Non-Gaussianity constrains hybrid inflation
  In hybrid inflationary models, inflation ends by a sudden instability
associated with a steep ridge in the potential. Here we argue that this feature
can generate a large contribution to the curvature perturbation on observable
scales. This contribution is almost scale-invariant but highly non-Gaussian.
The degree of non-Gaussianity can exceed current observational bounds, unless
the inflationary scale is extremely low or the hybrid potential contains very
large coupling constants. Non-linear effects on small scales may quench the
non-Gaussian signal, and while we find no compelling evidence that this occurs,
full lattice simulations are required to definitively address this issue.
  Note added: We now believe that nonlinear effects will invalidate the
original computation in this paper essentially instantaneously after the
short-wavelength modes reach the minimum of their potential. This means that
the mechanism described in this paper will not lead to appreciatable curvature
perturbations on long wavelengths, and no useful constraints on hybrid
inflation will result. We have inserted a brief calculation on p2 of this
manuscript to explain this fact, but have otherwise left the manuscript
unchanged.
