Re: Cosmology

[Leigh Palmer <palmer@SFU.CA>]

> At 6:12 PM -0700 2/11/01, Jim Green,  you wrote about Re: Cosmology:
>
>
> >   >There is no evidence that our galaxy is expanding. The universe is
> >   >expanding according to evidence of a couple of kinds, but none of
> >   >that evidence arises from observation of objects within our galaxy.
> >   >No external galaxy is seen to be expanding, either.
> >
> >   What makes a local cluster "gravitationally bound" and not a super cluster?
>
>
> Sounds like there is a certain mass density that cancels the expansion.
>
> Is there an accepted value for this density?
>
> Is it determined empirically?

The evidence that clusters of galaxies are not expanding is pretty
robust. Galaxies within clusters have peculiar velocities sufficiently
large that the clusters would not exist if the galaxies were not
gravitationally bound to their clusters. However, it seems that
insufficient mass is seen in clusters to bind them gravitationally
given those high velocities. This is one of the "missing mass" or
"dark matter" problems still plaguing our understanding of the
universe. With the detection of cluster gas this problem has been made
somewhat smaller, but we should recognize how great is our ignorance.

Leigh