New approach to the evolution of cosmological perturbations on large scales - art. no. 043527

We discuss the evolution of line ar perturbations about a Friedmann-Roberts on-Walker background metric, using only the local conservation of energy mo mentum. We show that on sufficiently large scales the curvature perturbatio n on spatial hypersurfaces of uniform density is conserved when the non-adi abatic pressure perturbation is negligible. This is the first time that thi s result has been demonstrated independently of the gravitational field equ ations. A physical picture of long-wavelength perturbations as being compos ed of separate Robertson-Walker universes gives a simple understanding of t he possible evolution of the curvature perturbation, in particular clarifyi ng the conditions under which super-horizon curvature perturbations may var y.