  We analyze four-dimensional Friedmann-Lemaitre-Robertson-Walker cosmologies
in type IIB, arising from a M-theory dual, and find that the null energy
condition (NEC) has to be obeyed by them (except for the negatively curved
case) in order for the M-theory action to have a Wilsonian effective
description. However, this does not imply that the M-theory metric has to obey
the 11d NEC. Thus, we propose a new swampland conjecture -- the 4d NEC is a
consistency condition for any theory to have a completion within M-theory --
with an explicit derivation of it for cosmological backgrounds from a top-down
perspective. We briefly discuss the cosmological consequences of such a
condition derived from M-theory.
