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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. –  Mew Sep 21 '12 at 9:55
    
@Chris: I am referring to the galaxies in context of collision...! –  Rorschach 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. –  Mew 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. –  Rorschach 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. –  Mew 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? –  Rorschach 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. –  Rorschach Sep 21 '12 at 12:07
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The expansion of the universe is such that the space added in-between A and B is such that A and B get further apart at rate, say R, proportional to the distance between A and B. To be precise R = H*D, where D is distance and H is a constant. This is Hubble's law. Therefore the further apart A and B are, the faster they will appear to move away from each other.

Now the above is assuming A and B are stationary relative to one another. It is possible for B to collide with A, if A is moving at a velocity greater R in the direction of B.

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So, the galaxies only appear to be moving away from each other, but in reality they are getting closer. So does that mean the red shift thing is bogus, or doesn't really mean what the current theories indicate? –  jmarkmurphy Nov 27 '13 at 17:07
    
No they are not getting closer they are getting further apart in general as outlined by hubbles law. This is due to universe expanding. This gives rise to red shift. However this does not mean galaxies could not collide just because the underlying space is expanding –  Mew Nov 28 '13 at 10:27
    
That does not make any sense at all. If the galaxies are gettting further apart, they can not collide. –  jmarkmurphy Nov 29 '13 at 5:49
    
@jmarkmurphy, it does make sense. Because I said they are getting further apart in general, but there are exceptions. Think of ants on an expanding baloon. If each ant is stationary, each ant appears to be moving away from each other ant as the baloon expands. But immagine if you have a rogue ant that walks along the balloon, it is possible for this ant to collide with the other ants, despite the expansion of the ants universe (ie the baloon). The expansion of space is more than simply galaxies moving away from each other, but rather the space fabric expanding. –  Mew Nov 29 '13 at 6:08
    
I understand the idea that the fabric of space is getting stretched, but even on the expanding balloon, if the rogue ant does not get closer to another ant, it will not collide with one. And considering the red shift thing, I see that as a kind of doppler shift. But the wave length will only be perceived as longer if the target and receiver are truly getting further apart. The thing I really don't fully understand is how you can measure the amount of shift if you don't know the original wavelength. Maybe you have a different way to measure the true wavelength. –  jmarkmurphy Nov 29 '13 at 22:32
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protected by Qmechanic Feb 25 at 11:04

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