Cosmological Perturbation Theory and the Spherical Collapse model - III. The Velocity divergence field and the $Ω$ dependence

Abstract

Cosmological Perturbation Theory (PT) is a useful tool to study the cumulants of the density and velocity fields in the large scale structure of the Universe. In Papers I & II of this series we saw that the Spherical Collapse (SC) model provides with the exact solution to PT at tree-level and gives a good approximation to the loop corrections (next to leading orders), indicating negligible tidal effects. Here, we derive predictions for the (smoothed) cumulants of the velocity divergence field $θ\equiv \nabla\cdot\vv$ for an irrotational fluid in the SC model. By comparing with the exact analytic results by Scoccimarro & Frieman (1996), it is shown that, at least for the unsmoothed case, the loop corrections to the cumulants of $θ$ are dominated by tidal effects. However, most of the tidal contribution seems to cancel out when computing the hierarchical ratios, $T_J = {<θ^J> / <θ^2>^{J-1}}$. We also extend the work presented in Papers I & II to give predictions for the cumulants of the density and velocity divergence fields in non-flat spaces. In particular, we show the equivalence between the spherically symmetric solution to the equations of motion in the SC model (given in terms of the density) and that of the Lagrangian PT approach (in terms of the displacement field). It is shown that the $Ω$-dependence is very weak for both cosmic fields even at one-loop (a 10% effect at most), except for the overall factor $f(Ω)$ that couples to the velocity divergence.