# I don't understand - Andromeda

1. Jun 30, 2011

### PlayUK

Excuse my ignorance. I don't know much about this subject, but there's something I've read about that I don't understand. If space is expanding and everything is getting further away from everything else, how is Andromeda heading towards us?

2. Jun 30, 2011

Staff Emeritus
Andromeda is close enough that the mutual gravitational attraction between it and our galaxy is enough to overcome it.

3. Jun 30, 2011

### Staff: Mentor

I thought it was because Andromeda is very close to us (on the scale of the entire universe), so the expansion of space all by itself produces only a very small "separation velocity" that is easily "hidden" by the random velocities that the two galaxies have anyway.

I think the overworked and often-misleading balloon analogy works here. Imagine ants crawling slowly and randomly around the balloon. If two ants happen to be close together, and approaching each other, the balloon surface in between doesn't produce enough expansion to overcome their approach.

4. Jun 30, 2011

### Nabeshin

I don't think so. I'm fairly certain the overall expansion is precisely halted in the presence of such such strong local gravitational fields. The way I think about it is that you can only really apply FRW (and hence, the expansion) on scales which the universe looks homogeneous and isotropic. Since this clearly isn't true for two galaxies as close as the milky way and andromeda, assigning an expansion velocity of the universe to the space around these galaxies does not make sense.

5. Jul 5, 2011

### mrspeedybob

While I am inclined to disagree with you, I don't know of any experiment that has been done to diferentiate between homogenous and inhomogenous expansion. As far as I know, we don't know anything about the distribution of dark enery. If I'm wrong then point me in the dirrection of the relevant info.

6. Jul 5, 2011

### PlayUK

Has anyone calculated the amount of expansion? Because the space I'm sitting in right now must be expanding too, surely?

Andromeda is so far away it's hard to image gravity overcoming expansion, especially when you read about galaxies moving away from us at substantial percentages of the speed of light (i.e. compounding expansion of space from here to a long way away). So the expansion must be very small.

7. Jul 5, 2011

### yenchin

It's not just Andromeda, at the scale of local cluster, galaxies will not be pulled apart by space expansion [unless of course the acceleration becomes much greater in the future]. See Section 2.6.3 of http://arxiv.org/abs/0707.0380" [Broken].

Last edited by a moderator: May 5, 2017
8. Jul 5, 2011

### PlayUK

Excellent paper. Clears up the misconceptions. Thanks.