The primordial "f_NL" non-Gaussianity, and perturbations beyond the present horizon

Understand

We show a primordial non-linear &#34;f_NL&#34; term may produce unphysically large CMB anisotropy for a red-tilted primordial power spectrum (n<1), because of coupling to primordial fluctuation on the largest scale.

• We consider a primordial power spectrum models of a running spectral index, and a transition at very low wavenumbers.
• We find that only negative running spectral index models are allowed, provided that there is no transition at a low wavenumbers (i.e.
• k<<1).