In article , Nicolaas Vroom
writes:

On Wednesday, October 3, 2012 9:58:43 AM UTC+2, Phillip Helbig
wrote:
Why do you expect the density to be as you described it?
I expect that a galaxy cluster more or less is the same as a
dwarf eliptical galaxy like: NGC 147

Why? Also, for comparison one needs the mass, which is usually less
precisely known than the light.

That means the highest density is in the center and slowly decreases.

OK, the NFW drops off more quickly.

They use the equation rho c = 3*H^2/8*pi*G to study the behaviour
of a single galaxy cluster while at the same type that same equation
is used to study the whole Universe.

The critical density, of course, is important for the entire universe.
However, in order for something to collapse and form structure, then, at
least to first order, it has to locally exceed the critical density, so
it is not surprising to see this crop up in the context of smaller
structures as well.