Numerical simulations of compact objects in general relativity and modified theories of gravity

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The numerical modelling of black holes and neutron stars in Einstein's theory of general relativity has made enormous progress during the last 10 years and these methods are now applied to a variety of scenarios in astropysics, high-energy physics and the newly emerging field of gravitational-wave physics. This project is dedicated to the computational modelling of neutron stars in the strong-field regime of general relativity as well as generalizations of this theory designed to addressfundamental questions in contemporary physics, most importantly modifications of gravity that may explain the dark-matter and dark-energy problem. Gravitational-wave physics, in particular, will provide the community with unprecedented opportunities to confront such modifications with observational evidence. For this purpose it is vital to develop a theoretical understanding of the gravitational-wave signals expected from astrophysical systems involving compact objects in the framework of general relativity as well its possible generalizations. Particular focus will be on so-called scalar-tensor theories of gravity where, in addition to the metric degrees of freedom, gravity is mediated by one or more additional scalar fields. The specific classes of theories to be considered involve standard scalar-tensor theories as well as more complex theories with so-called screening mechanisms; all these theories are currently considered under the umbrella of Horndesky theories. The physical systems considered include static and dynamic neutron stars, gravitational collapse including supernova core collapse and black-hole spacetimes. In all these cases gravitational-wave forms as well as the systems' dynamics will be studied and compared with the standard-general-relativistic analogs with the goal to identify possible "smoking guns" of modified theories of gravity.