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Space.com says intergalactic hydrogen gas absorbs all optical light

  1. Oct 29, 2009 #1
    What? Is there any truth to this? The caption on a space.com photo of the oldest object ever found says, "… since all the optical light has been absorbed by intergalactic hydrogen gas, leaving only infrared light." I'm thinking the expansion of space is a more likely explanation.

    http://bit.ly/2smLUp - photo with caption

    I found it in this article.
  2. jcsd
  3. Oct 29, 2009 #2


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    Considering we 'see' stars, it doesn't seem correct that all optical light has been absorbed.

    Is the photo caption refering to Milky Way stars in the foreground, as opposed to GRB which is far away?

    I wonder how the EM signature of GRB 090423 compares with others, and how much of the 'color' is due to redshift vs scatter or absorption.
    Last edited: Oct 29, 2009
  4. Oct 29, 2009 #3

    George Jones

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    From the Gemini Observatory website

  5. Oct 29, 2009 #4

    Jonathan Scott

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    The theory is that at present most of the intergalactic medium is ionized hydrogen, which lets most of the light through, but for a period in the early history of the universe, there would have been more neutral hydrogen gas around, which would absorb light. This is called the "Gunn-Peterson effect", and shows up as a "trough" feature in the spectra of the most distant quasars and galaxies.

    For a summary, see http://en.wikipedia.org/wiki/Gunn-Peterson_trough" [Broken] in Wikipedia.
    Last edited by a moderator: May 4, 2017
  6. Oct 29, 2009 #5
    Wow, you guys are great. That wikipedia page says it starts at about z=6. Ned Wright's calculator says that's about 12.7 billion years ago. So that GRB is well within that. Thanks.
  7. Oct 29, 2009 #6
    What I *think* they meant is that anything that we'd be seeing on earth now as visible light now, would have been extreme UV when it was produced and the neutral hydrogen would have absorbed all of that. Any light that the original object produced would end up as infrared by the time it gets to earth.
  8. Oct 29, 2009 #7
    Also neutral hydrogen isn't a huge absorber of visible light. If you shine light through a hydrogen balloon most of it makes it through. It is a major absorber of extreme UV since if you have neutral hydrogen and shine extreme UV in it, it will knock off the electrons and ionize the hydrogen.

    At z=8.2, anything that ends up as light on earth is going to start out as UV.
  9. Oct 29, 2009 #8
    That seems to be what happened....


    Lyman alpha is the amount of energy that it takes to take a hydrogen atom from the ground state to the next higher state. Once you start exposing hydrogen atoms to radiation that's more intense than Lyman-alpha, you start breaking up the atom and so the amount of radiation that the hydrogen lets through drops dramatically. For radiation that less intense than Lyman-alpha, all you end up doing is jiggling the atom so the hydrogen really doesn't block that much radiation.

    So we know what frequency you get this drop-off. If you see the drop-off in infrared, you then get a redshift. Get the redshift, you get a distance.
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