# how can galaxies collide if everything moving outwards

If the universe started from Big Bang and everything is expanding outwards and actually accelerating away from each other, than "How is it possible for two galaxies to collide as they all are moving in the same direction". To collide they must in opposite direction !

Edit:How is it possible for two galaxies to collide as those colliding galaxies are moving in the same direction.

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First they are not "all moving in the same direction". Imagine a balloon expanding. Each point on the balloon gets further away from each other point on the balloon, but each point isn't moving in the same direction as each other point. To answer your question, an ant on the balloon is able to collide with another ant on the balloon, even though their space is expanding, if the ant walks fast enough along the baloon. –  Chris Sep 21 '12 at 9:55
@Chris: I am referring to the galaxies in context of collision...! –  Blackbird Sep 21 '12 at 11:11
Yes, I should have been more explicit. The expansion of space is analogous to the 2-D surface on an expanding balloon. –  Chris Sep 21 '12 at 11:19
@Chris: what factors decide that one ant will walk faster than others? They all are equally strong and intention-less. –  Blackbird Sep 21 '12 at 11:28
Gravity. As I mentioned in my post, the expansion of the universe is at a velocity proportional to the distance two objects are from each other. So if two galaxies are close enough, they will not "expand" away from each other quick enough to escape gravity. –  Chris Sep 21 '12 at 11:30
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Galaxies interact with each other due to their mutual gravitation.

If matter had been exactly evenly distributed after the big bang then galaxies wouldn't collide, but there wouldn't be any galaxies anyway. The inhomogeneities in the matter distribution are believed to originate from quantum fluctuations that occurred when inflation ended. Some regions ended up with higher than average matter density and some with lower than average matter density. In the overdense regions the mutual gravity of the matter overcame the expansion of spacetime and those regions collapsed to form galaxy superclusters, then clusters then galaxies. Within a galaxy cluster the galaxies have essentially random velocities relative to each other, which is why there are sometimes collisions.

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:It seems more like overtaking rather than collision and If so than which is moving at high speed..earth or andromeda? and since they are 4 billion years away from each other, they don't have any gravitational effect on each other yet ! What can be the reason that if speed is relativistic...one might slow down and other speed up? –  Blackbird Sep 21 '12 at 9:38
Andromeda is only 2.5 million light years away. This may seem a long distance for humans, but for galaxies it isn't far. It's only about 25 times the diameter of the Milky Way, so if the Milky Way was a dinner plate the distance to Andromeda would be about 7 metres. Galaxies are generally quite close in relation to their size. At the time inflation ended, $10^{-33}$ seconds after the Big Bang, the Milky Way and Andromeda were about 10cm apart! Obviously space has expanded a lot since then :-) –  John Rennie Sep 21 '12 at 10:02
that's why I said just years and not light years..! I get your point that this much distance doesn't matter much but as far as relativistic speed is concerned..I need to read more on it ! And if they are just overtaking than why each other's gravitational pull shreds each other though the direction of force is same...actually the very strong gravitational pull of one might increase the velocity of another as well because of the pull it will create. –  Blackbird Sep 21 '12 at 12:07