  Recently a swampland criterion has been proposed that rules out de Sitter
vacua in string theory. Such a criterion should hold at all points in the field
space and especially at points where the system is on-shell. However there has
not been any attempt to examine the swampland criterion against explicit
equations of motion. In this paper we study four-dimensional de Sitter and
quasi-de Sitter solutions using dimensionally reduced M-theory. While on one
hand all classical sources that could allow for solutions with de Sitter
isometries are ruled out, the quantum corrections, on the other hand, are found
to allow for de Sitter solutions provided certain constraints are satisfied. A
careful study however shows that generically such a constrained system does not
allow for an effective field theory description in four-dimensions.
Nevertheless, if some hierarchies between the various quantum pieces could be
found, certain solutions with an effective field theory description might
exist. Such hierarchies appear once some mild time dependence is switched on,
in which case certain quasi-de Sitter solutions may be found without a
violation of the swampland criterion.
