Natural Quintessence in String Theory
  We introduce a natural model of quintessence in string theory where the light
rolling scalar is radiatively stable and couples to Standard Model matter with
weaker-than- Planckian strength. The model is embedded in an anisotropic type
IIB compactification with two exponentially large extra dimensions and
TeV-scale gravity. The bulk turns out to be nearly supersymmetric since the
scale of the gravitino mass is of the order of the observed value of the
cosmological constant. The quintessence field is a modulus parameterising the
size of an internal four-cycle which naturally develops a potential of the
order (gravitino mass)^4, leading to a small dark energy scale without tunings.
The mass of the quintessence field is also radiatively stable since it is
protected by supersymmetry in the bulk. Moreover, this light scalar couples to
ordinary matter via its mixing with the volume mode. Due to the fact that the
quintessence field is a flat direction at leading order, this mixing is very
small, resulting in a suppressed coupling to Standard Model particles which
avoids stringent fifth-force constraints. On the other hand, if dark matter is
realised in terms of Kaluza-Klein states, unsuppressed couplings between dark
energy and dark matter can emerge, leading to a scenario of coupled
quintessence within string theory. We study the dynamics of quintessence in our
set-up, showing that its main features make it compatible with observations.
