Nuclear, Particle, and Weak Interaction Physics of the Big Bang and Stellar Collapse

Research in theoretical physics and astrophysics uses advances in theoretical physics to understand how nature works. Specifically, this project uses quantum mechanics applied in the context of exotic environments provided by collapsing stars and the very early universe, to glean new insights into the fundamental properties and behaviors of elementary particles and of atomic nuclei. The calculations performed here go the other way as well, using what is known about elementary particles and atomic nuclei to understand the cosmos. Experiments and observations have revealed that roughly 95% of the mass and energy in the universe is not explained by ordinary atoms. That staggering fact demands investigation on many fronts. To that end, the research made possible by this project focuses on the properties and behavior of one particularly mysterious elementary particle, the neutrino. The objective is twofold: (1) Better understanding of how these particles interact with atomic nuclei and affect the evolution of the early universe, the collapse of massive stars, and associated production of elements and black holes; and (2) Use those particles as a portal to new and unknown physics. The calculations in this research are pioneering new approaches to treating complex quantum problems. They serve to train the next generation of physicists and astrophysicists. This theoretical research revolves around calculations of neutrino interactions, the weak interaction, and the structure of atomic nuclei in the extreme environments of collapsing massive stars and the early universe. These exotic environments, the targets of multi-messenger (electromagnetic, particle, gravitational waves) astrophysics observations and the Stage-4 Cosmic Microwave Background (CMB) efforts, respectively, can be energetically-dominated by neutrinos. As such, they could provide insights into the properties and interactions of these mysterious particles. This information could be complementary to that gleaned from, e.g., long-baseline oscillation and neutrino-less double beta decay experiments. The evolution of neutrino flavor (electron, muon, tau) and spin (neutrino or antineutrino), is fundamental to the dynamics of, and nucleosynthesis emerging from, these astrophysical sites, as well as for assessing the entropy history of the early universe, vetting beyond standard model physics possibilities with the CMB and nucleosynthesis, and the prospects for black hole formation. Novel calculations are underway to model the complicated, nonlinear quantum behavior of neutrinos in these neutrino-dense cosmic environments. These calculations comprise an excellent training opportunity for the next generation of physicists and astrophysicists. This project advances the objectives of "Windows on the Universe: the Era of Multi-Messenger Astrophysics", one of the 10 Big Ideas for Future NSF Investments. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.