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Originally Posted by KiwiPhil
As far as I'm aware the velocity of singular galaxies are not increasing, (except for the effect of dark energy) unless you mean that the further they are from us the faster they appear to be travelling. This is just the standard definition of expansion, which I agree with.
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Yes I am referring to recessional velocity, that is the velocity at which distant galaxies APPEAR to move away from us. This means, as you say, that the galaxies themselves are not moving (which by the way they would be if we were talking explosion) but the space between them is expanding. If we call that “the standard definition of expansion” it is only because the Big Bang theory has become the standard theory. The theory explains why galaxies SEEM to move faster the further away they are, namely because of the expansion of space-time itself and that makes it a very unique kind of expansion.
Quote:
Originally Posted by KiwiPhil
Now correct me if I'm wrong, but if an explosion happened in a frictionless environment wouldn't it have the same sort of expansion if there was nothing to slow it down? (apart from gravity)
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If you had a standard explosion inside an existing space you would NOT get that increasing effect because here it would be the galaxies that were moving (and let’s forget about gravity, dark matter and dark energy since these factors for now are not relevant and will only serve to confuse). The explosion would occur in a specific place inside that space and pieces of matter would fly away from that point in all directions at approximately the same speed. Each piece would maintain its speed. No force would act to increase it. What would you see if you could sit on one of these pieces? Well, you would certainly not find that all the other pieces were flying away from you. Those pieces behind you (in the direction of the original explosion point) and the pieces in front of you would maintain their distance to you since you were headed the same way and even if some of them were much farther away than others, that would not make them move faster. The ones that would recede at the highest speed would be the ones flying in the opposite direction. There would be no increase in speed whatsoever due to distance.
If you could remain at the original center of the explosion (and that’s where we appear to be) you would see all the other pieces of matter fly away from you at the same speed regardless of distance.
I think that if you agree with the standard definition of expansion as you say you do, then you have thereby already accepted that space itself is expanding and if you follow that backwards you will get to a point that contains not only all matter (like in an explosion) but all of space and all of time.
Sorry, I must keep it shorter next time...