I would expect that energy of photons in the Cosmic ray Microwave Background gets less and less because their wavelengths are stretched due to the expansion of space.

How can this be possible? Does it only look like their wavelength is increasing because we, as observers are not "expanding along"?

Where does the energy of the decaying CMB go?

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    $\begingroup$ See physics.stackexchange.com/q/259759/123208 & links therein. $\endgroup$
    – PM 2Ring
    Jun 8 at 16:51
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    $\begingroup$ It doesn't go anywhere. $\endgroup$
    – pela
    Jun 8 at 18:14
  • $\begingroup$ @planetmaker But the wavelength of each photon decreases. This means (the number of photons stays the same) that the total energy goes down. (as seen from our perspective). $\endgroup$
    – Methadont
    Jun 9 at 8:21
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    $\begingroup$ @pela That's a very nice article! $\endgroup$
    – Methadont
    Jun 9 at 8:33
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    $\begingroup$ Barbierium, you should check out Prof Davis's homepage. There's lots of great stuff in the Research section and videos in the Talks and Outreach section. FWIW, her PhD topic was on the expansion of space, and her papers and popular articles "Clarifying common misconceptions about the expansion of the universe and cosmic horizons" are very highly regarded. $\endgroup$
    – PM 2Ring
    Jun 9 at 8:54

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