  Bouchet et al. (1992) showed that in an open or closed Universe with only
pressureless matter, gravitational instability from Gaussian initial conditions
induces a normalized skewness, $S_3 \equiv \VEV{\delta^3} \VEV{\delta^2}^{-2}$,
that has only a very weak dependence on the nonrelativistic-matter density.
Here we generalize this result to a plethora of models with various
contributions to the total energy density, including nonrelativistic matter, a
cosmological constant, and other forms of missing energy. Our numerical results
show that the skewness (and bispectrum) depend only very weakly ($\lesssim 2%$)
on the expansion history. Thus, the skewness and bispectrum provide a robust
test of gravitational instability from Gaussian initial conditions, independent
of the underlying cosmological model.
