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How could visible light be in pitch-black? Can it be?

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    $\begingroup$ Read the sentence the correct way around. It's pitch black at visible wavelengths. As opposed to radio wavelengths, where it is not pitch black, but glowing. $\endgroup$ Commented Nov 25, 2016 at 10:36
  • $\begingroup$ By sky, they mean deep space, not our nice blue atmosphere. Olbers' paradox: en.wikipedia.org/wiki/Olbers'_paradox $\endgroup$ Commented Nov 25, 2016 at 15:57
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    $\begingroup$ It seems you interpret the sentence as "the light is black", but it's just a way of saying "there is no light (at optical wavelengths, but there is at radio wavelengths)". I don't see why people would downvote your question, though. $\endgroup$
    – pela
    Commented Nov 25, 2016 at 20:09
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    $\begingroup$ @pela you should make your comment the answer. $\endgroup$
    – LaserYeti
    Commented Nov 26, 2016 at 1:55

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Black is not a color; it is the absence of light.

I think you interpret the sentence as "the light is black", but it's just a way of saying "there is no light (at optical wavelengths, but there is at radio wavelengths)".

In my opinion this is not a good way of explaining the CMB. The sky is not black at any wavelengths, but has a background radiation at all wavelengths. In the optical, this background originates (primarily) from the integrated light from stars; in the microwave it originates (primarily) from the afterglow of Big Bang; in the infrared from dust, etc.

See the figure in this answer to a question about the number density of CMB photons, to see how large the background is in various wavelength regions.

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