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Could the CMBR Be Very Uneven?

  1. Dec 11, 2008 #1


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    In a recent article in ScienceNews, Ron Cowen states:

    (quote) Hidden in the peaks and valleys imprinted on the cosmic microwave background — the radiation leftover from the Big Bang — is a wealth of information not only about the early universe but the distribution of matter throughout the cosmos. (endquote)

    The CMB photons (on their way to Earth) must collide with many, many particles of gas, dust, etc. which must change their direction of travel a little bit from each collision. I would think that by the time they reach us, they are quite a bit out-of-line with the spot on the CMB from which they started. In other words, they are being scattered like a piece of sand-blasted plate glass diffuses to a flashlight beam.

    Is it possible that the CMBR is not nearly as uniform as it appears to us here on Earth?

  2. jcsd
  3. Dec 11, 2008 #2


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    For definiteness, the peaks and valleys he's talking about represent variation by one part in 100,000. If I remember right the variation rms is about 18 microkelvin and the average is about 2.7 kelvin. That's less than one part in a hundred thousand.

    AFAIK the CMB photons that we detect have not collided with dust to any significant extent, or been scattered by gas molecules. It is reasonable to suppose that there has been some attrition, but my understanding is that what we detect is pretty much pristine radiation from the time of last scattering.

    That's a good physical analogy. Indeed if a large percentage of the CMB photons were getting randomly scattered en route to us that would wash out patterns of temperature variation and make the map much more uniform.

    My understanding is that this is not happening to any significant extent, but I could be wrong and I'd be glad to be corrected if that is the case.


    Frank, I'll hazard a guess as to how we can be sure.
    We see a lot of fine speckling in the temperature map that just wouldn't be there if most of the photons were being knocked out of line.
    Technically there is a level power distribution---as much amplitude in the small angle variation as there is in the large angle variation----as much variation in the little speckles as there is in the large splotches.

    If your cloudy glass blurring business was happening, then the little speckles would take a hit, I should think.

    The case is not air-tight, but that's an attempt at an argument. the other thing is, astronomers keep getting better instruments and there may come a time when the instrument resolution is high enough to pick up speckles a tenth the size of what we map now and they may see that they DO take a hit. There may be some blurring effect, like what you imagine, but we just haven't seen it because we haven't looked with a big enough magnifying glass.
  4. Dec 11, 2008 #3


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    A variation of one part in 100,000 is what we see now with our present equipment. My point is that the variation in the CMB could be much more than this, maybe tens times as much, because of photon scattering on the way here.

    How do we know that there have not been significant collisions of photons with particles? There must be a lot of stuff in space between here and there.

    The fine speckling we see could be fine because of scattering. If there were no scattering, what we could see might just astound us because of significant variations from place to place.

  5. Dec 13, 2008 #4

    George Jones

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    Look up the Sunyaev-Zeldovich effect or distortion.
  6. Dec 13, 2008 #5


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