Quantum Mechanical Breakdown of Perfect Homogeneity in Reheating After   Inflation
  In the context of quantum fields in time dependent classical backgrounds, we notice that the number of created particles with a given momentum largely deviates about its mean value. Guided with this observation we use a complete orthonormal family of localized wave packets to calculate the deviations in the number and energy densities of particles produced in a volume of a given size during reheating. It turns out that at the end of reheating there exists (in general tiny) spatial variations in these densities on Hubble length scales over which local interactions are incapable of restoring homogeneity. This signals the destruction of perfect homogeneity attained after inflation due to the quantum nature of particle production process in reheating. 
Ali Kaya