How do we know the universe is expanding?

Question

I'm a CS graduate, not a physicist, so my question might be a bit silly, but I haven't found an answer in the internet yet.

I've read recently that the reason behind our observation that galaxies etc. seem to be moving away from us is because the space is expanding (and not that they are simply moving at high speed through the space).

The question is: what experiment or reasoning lead scientists to this conclusion - how one can tell if this is not simply a movement through space?

I've read somewhere an explanation, that it would be very peculiar that all objects are moving away from us, as we are not expected to be the center of the universe. But from my understanding of how velocity vectors addition/subtraction works, we would observe all galaxies to be going outward from us regardless of the original center of "explosion". So I guess this wasn't the best explanation/proof.

My personal problem with accepting the expansion idea, is that since objects keep their sizes intact (presumably to some forces keeping their atoms together) it should (?) provide the objects with more and more energy: consider a ball of gas which grows 1 meter in diameter and then "falls" back into it's previous size due to gravitation (or whatever other force) - it would heat up, and quite probably "bounce" again to the larger size, and oscillate like that for some period. (At least that's what I see on my simplistic 2-D simulations of balls). 

Comments

This is introductory general relativity, not research level. This question should rather be asked on physicsforums.

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I agree with @ArnoldNeumaier, the question is not bad as such, but PhysicsOverflow is for graduate-level+ questions. Therefore I am voting to close.

Answer 1

The galaxies, seen from here, move at high speed through space away from us as you say, and far galaxies move faster than close galaxies. So, the universe is expanding.

Saying that space is expanding, instead of simply saying that the galaxies are moving away, reflects Einstein's General Relativity model, in which matter and energy curve space and curved space pushes matter and energy around. Under some conditions, Einstein's equations have solutions that correspond to expanding space. Imagine a balloon that is expanding, and the galaxies as dots on the surface on the balloon. The dots move away from each other because the balloon is expanding. Note that in this lower-dimensional analogy the universe is only the 2D surface of the balloon, we couldn't visualize curved 3D space intuitively.