2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/9659The cosmological fluid equations are used to study the nonlinear mode coupling of density fluctuations. We find that for realistic cosmological spectra there is a significant contribution to the nonlinear evolution on scales of interest to large-scale structure from the long-wave part of the initial spectrum. A consequence of this mode coupling is that at high redshift, $z$, the nonlinear scale [defined by $σ(z)=1$] can be significantly larger than a linear extrapolation would indicate. For the standard CDM spectrum with a $σ_8=1$ normalization the mass corresponding to the nonlinear scale at $z=20, 10, 5$ is about $100, 10, 3$ times (respectively) larger than the linearly extrapolated value. We also investigate the possibility of divergent contributions to the density field from long-wave modes if the spectral index of the power spectrum $\ n<-1$. Using an approximate non-perturbative approach we find that for $n>-3\,$ the divergent contribution appears only in the phase. This can be related to the large-scale bulk velocity, and clarifies previous results from N-body simulations.2 pages, Latex file, Proceedings of IAP Colloq 9, "Cosmological Velocity Fields" ed. F Bouchet & M. Lachieze-Rey, CSR-AT-93-22Astrophysicstext