Based on what I have learned, I know that a theoretical physicist has to work in a specific field. But can one work in two? For instance, a person who has a PhD in cosmology but is working on both cosmology and field theories (quantum, unified). Or maybe even the same person working at a university in a cosmology research department. Conducting research on something like the origin of dark energy, but then personally working on something like quantum field theory. Would it be reasonable to wast time getting multiple PhD's? Or should they just get one in cosmology and read books their other fields. (I'm personally a fan of Dover, so I would be reading their books). Can a person that is fully committed (that is also highly intelligent) get a PhD faster than normal, opening the opportunity for perhaps another one? Sorry about all of the questions, but I am interested in multiple fields of theoretical physics.