Classical non-Gaussianity from non-linear evolution of curvature   perturbations
  We study the non-linear evolution of the curvature perturbations during matter dominated era. We show that regardless of the origin of the primordial perturbation, the Bardeen potential and curvature receive sizable contributions from the classical non-linear evolution effects, and quantify them exactly. On the super-horizon scales we have squeezed peak of the bispectrum with magnitude, in terms of the local non-linear parameters of Bardeen curvature, 1/6 < f_{NL} 19/15, and of Bardeen potential, -1/4 < f_{NL} < 7/5, depending on the configuration of momenta. On the sub-horizon scales the bispectrum show equilateral shape, and can serve as a potential probe of general relativity. 
Jinn-Ouk Gong, Jai-chan Hwang and Hyerim Noh
astro-ph.CO gr-qc hep-ph hep-th