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Cosmological Particle Production Without Bogolubov Coefficients

We present an efficient new technique for calculating the amount of particle production in a cosmological background. The expectation value of the number operator for a given momentum mode can be extracted from the Feynman propagator. We demonstrate the computational economy of the technique by applying it to various cosmologies. We also show that the appropriate Feynman propagator, with boundary conditions that encode the initial state, can be constructed by the method of images. Our technique uses a first-quantized approach so, unlike conventional Bogolubov transformations, it may be amenable to a string-theoretic generalization.