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Thread: CMB quadrupole

  1. #1
    Join Date
    Jan 2003
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    CMB quadrupole

    I know about the CMB dipole, caused by the movement of the sun around the galaxy, but there's something called CMB quadrupole? What's the cause of it?

  2. #2
    Join Date
    Dec 2003
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    2,305
    Our observations are mapped to a two-dimensional sphere around us, and the data is analyzed using spherical harmonic components. The single degree-zero component is just the average value over the entire sphere. Once that is subtracted out, the degree-one component (actually three components, but they resolve to a single component) is the direction of the strongest difference from one side to the other. It has a area of high value 180 degrees from an area of low value--the "dipole". The next components, the degree-two, have five coefficients, but they also resolve into a single component that is going to have a shape very similar to either a prolate or an oblate spheroid. That's the quadrupole, essentially two areas of high values surrounded by an equator (not necessarily our equator) of low values, or vice versa. (The five components are such that the quadrupole could be "pinched" at the equator, so that it has two areas of a value halfway between the high and the low. From another perspective, such a distribution would have alternating high-low-high-low around its equator--hence the "quadrupole" name.)

    No motion through space would produce the quadruple distribution.

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