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The age of the universe

  1. Nov 14, 2007 #1
    Edwin Hubble uses the red shifting technique on light emanating from the furthest known galaxies that are 12 billion light years away to estimate the age of the universe.

    With this technique they say the Universe is expanding. The further away the object the faster it is moving.

    My question is this.

    If the light is from 12 billion years ago, wouldn't it then make sense that it was expanding? Wouldn’t that put the object near the event of the big bang?
  2. jcsd
  3. Nov 14, 2007 #2
    I think it refers to time elapsed since the big bang.
  4. Nov 14, 2007 #3
    Right, it is 12 billion light years away.

    So 12 billion light years ago, that mass that we are seeing now was moving faster because it was at the brink of the big bang, if in fact the big bang happened around then.

    am i correct?

    If i am, then that is no evidence that the universe is expanding now. in fact, for all we know it could be retracting...
  5. Nov 14, 2007 #4
    You're not correct, redshift expansion is measured via Hubble's law ( v=hd )

    This states that velocity of an object is equal to Hubble's constant multiplied by the distance.

    Here's a good link
  6. Nov 14, 2007 #5


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    The hubble law is that the light from ALL galaxies is receding from us and the speed is proportional to the distance.
    So nearby galaxies only a few million light years away are also moving away from us.
  7. Nov 14, 2007 #6
    But isnt the light we are measureing billions of lightyears old?
  8. Nov 14, 2007 #7
    Of course, because light (photons) has a finite speed. Redshift of distant stars and galaxies are due to the general expansion of the universe. The rate of expansion is called the Hubble constant, and if the universe were expanding uniformly since its beginning, would tell us how old the universe is. Extrapolating backward would bring the galaxies together about 2 billion years ago, using Hubble's original figures.
    Last edited: Nov 14, 2007
  9. Nov 14, 2007 #8
    but what if we arnt seeing light from galaxies from lets say 13 billion light years away simply because its not here yet.
  10. Nov 14, 2007 #9


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    Basically you plot the speed that galaxies are going away from us agaisnt distance.
    Draw a line back to the origin, that's when all the galaxies were in one place = start of the universe = 13.8 billion years (or so)
  11. Nov 14, 2007 #10
    so what we are seeing now is the speed of that object 12 billion years ago..????


    So, we have no way of knowing that objects current speed or if it even still exists?

    thats my point.
  12. Nov 14, 2007 #11

    Yes, perhaps, and maybe*...

    *=chances are is that it still "exists" but might have collided with another galaxy thus not existing in a "current" state
  13. Nov 14, 2007 #12
    so how then can we use that method to say, today the universe is still expanding? we can say.. the universe 12 billion years ago was expanding at this rate but we have no way of saying that it still is..
  14. Nov 14, 2007 #13
    Here, here, here, and here
  15. Nov 28, 2007 #14
    How can we see light from "near" the Big Bang?

    This seems like the right thread to post my question on so here goes. I have trouble understanding the concept of looking back in time to near the big bang as is often stated when showing Hubble photographs of the most distant galaxies. Since all matter was centralized at the big bang and since the light emitted from that event is moving out from that point in all directions at the speed of light, shouldn't any light coming from that event be much farther out in space than we are and thus not be visible to us? Unless matter, including the stuff we are made of, was moving out much faster than the speed of light in the early universe I can't see how we can just now be seeing light emitted near the time of the big bang. Also, shouldn't the big bang be located in a particular direction from our location today and not visible in just any direction?
  16. Nov 28, 2007 #15


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    This is the problem. It is a very common misconception that the Big Bang was some kind of explosion that happened at a point, sending material 'outwards' from that point. That seems to be the picture in most people minds, probably the very name of the theory doesn't help!

    In fact what Big Bang theory says is that the Big Bang happened everywhere. There is no special centre of the Universe where the Big Bang happened, everything is moving away from everything else, rather than everything just moving away from one point.

    I'm sure this would have been explained with more clarity in some of the link already provided in this thread, but feel free to ask more questions if you don't understand.
  17. Nov 28, 2007 #16
    Wallace this construct makes a lot of sense. But "everywhere" 13.8 billion years ago was presumably compressed into a "smaller" space back then and that is why there theoretically was a short period of very rapid expansion of space right after the big bang, or have I got this all wrong?
  18. Nov 28, 2007 #17

    Chris Hillman

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    Universe once much "smaller" than it is now? Correct (according to current mainstream understanding). This caused inflation? Incorrect.

    There appear to be two current threads in this forum with almost identical titles--- how did that happen?
  19. Nov 28, 2007 #18
    Perhaps we can not see past the 13 billion light year mark because there is a curvature similiar to the horizon of our Earth.

    Or perhaps there is simply nothing there.

    Or perhaps that light has not yet reached us.

    either way, when we look at light that old, i do not understand how it can be said that it is is still accelerating away from us or anything else for that matter. 13 billion years ago, as we are seeing it now... of course it was accellorating
  20. Nov 28, 2007 #19
    Regarding an object 13 billion light years away from us: We aren't saying that it is still accelerating now. It probably doesn't even exist now. All we are saying is that relative to our position now, it was accelerating away from us back then. Additionally, we are accelerating now away from where it was then.
  21. Nov 28, 2007 #20

    Chris Hillman

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    That's more or less correct. The rest of what you proposed seems more or less incorrect or at least potentially misleading.

    Words don't easily capture what the models actually "say", but pictures give a vivid impression. I highly recommend the discussion of the FRW models (including nonzero Lambda) in D'Inverno's textbook.
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