Fuzzy Dark Matter Candidates from String Theory
  String theory has been claimed to give rise to natural fuzzy dark matter
candidates in the form of ultralight axions. In this paper we revisit this
claim by a detailed study of how moduli stabilisation affects the masses and
decay constants of different axion fields which arise in type IIB flux
compactifications. We find that obtaining a considerable contribution to the
observed dark matter abundance without tuning the axion initial misalignment
angle is not a generic feature of 4D string models since it requires a mild
violation of the $S f\lesssim M_P$ bound, where $S$ is the instanton action and
$f$ the axion decay constant. Our analysis singles out $C_4$-axions,
$C_2$-axions and thraxions as the best candidates to realise fuzzy dark matter
in string theory. For all these ultralight axions we provide predictions which
can be confronted with present and forthcoming observations.
