PhD defence: Compact Objects and Gravitational Waves: Exploring Fundamental Physics with Tidal Effects

In this thesis, we study the role of tidal effects–characterising the deformation of a compact object to an external gravitational field–in binary systems of compact objects such as black holes and neutron stars. In particular, we develop theoretical frameworks to connect the microphysics encoded in the interior of the compact object, with the dynamical spacetime of the binary, relevant for current and future gravitational-wave observations.

The thesis starts with an overview of gravitational wave theory and tidal effects before discussing our three lines of novel work. First, we generalise the relativistic treatment of tidal effects to gravity theories beyond general relativity, which gives rise to an extra particle or field, called scalar field. We uncover a new set of tidal deformabilities and compute numerical values for neutron stars in scalar-tensor theories. We assess degeneracies between internal structure and gravity theories and extract the key parameters relevant for gravitational wave observations.

Second, we calculate in detail the effects on the gravitational wave and study the role and implications that these new parameters have for gravitational wave signals of coalescing binary systems. Furthermore, we develop a novel formalism to include the internal vibrations of the compact object (quasi-normal mode structure) in a way that overcomes previous difficulties related to unambiguous identifications of information from strong-gravity regimes. As a case study, we apply our formalism to scalar perturbations of a Schwarzschild black hole and recover results from previous literature.