Macroscopic description of preheating

We present a macroscopic model of the decay of a coherent classical scalar field into statistical fluctuations through the process of parametric amplification. We solve the field theory (henceforth,"microscopic") model to leading order in a Large N expansion, and show that the macroscopic model gives satisfactory results for the evolution of the field, its conjuguate momentum and the energy momentum tensor of the fluctuations over many oscillations. The macroscopic model is substantially simpler then the microscopic one, and can be easily generalized to include quantum fluctuations. Although we assume here and homogeneous situation, the model is fully covariant, and can be applied in inhomogeneous cases as well. These features make this a promising model in exploring the physics of preheating.