  It has recently been suggested that "gravity is the weakest force" in any
theory with a suitable UV completion within quantum gravity. One formulation of
this statement is the scalar weak gravity conjecture, which states that gravity
is weaker than the force originating from scalar fields. We study the scalar
weak gravity conjecture in de Sitter space, and discuss its low-energy
consequences in light of the experimental searches for fifth forces and
violations of the equivalence principle. We point out that some versions of the
scalar weak gravity conjecture forbid the existence of very light scalar
particles, such as the quintessence and axion-like particles. The absence of
the quintessence field means that these versions of the scalar weak gravity
conjecture are in phenomenological tension with the recently-proposed de Sitter
swampland conjecture and its refinements. Some other versions of the scalar
weak gravity conjecture escape these constraints, and could have interesting
phenomenological consequences.
