# Why does a doppler red shift prove expansion?

My question boils down to this. If the Universe was contracting the stars closer to the center would move faster to the middle than stars that were further away from the center.

That would also produce a red doppler shift. So why is it that the red shift is always linked to expansion? Human optimism?

Thanks,

Andrew

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## 1 Answer

No, if the universe were contracting, then all distant galaxies would appear blueshifted to us, because they would be moving towards us.

Also, faraway galaxies would be moving more rapidly towards us than more nearby galaxies, so faraway galaxies would be more blueshifted: Hubble's Law $$v = HD$$ would still hold, but $H$ would be negative. The larger the distance $D$, the more negative $v$ would be.

Finally, the universe has no center: all distant galaxies are moving away from each other (or in your case, all galaxies would be moving towards each other).

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Doesn't gravity get weaker the further the distance? If that is the case then galaxies further out would be moving slower not faster than us. –  KingAndrew Aug 14 '13 at 0:28
Galaxies are not flying away from a central point. Instead, they are receding from each other because space itself expands. The more space between two galaxies, the faster they recede. Look up the balloon analogy. –  Pulsar Aug 14 '13 at 11:46