The Eulerian cosmological fluid equations are used to study the nonlinear mode coupling of density fluctuations .
We evaluate the second-order power spectrum including all four-point contributions .
In the weakly nonlinear regime we find that the dominant nonlinear contribution for realistic cosmological spectra is made by the coupling of long-wave modes and is well estimated by second order perturbation theory .
For a linear spectrum like that of the cold dark matter model , second order effects cause a significant enhancement of the high k part of the spectrum and a slight suppression at low k near the peak of the spectrum .
Our perturbative results agree well in the quasilinear regime with the nonlinear spectrum from high-resolution N-body simulations .

We find that due to the long-wave mode coupling , characteristic nonlinear masses grow less slowly in time ( i.e. , are larger at higher redshifts ) than would be estimated using the linear power spectrum .
For the cold dark matter model at ( 1 + z ) = ( 20 , 10 , 5 , 2 ) the nonlinear mass is about ( 180 , 8 , 2.5 , 1.6 ) times ( respectively ) larger than a linear extrapolation would indicate , if the condition rms \delta \rho / \rho = 1 is used to define the nonlinear scale .
At high redshift the Press-Schechter mass distribution significantly underestimates the abundance of high-mass objects for the cold dark matter model .
Although the quantitative results depend on the definition of the nonlinear scale , these basic consequences hold for any initial spectrum whose post-recombination spectral index n decreases sufficiently rapidly with increasing k , a feature which arises quite generally during the transition from a radiation- to matter-dominated universe .