How Gaussian can our Universe be?
Abstract
Gravity is a nonlinear theory, and hence, barring cancellations, the initial superhorizon perturbations produced by inflation must contain some minimum amount of mode coupling, or primordial nonGaussianity. In singlefield slowroll models, where this lower bound is saturated, nonGaussianity is controlled by two observables: the tensortoscalar ratio, which is uncertain by more than fifty orders of magnitude; and the scalar spectral index, or tilt, which is relatively well measured. It is well known that to leading and nexttoleading order in derivatives, the contributions proportional to the tilt disappear from any local observable, and suspicion has been raised that this might happen to all orders, allowing for an arbitrarily low amount of primordial nonGaussianity. Employing Conformal Fermi Coordinates, we show explicitly that this is not the case. Instead, a contribution of order the tilt appears in local observables. In summary, the floor of physical primordial nonGaussianity in our Universe has a squeezedlimit scaling of k_{l}^{2}/k_{s}^{2}, similar to equilateral and orthogonal shapes, and a dimensionless amplitude of order 0.1 × (n_{s}1).
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 January 2017
 DOI:
 10.1088/14757516/2017/01/003
 arXiv:
 arXiv:1612.00033
 Bibcode:
 2017JCAP...01..003C
 Keywords:

 High Energy Physics  Theory;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology
 EPrint:
 26 + 18 pages, 2 figures. References added and minor typos corrected. Matches published version