  We point out that in the context of quantum fields in time dependent
classical backgrounds, the number of created particles with a given momentum
largely deviates about its mean value. Since the corresponding Fourier modes
are nonlocal, this deviation shows that the expectation value of the number
operator can only make sense in an ensemble of spacetimes. Using a complete
orthonormal family of localized wave packets, we show how an ensemble
interpretation can be given to cosmological particle creation in local terms.
The reheating process following inflation is reexamined in the light of this
construction.
