2016/11/30 by Giovanni Cabass, G. Cabass, Enrico Pajer +3 · 1 voice · 1 citation
Physics and Astronomy · #Cosmology and Gravitation Theories #Galaxies: Formation, Evolution, Phenomena #Particle physics theoretical and experimental studies #astro-ph.CO #gr-qc #hep-th
paper · pdf · doi:10.1088/1475-7516/2017/01/003
26 + 18 pages, 2 figures. References added and minor typos corrected. Matches published version
openalex created_date 2016/12/08 · arxiv created 2016/12/29 · openalex publication_date 2017/01/02 · arxiv updated 2017/01/11 · openalex updated_date 2026/08/01
Gravity is a non-linear theory, and hence, barring cancellations, the initial super-horizon perturbations produced by inflation must contain some minimum amount of mode coupling, or primordial non-Gaussianity. In single-field slow-roll models, where this lower bound is saturated, non-Gaussianity is controlled by two observables: the tensor-to-scalar 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 next-to-leading 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 non-Gaussianity. 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 non-Gaussianity in our Universe has a squeezed-limit scaling of k_ℓ2/ks2, similar to equilateral and orthogonal shapes, and a dimensionless amplitude of order 0.1×(ns-1).