# Does the universe move as one?

1. Aug 21, 2013

### nst.john

I know the universe is expanding and the expansion is accelerating but I'd the entire universe accelerating at the same rate? Or are different parts moving at different rates?

2. Aug 21, 2013

### phinds

No sure what your exact question is. There are two possible answers.

(1) Yes, the rate is uniform per volume of space outside of gravitationally bound objects such as galactic clusters.

(2) No, if cluster A and B are twice as far from each other as clusters C and D are from each other, then A and B are receding from each other twice as fast as C and D are receding from each other.

EDIT: on the other hand, for (2), if galaxies E and F are the same distance apart as A and B then they are receding from each other at the same speed as A and B are from each other, so in that sense, it's a yes.

Last edited: Aug 21, 2013
3. Aug 22, 2013

### nst.john

OK I understand the examples and see how they fit. Thanks!

4. Aug 22, 2013

### Naty1

phinds sure covered the observable universe well....what's going on beyond that is assumed
to be the same....such scientific assumptions have shown themselves in the past to be questionable....

We assume it is uniform at all points in time because the negative pressure vacuum energy, aka cosmological constant, seems to have constant energy density in all of homogeneous and isotropic space...
which usually means at intergalactic scales and larger.

Also, the acceleration of the universe varies over time and has been increasing since about 6B years ago as we entered an energy dominated expansionary phase. THAT was a rather recent and unanticipated revelation.

5. Aug 22, 2013

### Viracocha

In general the universe expands homogeneously, but there could easily be asymmetries in this expansion (on a smaller scale), I'd be surprised if there weren't