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Microwave BG vs Expansion vs Hydrogen Microwave Emission

  1. May 18, 2008 #1
    With 90% of the observable matter in the universe being Hydrogen (H2), why is it that the microwave background is attributed to the big band and not the emission of microwaves from excited state hydrogen?
  2. jcsd
  3. May 18, 2008 #2
    Good question. Assuming the cosmic background radiation is coming from the temperature of hydrogen gas within the last billion years, we would expect to see it gathered around visible galaxies. This is not the case, as the cosmic background radiation is nearly uniform in every direction. Furthermore, the intensity of the cosmic background radiation at 2.725 kelvin happens to fit perfectly with the big bang theory's prediction!

    Another way of answering you is to say that the cosmic background radiation IS being attributed to BOTH "the big bang" and to "the emission from excited hydrogen". You see, about half a million years after the big bang, the universe had cooled enough for it to be transparent to light. When this happened, photons generated by hot hydrogen ions (read: protons) were created in abundance. These photons are still out there to this day, on their long journey to end up in one of our telescopes.
  4. May 18, 2008 #3
    From the "inverse emission" image provided on Wiki (link http://upload.wikimedia.org/wikipedia/commons/2/28/WMAP_2008.png), I can not easily resolve where the microwaves are coming from. Based on that image, it seems difficult to justify the interpretation.

    When you write that the CMB is nearly uniform in every direction, should I interpret that as "as far as we can measure from the surface of the planet earth" ?

    If the CMB is due to the Big Bang, then shouldn't we be able to locate the point of origin, or should I believe that the Big Bang was spread uniformly across the universe?

    When you write "end up in one of our telescopes." Does that mean that there are many telescopes equipped to receive Microwaves?
  5. May 18, 2008 #4


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    CMB are located everywhere. Compare with the CNB (Cosmic neutrino background).

    Big bang happened everywhere, there are MANY threads on this fourm that deals with that question "where did big bang occur?", "where is the centre of the universe" and so on.
  6. May 18, 2008 #5


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    That's a plot of the microwave background in galactic coordinates.

    Well, no, since the satellites that measure it are not on the earth.

    There is no "point of the big bang". This is a common misconception of the big bang theory: there is neither a centre of the universe, nor a point at which the big bang happened.

    Note that I have moved this to the Cosmology forum.
  7. May 18, 2008 #6
  8. May 19, 2008 #7
    Another way of answering you is to say that the cosmic background radiation IS being attributed to BOTH "the big bang" and to "the emission from excited hydrogen".

    If we were in a static universe we would see radiation from all the matter around us at whatever the average temperature was, say 2.7 kelvin (maybe this is not correct?). There must be some difference between that black body radiation and the big bang radiation, otherwise people would not have been so startled by the discovery. How would/did/do they differ? I realize that we are demonstrably not in a static universe, but I'm not sure it was such a secure fact when the CMB was first discovered. Something about it got everybody's attention, but I've never understood exactly what it was.
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