Through the research contained in this proposal we want to address theoretical nuclear physics aspects that relate to two experimental programs of very high scientific profile: 1. High-precision measurements of beta decay particles. 3. Gravitational wave signals from neutron star mergers. The first program aims at searching for signals of physics beyond the Standard Model, and the second can improve our understanding of nuclear matter, neutron stars and the synthesis of elements. The achievement of such ambitious goals relies on progress at the frontier of modern ab initio nuclear theory. We will pursue three computational lines of research with mutual connections (see Resource usage for details): 1. The development of state-of-the-art many-body solvers for quantum systems of strongly-interacting particles. 2. The development of emulators, trading precision for speed, for the many-body solvers. 3. The study of nuclear interactions and currents from chiral effective field theory. This research project is connected to the following research grant: (i) VR project grant: "Precision Nuclear Theory" (C.Forssen, PI)