On Friday, October 5, 2012 9:17:32 PM UTC+2, Phillip Helbig---undress to reply wrote:
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.
The problem is that the density of the Universe is smaller
than the density of a galaxy cluster which inturn
is smaller than the density of a single large galaxy.
For the critical density I expect the same relation.
If that is true it means that you can not compare one
with the other.

This picture is even more complex if you assume that 80%
of all the matter in the Universe is non-baryonic.
Table 4 in
http://iopscience.iop.org/0004-637X/...1038/fulltext/
shows the amount of gas in a galaxy cluster is roughly
10% (20%) of the total mass.
Is my assumption understanding correct that the total
mass of a galaxy cluster including all baryonic and non
baryonic mass is a factor 5 higher than the total mass ?

Nicolaas Vroom