<p>As you say, expansion is proportional to distance. And in fact, compared to the size of the (observable) Universe, Andromeda and our Milky Way are <strong>close together</strong>. Indeed, farthest objects are <em>now</em> more than 20 <strong>b</strong>illion Ly from us, whereas Andromeda is only 2.5 <strong>m</strong>illion Ly away. That is a <strong>1/10000</strong> factor.</p>

<p>The space expansion between them is not enough to set them apart, because of their general motion. On small scale (until galaxy cluster scale), the motion is primarily ruled by gravity. <a href="https://en.wikipedia.org/wiki/Galaxy_groups_and_clusters" rel="noreferrer">Galaxy cluster</a> are in fact the largest gravitationally bound systems we know. </p>

<blockquote>
  <p><em>They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs</em></p>
</blockquote>

<p>Since Andromeda is the closest galaxy, it is inside "our" galaxy cluster, and stays bound to ours by gravity.</p>

CC BY-SA 3.0
2017-10-09T08:40:49.327Z
2017-10-09T08:40:49.327Z
J. Chomel
<p>According to Hubble's law, the universe is expanding and this expansion is proportional to distance between the two objects and thus objects are moving away at a very fast rate,  then why is it predicted that after 4 billion years the Milky Way shall collide with Andromeda galaxy?</p>

<cosmology><milky-way>
Collision with Andromeda Galaxy
Holy Answerer
# Collision with Andromeda Galaxy

(Asked by: username_0 on 2017-10-09)

According to Hubble's law, the universe is expanding and this expansion is proportional to distance between the two objects and thus objects are moving away at a very fast rate, then why is it predicted that after 4 billion years the Milky Way shall collide with Andromeda galaxy?

**Tags:** cosmology, milky-way

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## 1 answers

### Answer by username_1 on 2017-10-09. Score: 10 | Accepted answer

As you say, expansion is proportional to distance. And in fact, compared to the size of the (observable) Universe, Andromeda and our Milky Way are **close together**. Indeed, farthest objects are *now* more than 20 **b**illion Ly from us, whereas Andromeda is only 2.5 **m**illion Ly away. That is a **1/10000** factor.

The space expansion between them is not enough to set them apart, because of their general motion. On small scale (until galaxy cluster scale), the motion is primarily ruled by gravity. [Galaxy cluster](https://en.wikipedia.org/wiki/Galaxy_groups_and_clusters) are in fact the largest gravitationally bound systems we know.

> *They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs*

Since Andromeda is the closest galaxy, it is inside "our" galaxy cluster, and stays bound to ours by gravity.


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