  Eternal inflation requires upward fluctuations of the energy in a Hubble
volume, which appear to violate the energy conditions. In particular, a scalar
field in an inflating spacetime should obey the averaged null energy condition,
which seems to rule out eternal inflation. Here we show how eternal inflation
is possible when energy conditions (even the null energy condition) are obeyed.
The critical point is that energy conditions restrict the evolution of any
single quantum state, while the process of eternal inflation involves
repeatedly selecting a subsector of the previous state, so there is no single
state where the conditions are violated.

General Relativity and Quantum Cosmology