How Gaussian can our Universe be?
  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
$k_\ell^2/k_s^2$, similar to equilateral and orthogonal shapes, and a
dimensionless amplitude of order $0.1\times(n_\mathrm{s}-1)$.
