Vanishing of local non-Gaussianity in canonical single field inflation
  We study the production of observable primordial local non-Gaussianity in two
opposite regimes of canonical single field inflation: attractor (standard
single field slow-roll inflation) and non attractor (ultra slow-roll
inflation). In the attractor regime, the standard derivation of the
bispectrum's squeezed limit using co-moving coordinates gives the well known
Maldacena's consistency relation $f_{NL} = 5(1-n_{s})/12$. On the other hand,
in the non-attractor regime, the squeezed limit offers a substantial violation
of this relation given by $f_{NL} = 5/2$. In this work we argue that,
independently of whether inflation is attractor or non-attractor, the size of
the observable primordial local non-Gaussianity is predicted to be
$f_{NL}^{obs} = 0$ (a result that was already understood to hold in the case of
attractor models). To show this, we follow the use of the so-called Conformal
Fermi Coordinates (CFC), recently introduced in the literature. These
coordinates parametrize the local environment of inertial observers in a
perturbed FRW spacetime, allowing one to identify and compute gauge invariant
quantities, such as $n$-point correlation functions. Concretely, we find that
during inflation, after all the modes have exited the horizon, the squeezed
limit of the 3-point correlation function of curvature perturbations vanishes
in the CFC frame, regardless of the inflationary regime. We argue that such a
cancellation should persist after inflation ends.
