An attempt is made to clarify the approximate nature of the commonly used Omega exp 0.6 formulas. The velocity of a shell surrounding a spherical density perturbation with average fractional overdensity Delta rho/rho in an otherwise Friedmann universe is expanded here in powers of Delta rho/rho. An expression is obtained for the fractional velocity change of the perturbed shell.
No takes yet. Share an insight, caveat, or question.
Lightman et al. (1990) studied this question.