High Momentum Behavior of Geometric Bremsstrahlung in the Expanding   Universe
  We shall discuss various kinds of geometric bremsstrahlung processes in the spatially flat Robertson-Walker universe. Despite that the temperature of the universe is much higher than particle masses and the Hubble parameter, the transition probability of these processes do not vanish. It is also pointed out that explicit forms of the probability possess a new duality with respect to scale factor of background geometry. 
M. Hotta, H. Inoue, I. Joichi and M. Tanaka