  We present a second-order gauge-invariant formalism to study the evolution of
curvature perturbations in a Friedmann-Robertson-Walker universe filled by
multiple interacting fluids. We apply such a general formalism to describe the
evolution of the second-order curvature perturbations in the standard
one-single field inflation, in the curvaton and in the inhomogeneous reheating
scenarios for the generation of the cosmological perturbations. Moreover, we
provide the exact expression for the second-order temperature anisotropies on
large scales, including second-order gravitational effects and extend the
well-known formula for the Sachs-Wolfe effect at linear order. Our findings
clarify what is the exact non-linearity parameter f_NL entering in the
determination of higher-order statistics such as the bispectrum of Cosmic
Microwave Background temperature anisotropies. Finally, we compute the level of
non-Gaussianity in each scenario for the creation of cosmological
perturbations.
