Beyond \delta N formalism

Understand

We develop a theory of nonlinear cosmological perturbations on superhorizon scales for a multi-component scalar field with a general kinetic term and a general form of the potential in the context of inflationary cosmology.

• We employ the ADM formalism and the spatial gradient expansion approach, characterised by O(\epsilon^2), where \epsilon=1/(HL) is a small parameter representing the ratio of the Hubble radius to the characteristic length scale L of perturbations.
• We provide a formalism to obtain the solution in the multi-field case.
• This formalism can be applied to the superhorizon evolution of a primordial non-Gaussianity beyond the so-called \delta N formalism which is equivalent to O(\epsilon^0) of the gradient expansion.