A First-Quantized Formalism for Cosmological Particle Production

We show that the amount of particle production in an arbitrary cosmologicalbackground can be determined using only the late-time positive-frequency modes.We don't refer to modes at early times, so there is no need for a Bogolubovtransformation. We also show that particle production can be extracted from theFeynman propagator in an auxiliary spacetime. This provides a first-quantizedformalism for computing particle production which, unlike conventionalBogolubov transformations, may be amenable to a string-theoreticgeneralization.Comment: 18 pages, LaTeX; v2: significantly revised for clarity; conclusions unchange