Shift-symmetric orbital inflation: Single field or multifield?

Physical review. D/Physical review. D., vol. 102

Abstract

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.

Authors 6

1. Ana Achúcarro Aachen Lorentz Institute for Theoretical Physics

   Leiden University · University of the Basque Country

   Affiliation as printed

   Department of Theoretical Physics, University of the Basque Country, 48080 Bilbao, Spain

   Lorentz Institute for Theoretical Physics, Leiden University, 2333 CA Leiden, Netherlands

2. Edmund J. Copeland

   University of Nottingham

   Affiliation as printed

   School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom

3. Oksana Iarygina Aachen Lorentz Institute for Theoretical Physics

   Leiden University

   Affiliation as printed

   Lorentz Institute for Theoretical Physics, Leiden University, 2333 CA Leiden, Netherlands

4. Gonzalo A. Palma

   University of Chile

   Affiliation as printed

   Grupo de Cosmología y Astrofísica Teórica, Departamento de Física, FCFM, Universidad de Chile, Blanco Encalada 2008, Santiago, Chile

5. Dong-Gang Wang Aachen Leiden Observatory Lorentz Institute for Theoretical Physics

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, 2300 RA Leiden, Netherlands

   Lorentz Institute for Theoretical Physics, Leiden University, 2333 CA Leiden, Netherlands

6. Yvette Welling Aachen Leiden Observatory Lorentz Institute for Theoretical Physics

   Leiden University · Deutsches Elektronen-Synchrotron DESY · Leiden Observatory

   Affiliation as printed

   Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany

   Leiden Observatory, Leiden University, 2300 RA Leiden, Netherlands

   Lorentz Institute for Theoretical Physics, Leiden University, 2333 CA Leiden, Netherlands