Shift-Symmetric Orbital Inflation: single field or multi-field?
  We present a new class of two-field inflationary attractor models, known as
`shift-symmetric orbital inflation', whose behaviour is strongly multi-field
but whose predictions are remarkably close to those of single-field inflation.
In these models, the field space metric and potential are such that the
inflaton trajectory is along an `angular' isometry direction whose `radius' is
constant but arbitrary. As a result, the radial (isocurvature) perturbations
away from the trajectory are exactly massless and they freeze on superhorizon
scales. These models are the first exact realization of the `ultra-light
isocurvature' scenario, previously described in the literature, where a
combined shift symmetry emerges between the curvature and isocurvature
perturbations and results in primordial perturbation spectra that are entirely
consistent with current observations. Due to the turning trajectory, the radial
perturbation sources the tangential (curvature) perturbation and makes it grow
linearly in time. As a result, only one degree of freedom (i.e. the one from
isocurvature modes) is responsible for the primordial observables at the end of
inflation, which yields the same phenomenology as in single-field inflation. In
particular, isocurvature perturbations and local non-Gaussianity are highly
suppressed here, even if the inflationary dynamics is truly multi-field. We
comment on the generalization to models with more than two fields.
