Isocurvature Perturbations of Dark Energy and Dark Matter from the
  Swampland Conjecture
  We point out that the recently proposed Swampland conjecture on the potential
gradient can lead to isocurvature perturbations of dark energy, if the
quintessence field acquires large quantum fluctuations during high-scale
inflation preferred by the conjecture. Also, if the quintessence field is
coupled to a dark sector that contains dark matter, isocurvature perturbation
of dark matter is similarly induced. Both isocurvature perturbations can be
suppressed if the quintessence potential allows a tracker solution in the early
Universe. We find that a vector field of mass $\lesssim {\cal O}(1)$ meV is an
excellent dark matter candidate in this context, not only because the right
abundance is known to be produced by quantum fluctuations during high-scale
inflation without running afoul of isocurvature bounds, but also because its
coupling to the quintessence does not spoil the flatness of the potential.
