Weakly nonlinear clustering for arbitrary expansion histories

Bouchet et al. showed that in an open or closed universe with only pressure less matter, gravitational instability from Gaussian initial conditions ind uces a normalized skewness, S-3 equivalent to <delta(3)><delta(2)>(-2), whi ch has only a very weak dependence on the nonrelativistic matter density. H ere we generalize this result to a plethora of models with various contribu tions to the total energy density, including nonrelativistic matter, a cosm ological constant, and other forms of missing energy. Our numerical results show that the skewness (and bispectrum) depend only very weakly (less than or similar to 2%) on the expansion history. Thus the skewness and bispectr um provide a robust test of gravitational instability from Gaussian initial conditions, independent of the underlying cosmological model.