Without invoking a perturbative expansion, we define the cosmological curvature perturbation, and consider its behaviour assuming that the universe is smooth over a sufficiently large comoving scale. The equations are simple, resembling closely the first-order equations, and they lead to results which generalize those already proven in linear perturbation theory and (in part) in second-order perturbation theory. In particular, the curvature perturbation is conserved provided that the pressure is a unique function of the energy density.
This pioneering paper showed that the so-called delta N formalism can be radically generalised, to provide a powerful tool for calculating the non-gaussianity of the primordial perturbation. It has already been used as the starting point for new directions of research by several groups. 78 citations (SPIRES). RAE_import_type : Journal article RAE_uoa_type : Physics