The Ω dependence in equations of motion

The equations of motion governing the evolution of a collisionless gravitating system of particles in an expanding universe can be cast in a form which is almost independent of the cosmological density parameter, Ω, and the cosmological constant, Λ. The new equations are expressed in terms of a time variable τ=ln D , where D is the linear rate of growth of density fluctuations. The dependence on the density parameter is proportional to ε=Ω −0.2 −1 times the difference between the peculiar velocity (with respect to τ) of particles and the gravity field (minus the gradient of the potential); or, before shell‐crossing, times the sum of the density contrast and the velocity divergence. In a one‐dimensional collapse or expansion, the equations are fully independent of Ω and Λ before shell crossing. In the general case, the effect of this weak Ω dependence is to enhance the rate of evolution of density perturbations in dense regions. In a flat universe with Λ7ne;0, this enhancement is less pronounced than in an open universe with Λ=0 and the same Ω. Using the spherical collapse model, we find that the increase of the rms density fluctuations in a low‐Ω universe relative to that in a flat universe with the same linear normalization is ∼0.01ε(Ω)〈δ 3 〉, where δ is the density field in the flat universe. The equations predict that the smooth average velocity field scales like Ω 0.6 , while the local velocity dispersion (rms value) scales, approximately, like Ω 0.5 . High‐resolution N ‐body simulations confirm these results and show that density fields, when smoothed on scales slightly larger than clusters, are insensitive to the cosmological model. Haloes in an open model simulation are more concentrated than haloes of the same M /Ω in a flat model simulation.