Quasi-normal modes of rotating black holes in alternative theories of gravity

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In recent years there has been enormous progress in the gravitation and astrophysics of compact objects. LIGO/VIRGO observations have opened the new window of gravitational wave multimessenger astronomy and provided first glimpses of such cataclysmic events as stellar type black hole and neutron star mergers. Such merger events can and will strongly improve our understanding of black holes and neutron stars on the one hand and gravity on the other. Black holes and neutron stars represent ideal laboratories to test general relativity and alternative theories of gravity because of their extreme compactness. Alternative gravitational theories typically introduce further degrees of freedom, with many featuring a gravitational scalar field. The compact objects in these theories can then differ substantially from their general relativity counterparts. The presence of a gravitational scalar field, for instance, yields a much richer gravitational wave spectrum, allowing also for scalar radiation. By requiring consistency with present and future observations, the presence of such new degrees of freedom can lead to important new constraints on these theories and test their viability. Quasi-normal modes have received much attention in recent years, since they describe gravitational waves emitted during the ringdown phase of merger events and should be quite well discernible by future gravitational wave detectors. Thus quasi-normal modes represent crucial observables. Moreover, quasi-normal modes reveal the linear stability of black holes or neutron stars. The research we propose aims at developing theoretical and numerical tools to obtain quasinormal modes in the case of slow (perturbative) and fast (nonperturbative) rotation, and to apply these tools to black holes in alternative gravity theories. Later these tools can be generalized for neutron stars. This will allow us to extract the relevant signatures of alternative gravity theories in the ringdown of rotating compact objects, providing new constraints on these theories.