  Eternal inflation is shown to require violations of the Null Energy Condition
(NEC) on superhorizon scales. With light scalar fields as the matter sources in
a de Sitter background, there can be no classical or semiclassical violations
of the NEC. However, quantum fluctuations of the energy-momentum tensor,
described by an expectation value of four field operators, do lead to
large-scale NEC violations. The backreaction of such quantum fluctuations on
the inflating spacetime is generally deduced at a heuristic level and leads to
the eternal inflation scenario. A rigorous treatment of the backreaction will
necessarily include fluctuations of the metric, and several new effects are
expected to come into play.
