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Question about the CMB and WMAP

  1. May 22, 2014 #1
    A very basic question that is stalking me for more than 20 years.

    Every popular science cosmology book states:

    A variation in temperature in the CMB of 1 part in 100.000, and then, from place to place or even from point to point.

    And then the WMAP picture. Everywhere hot en cold spots many times greater than:from point to point.

    Can anybody explain what is the relation? Or what is the flaw in my longtime thinking? Is 1 part in 100.000 not 1/100000 th (I doubt it)?
  2. jcsd
  3. May 22, 2014 #2


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    The WMAP picture is rescaled to magnify the variations so that you can see them. If it were shown with a non-magnified scale (say with the colors running from 0.0-3.0K), it would be a featureless smooth map, with all pixels the same color. Instead, the scale is centered around 2.74K and runs +/- 200-300 μK so that you can see the variations. Is this your question?
  4. May 22, 2014 #3
    Thank you, much clearer.

    Why do they say that the amplitude of the temperature variation (I suppose difference maximum-mean) has just the right size to create structure?
  5. May 22, 2014 #4


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    I wouldn't state it in that way. The very early universe had very small non-uniformities consistent with quantum fluctuations. In other words, the uncertainty principle does not allow the early universe to be perfectly uniform. Gravity causes these non-uniformities to grow with time. By the time of the emission of the CMB radiation (when the age of the universe was about 400,000 years), the non-uniformities had grown to about 1 part in 100,000, as you said. They then continued to grow to give rise to the structure that we see today. So instead of saying the the CMB non-uniformities were just the right size to create structure, I would say that the observed non-uniformities that we see in the CMB are consistent with the observed non-uniformities that we see in the large-scale structure of the universe today.

    By the way, if you want to see some videos of the structure growth, try going to the Illustris Project and watching some of the video links. These are fascinating videos of simulations of the growth of cosmic structure.

    Here is the link: http://www.illustris-project.org/
    Last edited: May 22, 2014
  6. May 22, 2014 #5
    Thanks, I wil watch the video.
    One more question about the amplitude. Sound waves have maximal compressions and maximal rarefactions. So are we speaking then about total amplitudes?
  7. May 22, 2014 #6


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    I don't know how to answer that. The amplitude can be specified in many different ways, such as some number (1,2,3,...) of standard deviations, FWHM, total deviation, etc. It would depend on which source you are referring to.
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