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

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 Ach&#xfa;carro</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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