Issues concerning the waterfall of hybrid inflation
  We discuss the waterfall that ends hybrid inflation. Making some simplifying
assumptions, that may be satisfied by GUT inflation models, two issues are
addressed. First, the procedure of keeping the quantum fluctuation of the
waterfall field only in the regime where it can be regarded as classical.
Second, the contribution to the primordial curvature perturbation that is
generated during the waterfall. Because the waterfall field is heavy during
inflation, the spectrum of this contribution is strongly blue and hence is
negligible on cosmological scales.
