  It is shown that density fluctuations obey a scaling law in an open Friedmann
universe. In a flat universe, the fluctuations are not scale-invariant. We
compute the growth rate of adiabatic scale-invariant density fluctuations in
flat, open and inflationary universes. We find that, given a sufficiently long
time, the density perturbations decay away in the Einstein-de-Sitter universe.
On the contrary, the rapid growth of the density instabilities makes an open
universe inhomogeneous in a time scale comparable to the age of our universe.
We also find that the fluctuations grow exponentially in a flat inflationary
universe.
