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How come we can see galaxies 9 B l/y away if universe is only 14 B y/o

  1. Feb 18, 2007 #1
    this might be a dumb question. but how come we can see objects that are 9 or more billion light years away from us?
    seeing as the universe is only 14 billion years old, and nothing can travel at the speed of light... then how can we see objects that would have taken 6,7,8 + billion years to get to where they are compared to us... then the light from those objects would have taken that same amount of time to reach us...
    wouldn't that take more time than is available?

    I'm probably overlooking something really obvious as usual...
     
  2. jcsd
  3. Feb 18, 2007 #2

    ranger

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    I believe we've seen gamma ray bursts which occurred 10 billion light years away from us. Gamma rays are EM waves wand travel at the speed of light. I dont why we would need more time.
     
  4. Feb 18, 2007 #3
    but wouldn't those gamma rays come from something that took at least 10 billion years to get there?
    if something 10 billion L/y away reacted to create those bursts, even if it could travel at the speed of light, it would take at least 10 billion years to get there, and then 10 billion more years for those rays to reach us, wouldn't it?

    I mean, if energy can't be created or destroyed, then those gamma rays couldn't have just come out of nowhere... there had to have been something there to create those gamma rays, and that something would have had to get there somehow.

    and even closer objects... something 7 billion L/y away, would still take 7 billion years to get there, plus 7 billion years for the light to reach us... even if it traveled at the speed of light that's more than the age of the universe.
     
    Last edited: Feb 18, 2007
  5. Feb 18, 2007 #4

    D H

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    To get where? The GRB emitted photons from where it was 10 billion years ago. It took those photons 10 billion years to catch up to us. There is no telling where that galaxy is "now". Remember that we are moving also, as is space itself.
     
  6. Feb 18, 2007 #5
    no, I understand that it is not "there" anymore.
    but here's my problem:

    - the light took 10 billion years to get here.
    - so, 10 billion years ago, that reaction happened 10 billion light years away from where we are now.
    - that would give that matter less than 4 billion years to travel 10 billion light years in order to get to where it was and generate those light waves... which is impossible.
     
  7. Feb 18, 2007 #6

    DaveC426913

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    The two galaxies did not travel to get 10 billion light years away from each other, the space between them expanded. The rate at which space expands is not limited by the speed of light.
     
  8. Feb 18, 2007 #7

    D H

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    We have been moving for 10 billion years since the light was emitted.

    Assume, for example, that two objects are 4 billion light years apart when one of the objects explodes. If the two objects are moving away from each other with a constant velocity of 0.6 c, the second object won't see the explosion for another ten billion years.
     
  9. Feb 18, 2007 #8
    O... ok, I think i understand now... tell me if i got it right:

    - light always travels at the same speed in the vacuum of space
    - the distance between those two points is actually growing (the earth and the gamma burst), but they are not actually "moving" in the stricter sense.
    - so the light ends up having to "travel" a longer distance than the object had to in order to get back to the earth, because the actual distance between those two points kept growing.
     
    Last edited: Feb 18, 2007
  10. Feb 19, 2007 #9

    DaveC426913

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    Bang on.
    (10 char)
     
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