Cosmic Perturbations from a Rotating Field
  Complex scalar fields charged under approximate $U(1)$ symmetries appear in
well-motivated extensions of the Standard Model. One example is the field that
contains the QCD axion field associated with the Peccei-Quinn symmetry; others
include flat directions in supersymmetric theories with baryon, lepton, or
flavor charges. These fields may take on large values and rotate in field space
in the early universe. The relevant approximate $U(1)$ symmetry ensures that
the angular direction of the complex field is light during inflation and that
the rotation is thermodynamically stable and is long-lived. These properties
allow rotating complex scalar fields to naturally serve as curvatons and
explain the observed perturbations of the universe. The scenario imprints
non-Gaussianity in the curvature perturbations, likely at a level detectable in
future large scale structure observations. The rotation can also explain the
baryon asymmetry of the universe without producing excessive isocurvature
perturbations.
