13 pages, 13 figures

The most accurate model to describe the gravitational interaction is the well-known theory of General Relativity. Several observational evidences corroborate the legitimacy of the theory compared to the older Newtonian gravity. General Relativity furthermore predicts the existence of gravitational waves, i.e. spacetime ripples produced by accelerated masses. Thanks to a connected network of interferometers called LIGO/Virgo, gravitational waves from the coalescence of massive and compact astrophysical bodies have been measured directly. These recent observations paved the way to a completely new route to test the gravitational interaction. The possibility of using gravitational waves to obtain a deeper understanding of open problems within General Relativity motivates the work developed in this thesis. Each part is essentially devoted to challenging the current model of gravitation, sometimes including yet undiscovered new matter fields, and other times modifying the theoretical framework of General Relativity. In the first part of this manuscript, I discuss the astrophysical consequences of the presence of scalar fields permeating galaxies. A detailed picture of the interaction of massive black holes and scalar dark matter structures is provided. The second part is dedicated to the analysis of the generation and propagation of gravitational waves. As an example, I examine the close limit approximation as a promising tool to investigate the collision of extreme compact objects. The last part of this thesis instead focuses on unstable mechanisms around black holes and stars (scalarization and vectorization), in two alternative models of gravitation., 224 pages, PhD Thesis

The close limit approximation of binary black hole is a powerful method to study gravitational-wave emission from highly non-linear geometries. In this work, we use it as a tool to model black hole spacetimes in theories of gravity with a new fundamental scalar degree of freedom. As an example, we consider Einstein-scalar-Gauss-Bonnet gravity, which admits as solution the Schwarzschild geometry as well as black holes with scalar hair. Accordingly, we find scalar perturbations growing unbounded around binary systems. This "dynamical scalarization" process is easier to trigger (i.e. occurs at lower values of the coupling constant of the theory) than the corresponding process for isolated black holes. Our results and framework highlight the fundamental role of the interaction during the collision of compact objects. They also emphasize the importance of having waveforms for black hole binaries in alternative theories, in order to consistently perform tests beyond General Relativity., 8 pages, 2 figures. Comments are welcome