# Shift-Symmetric Orbital Inflation: single field or multi-field?

## TL;DR

This paper introduces shift-symmetric orbital inflation, a class of multi-field models that mimic single-field inflation predictions by having massless isocurvature modes and suppressed non-Gaussianity, despite their multi-field nature.

## Contribution

The paper presents the first exact realization of ultra-light isocurvature inflation, showing how multi-field dynamics can produce single-field-like primordial spectra with suppressed non-Gaussianity.

## Key findings

- Radial (isocurvature) perturbations are massless and freeze superhorizon.
- The model's predictions align with current observational constraints.
- Only one degree of freedom influences primordial observables.

## Abstract

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.