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I have been reading a lot about Cosmological experiments such as Planck or WMAP that aim to investigate the CMB's power spectrum. I am unclear though as to what theses sky surveys actually measure, is it the distance to regions of temperature fluctuations?

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They measure the spectrum of microwaves (with pretty good angular resolution) over the whole sky.

The spectrum represents the red-shifted black-body of the hot, early universe plus local contributions. They separate out the underlying black-body spectrum and that is the signal.

Much information can be extracted from the temperature- and angular-fluxuations of the signal.

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