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Published online by Cambridge University Press: 03 August 2017
We consider two density perturbation modes with significantly different length scales λ and L (λ ≪ L) in a homogeneous Universe within Newtonian approximation. For the two modes the coupling terms in the corresponding Euler-Lagrange and Poisson equations are taken into account within lowest order of approximation. We assume that the λ - nods (high-frequency mode) is superimposed on the large-scale mode in such way that by an appropriate averaging procedure, the global behaviour is determined only by the single L-mode. Locally (Δ x ≈ λ ≪ L) the space dependence of the L-mode can be neglected in comparison with the λ -mode, but its time evolution remains important for the evolution of the λ -mode perturbation δ. We obtain for δ the equation: where , b is the sound velocity with respect to the undisturbed homogeneous natter distribution, H is the Hubble parameter, .