On the de Sitter swampland criterion
  A new swampland criterion has recently been proposed. As a consequence, it
forbids the existence of de Sitter solutions in a low energy effective theory
of a quantum gravity. However, there exist classical de Sitter solutions of
ten-dimensional (10d) type II supergravities, even though they are unstable.
This appears at first sight in contradiction with the criterion. Beyond
possible doubts on the validity of these solutions, we propose two answers to
this apparent puzzle. A first possibility is that the known 10d solutions
always exhibit an energy scale of order or higher than a Kaluza-Klein scale, as
we argue. A corresponding 4d low energy effective theory would then differ from
the usual consistent truncations, and as we explain, would not admit a de
Sitter solution. This would reconcile the existence of these 10d de Sitter
solutions with the 4d criterion. A second, alternative possibility is to have a
refined swampland criterion, that we propose. It forbids to have both the
existence and the stability of a de Sitter solution, while unstable solutions
are still allowed.
