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 behavior is strongly multifield 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 multifield. We comment on the generalization to models with more than two fields.