Simple method for computing the nonlinear mass correlation function with implications for stable clustering

We propose a simple and accurate method for computing analytically the mass correlation function for cold dark matter and scale-free models that fits N-body simulations over a range that extends from the linear to the strongl y nonlinear regime. The method, based on the dynamical evolution of the pai r-conservation equation, relies on a universal relation between the pairwis e velocity and the smoothed correlation function valid for high-and low-den sity models, as derived empirically from N-body simulations. An intriguing alternative relation, based on the stable-clustering hypothesis, predicts a power-law behavior of the mass correlation function that disagrees with N- body simulations but conforms well to the observed galaxy correlation funct ion if negligible bias is assumed. The method is a useful tool for rapidly exploring a wide span of models and, at the same time, raises new questions about large-scale structure formation.