Runaway Quintessence, Out of the Swampland
  We propose a simple, well-motivated and robust alternative to a metastable de
Sitter vacuum in string theory, consistent with current observations of dark
energy and naturally satisfying conjectured swampland constraints. Inflation
ends in a supersymmetric Minkowski minimum, with a flat direction that is
protected by non-renormalisation theorems. At some scale non-perturbative
effects kick in, inducing a runaway scalar potential. The tail of this runaway
potential cannot sustain a late-time dominating, slow-roll quintessence.
However, the potential always contains a dS maximum. If the modulus starts
close to the hilltop, it remains frozen there by Hubble friction for much of
the cosmological history, at first sourcing a tiny classical vacuum energy and
then constituting a rolling quintessence, with observable consequences. So long
as the modulus is localised away from the Standard Model in the extra
dimensions, there are no observable fifth forces nor is there time-variation of
fundamental constants, and the modulus mass is protected from radiative
corrections. We revisit concrete string models based on heterotic orbifold
compactifications, and show that their de Sitter extrema satisfy the recently
refined dS Swampland Conjecture.
