How Is Andromeda Approaching If Space Is Expanding?

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Discussion Overview

The discussion revolves around the apparent contradiction of the Andromeda galaxy approaching the Milky Way while the universe is expanding. Participants explore the implications of gravitational attraction versus cosmic expansion, examining the scales at which these forces operate and the relevance of local versus global dynamics in cosmology.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that the gravitational attraction between Andromeda and the Milky Way is sufficient to overcome the expansion of space.
  • Others argue that the proximity of Andromeda means that the expansion of space contributes only a minor separation velocity, which is overshadowed by the random velocities of the galaxies.
  • One participant introduces the balloon analogy to illustrate how local dynamics can dominate over cosmic expansion when galaxies are close together.
  • There is a contention regarding the applicability of Friedmann-Robertson-Walker (FRW) models, with some asserting that these models only apply on scales where the universe appears homogeneous and isotropic.
  • Concerns are raised about the lack of experimental differentiation between homogeneous and inhomogeneous expansion, particularly in relation to dark energy distribution.
  • Participants inquire about calculations of the amount of expansion affecting local space, questioning how gravity can overcome expansion at such distances.
  • One participant references a paper that they believe clarifies misconceptions about the topic.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between gravitational forces and cosmic expansion, with no consensus reached on how these forces interact at the scale of the Milky Way and Andromeda galaxies.

Contextual Notes

Limitations include the lack of consensus on the applicability of cosmological models to local structures and the unresolved nature of dark energy's distribution.

PlayUK
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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?
 
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Andromeda is close enough that the mutual gravitational attraction between it and our galaxy is enough to overcome it.
 
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.
 
jtbell said:
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.

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.
 
Nabeshin said:
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.

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.
 
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.
 
PlayUK said:
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.

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" .
 
Last edited by a moderator:
Excellent paper. Clears up the misconceptions. Thanks.
 

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