# Expaning universe and colliding galaxies

## Main Question or Discussion Point

I have some trouble seeing these two things. Lets say the big bang did happen, and the universe is expanding... ie all galaxies are moving away from each other. If all galaxies are moving away from each other, how can two gallaxies collide?

Also... If again the big bang happened, and the universe is expanding, Somewhat like dots painted the surface of a balloon being blown up. Then we would be on a point on that balloon, and if we look back in time to the big bang we would be about 14 billion years back. BUT that would only be half way across the universal sphere. Therefore we should be able to see galaxies etc that are farther away then the point of the big bang?

Just some thoughts...

jam

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Janus
Staff Emeritus
Gold Member
The first question is asked alot. (We really should look into a FAQ subforum)
The answer is that galaxies form gravitationally bound groups which clump together and resist the universal tendancy to expand. It is these clusters of galaxies which are moving apart. The indivdual galaxies of a cluster orbit their common CoG, and sometimes their paths cross such that there is a collision.

For the second question, you must remember that as we look futher away, we are also looking back in time. Thus as we look further away we are looking at the universe when it was smaller. So while a spherical Balloon might describe the present shape of the universe, the visible] shape to us would be more like a balloon which someone has pushed their finger into, causing a depression that goes all the way to the center ofthe balloon. Our sight lines follow this depression to the center, which would represent the big bang.

Phobos
Staff Emeritus
Gold Member
Exactly. Imagine that each dot on the balloon represents a cluster of galaxies instead of individual galaxies. The expansion of space is most significant in the vast empty regions between galaxy clusters. On smaller scales, gravity wins out.

For the second point, keep in mind that the Big Bang did not happen at one point in space...it happened at every point in space.

cepheid
Staff Emeritus