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Assuming a flat FLRW universe that is expanding: In comoving space, does the energy of a photon decrease or stay constant? A physical argument for this would be nice.

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The energy of the photon decreases as the reciprocal of the scale factor.

Typically, this can be interpreted as the wavelength being "stretched" by the expansion, but alternatively you can think of it representing the accumulated Doppler shift between the frame in which the photon was emitted and the frame in which its energy is measured.

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  • $\begingroup$ Thanks. What if the question was looking at physical space rather than comoving space? Does the energy of the photon in physical space also decrease by a factor of the scale factor? $\endgroup$
    – Matrix23
    Commented May 19 at 6:24

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