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Hubble's law says that the universe is expanding.How come the milky way and the andromeda are on a collision course?How will they end up colliding with each other?

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The large-scale structure of the Universe is expanding. However, gravity still works, and it's especially powerful if the distance is small. E.g., the Earth is still pulling your body closer to it, even though the Universe is expanding. The Earth and the Moon still attract each other, even though the Universe is expanding. Our whole galaxy is still held together by gravity, even though... You get it.

M31 (a.k.a. the Andromeda Galaxy) is "only" 2.5 million light years away, which is pretty close, by cosmic standards. At that scale, for large objects such as galaxies, gravity overrides cosmic expansion. There is a net positive attraction between us and M31. Our galaxies are being pulled together by gravity.

As to what exactly will happen when they collide, take a look at this simulation:

http://www.youtube.com/watch?v=MhKP4llTyYI

Or just download the software yourself, it's free:

http://universesandbox.com/

Only galaxies which are very far away from each other are being pulled apart by the cosmic expansion faster than gravity could pull them together.

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""At that scale, for large objects such as galaxies, gravity overrides cosmic expansion. "" This does not depend on size of objects! Even a single atom located at that distance will be attracted and "caught" by milky way. It depends on relative velocities! If it exeeds a critical value, the object will vanish to infinity, as it is for the pioneer spacecraft with respect to our sun. –  Georg Sep 15 '11 at 16:40
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Ok. Now replace the Milky Way with another atom. You've got two atoms, a large distance in between, and some relative velocity. They will not "catch" each other. When I said "objects" I was thinking of both sides, not just one. But anyway, it depends on all those things: distance, masses, velocities, etc. –  Florin Andrei Sep 15 '11 at 17:14
    
People always say that the Milky Way and M31 are going to collide with each other, but I've never been sure that there's evidence for that claim. It's true that the two galaxies are moving towards each other right now, but as far as I know there's no way to measure the transverse component of the velocity. If there's a significant transverse component (which seems very plausible), then they won't hit each other. –  Ted Bunn Sep 15 '11 at 17:52
    
@Ted Bunn There are indirect ways to measure the transverse velocity, but the large uncertainty in its magnitude and in the masses of M31 and the Milky Way make the collision just a possibility. See "Goodbye Andromeda?" and "On the collision between the Milky Way and the Andromeda Galaxy" for more information. –  mmc Oct 2 '11 at 18:30
    
Youtube link is dead. </shame> –  corsiKa Apr 30 at 21:31

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