Shift-Symmetric Orbital Inflation: single field or multi-field?. (arXiv:1901.03657v1 [astro-ph.CO])

<a href="http://arxiv.org/find/astro-ph/1/au:+Achucarro_A/0/1/0/all/0/1">Ana Achúcarro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Copeland_E/0/1/0/all/0/1">Edmund J. Copeland</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Iarygina_O/0/1/0/all/0/1">Oksana Iarygina</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Palma_G/0/1/0/all/0/1">Gonzalo A. Palma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_D/0/1/0/all/0/1">Dong-Gang Wang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Welling_Y/0/1/0/all/0/1">Yvette Welling</a>

We present a class of two-field inflationary 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.

We present a class of two-field inflationary 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.

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