  The dynamics of preheating after inflation has not been clearly understood
yet.In particular, the issue of the generation of metric perturbations during
preheating on super-horizon scale is still unsettled. Large scale perturbations
may leave an imprint on the cosmic microwave background, or may become seeds
for generation of primordial black holes. Hence, in order to make a connection
between the particle physics models and cosmological observations,
understanding the evolution of super-Hubble scale perturbations during
preheating is important. Here, we propose an alternative treatment to handle
this issue based on the so-called separate universe approach, which suggests
less efficient amplification of super-Hubble modes during preheating than was
expected before. We also point out an important issue which may have been
overlooked in previous treatments.
