Natural vacua in hyperbolic Friedmann-Robertson-Walker spacetimes

Recent evidence indicates that the Universe is open, i.e., spatially hyperbolic, long-standing theoretical preferences to the contrary notwithstanding. This makes it possible to select a vacuum state, Fock space, and particle definition for a quantized field by requiring concordance with ordinary flat-spacetime theory at late times. The particle-number basis states thus identified span the physical state space of the field at all times. This construction is demonstrated here explicitly for a massive, minimally coupled, linear scalar field in an open, radiation-dominated Friedmann-Robertson-Walker spacetime.