PDA

View Full Version : another thing that puzzles me (clusters not expanding)



speach
2018-Apr-13, 11:51 PM
In the expanding universe all the galaxies are moving away from each other. Why is then is our local group coalescing? Can I take it from this, that in the wider universe there are local groups moving together and at the same time as a unified group they are move away from all other galaxies?

Hornblower
2018-Apr-14, 12:01 AM
The local group galaxies are close enough together for their gravity to overpower the cosmological expansion tendency.

DaveC426913
2018-Apr-14, 04:09 AM
Because the expansion of the universe is very weak indeed. It is weaker than the gravitational attraction of local galaxies. They stick together as a bunch. Expansion is more of an effect at the scale of galactic clusters.


Stick a bunch of pennies on a balloon. Tape a few of them together edge-to-edge with some strong tape.

Blow up the balloon.

(The pennies themselves certainly don't disintegrate - their bonds are far too strong.)

But neither does the tape. The local group of pennies stays together even as the surface underneath them expands.

Strange
2018-Apr-14, 07:59 AM
Perhaps another way of looking at it is that the expansion only appears where you have a homogeneous distribution of matter. This isn't true within galaxies or clusters of galaxies.

Cougar
2018-Apr-14, 01:25 PM
Can I take it from this, that in the wider universe there are local groups moving together and at the same time as a unified group they are move away from all other galaxies?

Almost. They are not "moving away" from other galaxies. The other galaxies appear to be moving away from them due to the expansion.

George
2018-Apr-14, 07:12 PM
Almost. They are not "moving away" from other galaxies. The other galaxies appear to be moving away from them due to the expansion. There is a nuance there but since the distance between clusters is increasing then perhaps they are being "carried away".

I assume that there is some gained separation due to the expansion for each galaxy in a cluster, so if we were to remove this expansion factor (at some point in the distant past but not in the beginning, of course), and given DM as first noted in Zwicky's cluster observations, would we see much difference in the configuration of the tighter clusters, at least, today? IOW, if we use warm chewing gum in lieu of pennies (for each galaxy cluster) on the balloon, would there be any noticeable size difference today in the gum?

PetTastic
2018-Apr-14, 08:57 PM
I am never exactly sure about the correct way to think about this.

Is it that the local galaxies are being stopped from expanding away from each other by gravity, or is it space between them that it being stopped from expanding by gravity?

The expansion of space causes the cosmic redshift so there is a difference in what we see looking out though local space that is still expanding while galaxies are being pulled through it against the expansion, compared to local space not expanding.

DaveC426913
2018-Apr-14, 09:23 PM
Is it that the local galaxies are being stopped from expanding away from each other by gravity, or is it space between them that it being stopped from expanding by gravity?

The first.