  The expansion of spacetime alters the energy-time uncertainty principle,
allowing virtual particles to persist longer than in flat space. For a given
expansion the effect is largest for massless particles which are not
conformally invariant. For a given particle type the effect is largest for
inflation. I exhibit these two principles in the context of massless fermions
which are Yukawa-coupled to massless scalars. I also review the many related
examples which have been studied recently.
