Chain Inflation Reconsidered
  We investigate density perturbations in the chain inflation scenario, where
the inflaton undergoes successive tunneling transitions along one field
direction. First we show that when the bubble walls associated with such a
phase transition meet, they induce the next phase transition and continue to
propagate as phase interfaces. Then we present an analytical calculation of the
density fluctuations and an estimate of non-Gaussianities such as f_{NL}, which
we find to be small (of order 1). To get the right amplitude for the power
spectrum, there have to be 10^8 phase transitions per Hubble time, a
significant model building challenge. We find that working models of chain
inflation must be rather strongly coupled, and thus have a very limited range
of validity as effective field theories. We discuss generalizations to the
multiple field case, the curvaton scenario, and noncanonical kinetic terms.
