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Light that reaches us

  1. May 28, 2012 #1
    Please correct me if I am wrong:
    the oldest light that we just begin to receive is around 14 billion light years old.

    This light originated from a lightsource that was about
    30 million light years away (380,000 years after big bang.)

    So although the lightsource was 30 million light years away, the light took about 14 billion light years to reach us, because of the expansion of the universe.

    At the present, the lightsource is physically located about 46 billion light years away from us. (although its light has traveled 14 billion years only).

    However, by the same reasoning, the light that is just leaving the light source will take more than 46 billion light years to reach us.

    Therefore, the new light from the identical light source takes significantly longer time to reach us. As more and more new light is formed, then, the time it takes for the new light to reach us will evermore increase, until it increases to infinity.

    Therefore, the object will some day no longer be visible.

    Is my reasoning correct? Also, if this is correct, does that mean
    the object that we barely begin to see
    (the first light from big bang reaching us as a cosmic microwave background),
    will appear as a cosmic microwave background
    and then disappear completely in the later future?
  2. jcsd
  3. May 29, 2012 #2


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    Gold Member

    It will continue to redshift until its wavelength exceeds the cosmic background radiation. It will then be undetectable.
  4. May 29, 2012 #3
    Also, I think the time to reach us does approach infinity, but I think it will remain finite for any finite amount of time. So it would take an infinite amount of time for it to no longer be visible.
  5. May 29, 2012 #4


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    Staff: Mentor

    Would it ever exceed the wavelength of the CMB?
  6. May 29, 2012 #5
    No. Your reasoning is too loose. It is more correct to say that the time it takes for the light to reach us increases without bound. But it will never reach infinity. It will always be a finite number.
  7. May 29, 2012 #6
    4ever...that explanation seems really good....congrats!!!!
    Here is another I keep in my notes [from these forums] almost like yours, but you'll see it starts a little earlier, uses 41mly instead of your 30mly at age 380,000 yrs, and shows how the scalefactor works:

    Also if you are interested, see the illustrations by Lineweaver and Davis here, figure 1:


    They take some careful reading, some thought and study, but if you can figure them out, you'll have a throrough understanding of cosmological expansion and measures....and then you can explain them to me!!!!
  8. May 29, 2012 #7
    Thank you so much for all your help everyone! and Naty1, thank you for such a strong enthusiasm. I will sure read that article and ask you more if you do not mind!
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