Creation of unstable particles and decoherence in semiclassical   cosmology
  We consider a simple cosmological model in order to show the importance of unstable particle creation for the validity of the semiclassical approximation. Using the mathematical structure of rigged Hilbert spaces we show that particle creation is the seed of decoherence which enables the quantum to classical transition. 
Mario Castagnino, Susana Landau, and Fernando C. Lombardo