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Cosmic quadrupole

  1. Dec 1, 2004 #1
    When cosmologists say that the cosmic quadrupole could have a smaller amplitude than expected (there is a 1 in 7 chance this is true according to statistics),does this mean less energy in cmbr photons or that the distance between the poles of the quadrupole is smaller than expected?
  2. jcsd
  3. Dec 1, 2004 #2


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    I think the distance between the poles of anisotropy of the quadrupole is always 360°/4 = 90° in the sky. The quadrupole, or any of the other multipoles, are terms of an expansion of the temperature anisotropies in spherical harmonics and their amplitude is therefore related to the energy of the photons. To assign amplitudes to each of the multipoles in the expansion, the empirical data of the temperature anisotropies in the CMB has to be processed in order to find correlations over different angular scales (for a fixed angle the average of the temperature anisotropies is computed). The bigger the angle chosen, the fewer the information which is available to perform an average and get a correlation value. For very high angular scales this problem is called "cosmic variance" and may lead to a wrong amplitude of the octopole and the quadrupole. This is my (rather qualitative) understanding.
  4. Dec 2, 2004 #3
    Interestingly, the Cosmic Variance is less important for higher multipoles than the quadrupole, as explains this page:

    "Take the quadrupole anisotropy as an example. This is a thing on the sky which has five coefficients. Each of those coefficients is some random number (a Gaussian random variate in the standard models), and so the statistical distribution expected for the quadrupole amplitude is like the sum of five Gaussians (a 2 distribution with 5 degrees of freedom), and so is pretty wide. This means that measuring the actual quadrupole on our sky doesn't tell you a whole lot about the expectation value of quadrupoles for a bunch of observers. You can think of this uncertainty as a "theoretical error bar" if you like. For higher multipoles (smaller angular scales) there are a lot more samples, and so the "Cosmic Variance" is less important. So if you're trying to determine the angular power spectrum, then at large angles you tend to be dominated by "Cosmic Variance", while at small angles you're dominated by instrumental noise."
  5. Dec 2, 2004 #4
    Why does the low power (if there is indeed a discrepancy between theory and experiment) seem to be along one axis of the quadrupole and one axis of the octopole? Has this got something to do with dark matter being unevenly distributed around our galaxy, perhaps?
  6. Dec 2, 2004 #5
    There are outstanding images of the quadrupole and octopole and hexadecapole at:

    http://arxiv.org/abs/http://www.arxiv.org/abs/astro-ph/0302496 [Broken]
    Last edited by a moderator: May 1, 2017
  7. Dec 2, 2004 #6


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    I took a look to the paper you have mentioned in your previous post and it is quite complicated for me to understand. Especially, it is not clear for me why the features of the quadrupole which are mentioned in the paper should be a reason to assume a preferred direction in the sky and how far this may be related to the cosmic variance problem. Anyway, I would wonder if the conclusions of the paper were a widely accepted result. I will try to spend some time with this.
  8. Dec 2, 2004 #7
    This weeks nature has some answers, the author's of a recent paper have called into question the delay of WMAP year 2 data, being that there has been a delay, reasons for delay has not been forthcoming.

    I believe Thomas Larsson has highlighted this in SPR boards:https://www.physicsforums.com/showthread.php?t=54886

    I do recall that when the year 1 WMAP came to press, there was a great deal of over-enthusiastic backslapping, countered by the revelation within 48hrs of :They also looked for correlations with any known directions on the sky. No significant correlation with the Milky Way was found, but a strong correlation with the orientation of the solar system (ecliptic plane) and with its motion (measured as the CMB dipole) showed up.

    now as It was I who had been doing the 'handwaving' with respect to the above effect anomaly originaly at 'superstringtheory' boards, and latter at the original PF forum, I find the recent scientific 'kefuffle' really amusing :biggrin:
    Last edited: Dec 3, 2004
  9. Dec 2, 2004 #8


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  10. Dec 3, 2004 #9
    Of course!..my apologies :eek:
  11. Dec 4, 2004 #10


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    As far as I have understood there are two different aspects here (just to clarify).

    First, the significance of a small quadrupole, this means, a small amplitude in the spots of the quadrupole. This is sometimes interpreted as a possible indication of a finite geometry of the universe, but is not accepted as an evidence due to the cosmic variance problem.

    On the other hand, the significance of the planarity of the quadrupole, this means, the spots of the quadrupole are located nearly on the same plane in the sky. This is the aspect which is treated in the paper of Tegmark. It seams that Tegmark computes the scalar product of the vectors pointing to the spots in the quadrupole and octopole and uses a statistical method to understand the relevance of the high value this product.

    The intepretation of this is still unclear for me. Of course, this suggests that there is a non isotropic orientation of the quadrupole and octopole, as pointed out by Tegmark, but I still do not see which physical interpretation follows from this. For example, may be the density perturbations were aligned due to geometric constraints in the primitive universe? Any suggestions?
  12. Dec 4, 2004 #11


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    The universe is a large donut?


    (Toroidal topology)
  13. Dec 4, 2004 #12
    For example, may be the density perturbations were aligned due to geometric constraints in the primitive universe? Any suggestions?

    Rothie M:
    The authors mentioned close to the end of the paper that a universe which had one spatial dimension suppressed would account for the unexpected features of the quadrupole.
  14. Dec 4, 2004 #13
    The gravity of a second universe of missing antimatter, to one side of our universe, would flatten one spatial dimension in our universe,by stretching it in one direction.
    Last edited: Dec 4, 2004
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