A critique of scaling behaviour in non-linear structure formation scenarios

Abstract

Moments of the BBGKY equations for spatial correlation functions of cosmological density perturbations are used to obtain a differential equation for the evolution of the dimensionless function, $h = - ({v/{\dot{a}x}})$, where $v$ is the mean relative pair velocity. The BBGKY equations are closed using a hierarchical scaling ansatz for the 3-point correlation function. Scale-invariant solutions derived earlier by Davis and Peebles are then used in the non-linear regime, along with the generalised stable clustering hypothesis ($h \to $ const.), to obtain an expression for the asymptotic value of $h$, in terms of the power law index of clustering, $\gamma$,and the tangential and radial velocity dispersions. The Davis-Peebles solution is found to require that tangential dispersions are larger than radial ones, in the non-linear regime; this can be understood on physical grounds. Finally, stability analysis of the solution demonstrates that the allowed asymptotic values of $h$, consistent with the stable clustering hypothesis, lie in the range $0 \leq h \leq 1/2$. Thus, if the Davis-Peebles scale-invariant solution (and the hierarchical model for the 3-pt function) is correct, the standard stable clustering picture ($h \to 1$ as $\bar\xi \to \infty$) is not allowed in the non-linear regime of structure formation.