From spacetime to nucleus: Probing nuclear physics and testing general relativity

Our knowledge about dense matter occurring in the cores of neutron stars remains limited, as those densities are beyond reach on Earth. Fortunately, the dense matter can be probed not only with astrophysical observations but also in terrestrial heavy-ion collision experiments. In this thesis, we developed a Bayesian inference method to combine data from astrophysical observations of neutron stars with gravitational waves, electromagnetic waves from radio to X-ray, and heavy-ion collisions of gold nuclei at relativistic energies, with information from microscopic nuclear theory calculations to improve our understanding of dense matter. This way we arrived at state-of-the-art constraints on the properties of supranuclear matter. Besides the nuclear physics community, the astrophysics community also benefits from an accurate understanding of neutron star matter. This thesis showcases two such applications. First, we have shown how one can distinguish a low-mass black hole from a neutron star when no light is observed. Secondly, we introduced methods to check if a binary neutron star merger signal is gravitationally lensed even if we only see one image. In addition, we developed methods to test the validity of general relativity. First, we devised tools for establishing the presence of polarizations beyond the ones of general relativity, with a limited number of detectors. Secondly, we demonstrated how one could distinguish an exotic compact object from a black hole by looking for the signatures of resonant excitations. Einstein's theory withstood all of our tests.