# Space Time

1. Feb 17, 2006

### wolram

I have read that, on the scale of clusters (million light years) the effect of
the expansion of the universe is 10 million times smaller than the gravity
binding the cluster.
So space time, (flows) through the cluster (like a super fluid) or akin to a
group of logs staying in a fixed possition in a flowing stream, all poor
analogies but the best i can come up with.
This suggests that the (coulping) between matter and space time is
extremely weak.
My question, how can space time if viewed as a (super fluid or some thing
that gravitationaly bound bodies can ignore) carry massive bodies ie
gallaxies with it?

2. Feb 17, 2006

### Garth

In GR the coupling of the general expansion of space to objects within it is generally understood to be dependent on the average density of the universe compared with that of the object concerned.

If the average cosmological density is ~ 10-29 gms/cc and the average density of the cluster is ~ 10-22 gms/cc then the cosmological expansion effect (Hubble flow) is 10 million times smaller than the gravitational binding of the cluster.

However, the centre of mass of the cluster itself, or the super-cluster it belongs to, is carried along with the Hubble flow.

There is a question about how the gravitational field of the cluster/galaxy/stellar system within the cosmological field is treated. The local field is treated as Newtonian, which is the weak field limit of the Schwarzschild solution that itself is embedded in flat non-expanding Minkowskian space-time (at $r \rightarrow \infty$). If it is embedded in a cosmological metric as $r \rightarrow \infty$ then bound orbits might themselves be expanding with the universe.

This is generally not thought to be the case, but with the present non-identification of DM and the non-explained Pioneer anomaly (~ equal to the Hubble acceleration cH) the standard understanding may be in need of revision.

Garth

Last edited: Feb 17, 2006
3. Feb 17, 2006

### wolram

Thanks Garth, all very confusing, to me any way, i wonder how the system
(space time and mass) stayed together when dark energy started an
accelerated expansion.

4. Feb 17, 2006

### Garth

DE just affects the cosmological expansion. If gravitationally bound objects are not affected by that expansion (as is normally thought to be the case) then they will not be affected by an accelerated expansion caused by DE.

Garth

5. Feb 17, 2006

### wolram

Just had a look on the latest for the PA, a lot of talk here but not much on
new science, as usual it may be years befor an ansewer is forthcomming.

http://www.planetary.org/programs/projects/pioneer_anomaly/update_200511.html [Broken]

Last edited by a moderator: May 2, 2017
6. Feb 27, 2006