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Red shift question

  1. Oct 12, 2012 #1
    This might be obvious but i was listening to someone talk about how he believes the big bang is cyclical and the universe is constantly expanding and contracting. now i dont believe this but one of the points he raised was (and i know it doesnt support his argument) we dont know if galaxies are still moving away from us since the light we see from an object a million light years away takes a million years to reach us, we may see it red shifted now but it could be coming towards us but the light wouldnt be blue shifted until a million years after it started to move towards us. Is this correct?
  2. jcsd
  3. Oct 13, 2012 #2


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    As the expansion of the universe in recent years has been speeding up instead of slowing down, it really isn't reasonable to suggest that this trend has suddenly reversed itself in the (comparatively) small amount of time it has taken the light to reach us from the relatively nearby universe.

    Thinking our universe has already started to collapse would really be akin to thinking that the Sun might not be there any longer because the light from the Sun takes around seven minutes to get to us: the Sun might have suddenly rocketed off, away from the rest of the Solar System, leaving the planets to fly off in its wake. There's basically no way this can happen.

    Might the acceleration of the expansion reverse itself at some time in the far future, however? Well, it's conceivable. But it doesn't seem likely. it would require that the dark energy starts, for some unknown reason, behaving very differently from what it's done in the past. It might be possible to come up with a speculative model of dark energy where this happens, but it certainly isn't easy.
  4. Oct 14, 2012 #3


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    The Sun could have it's gravity magically turn off and we wouldn't know about it yet. That doesn't mean it is likely. The universe could very well slow down and reverse in the future, but currently it simply doesn't look that way, it looks like the opposite. When making theories and models using science we MUST rely on things that are observable, not things that we would like to happen.
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