Enhancement of Non-Gaussianity after Inflation

TL;DR

This work provides a relativistic, second-order treatment of cosmological perturbations in a flat FRW universe with a perfect fluid to track non-Gaussianity on super-horizon scales after inflation. By leveraging super-horizon conservation of the gauge-invariant curvature perturbation and solving the second-order Einstein equations, it shows that gravitational dynamics amplify the small inflationary non-Gaussianity in single-field slow-roll models, yielding a momentum-dependent kernel for $f^{\phi}_{\rm NL}$. The authors derive an explicit large-scale expression for $f^{\phi}_{\rm NL}$ and demonstrate that the post-inflationary era can generate a sizable non-Gaussian component in the gravitational potential, even if the inflationary $f_{\rm NL}$ was negligible. The results have implications for interpreting CMB and large-scale structure data, and they emphasize that a detected non-Gaussian signal does not uniquely rule out simple single-field inflation, since multi-field scenarios can produce larger primordial non-Gaussianity as well.

Abstract

We study the evolution of cosmological perturbations on large scales, up to second order, for a perfect fluid with generic equation of state. Taking advantage of super-horizon conservation laws, it is possible to follow the evolution of the non-Gaussianity of perturbations through the different stages after inflation. We find that a large non-linearity is generated by the gravitational dynamics from the original inflationary quantum fluctuations. This leads to a significant enhancement of the tiny intrinsic non-Gaussianity produced during inflation in single-field slow-roll models.