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B Conservation of Energy and Expansion Redshift

  1. Apr 1, 2017 #1
    Since the universe is expanding, photons emitted by distant stars are red-shifted, having their wavelengths stretched out. But, since the energy of a photon is dependent on the inverse of its wavelength, doesn't that mean that the expansion of the universe is causing photons to lose energy? How does this not violate the conservation of energy?

    A similar question deals with the admittedly minuscule but still present changes in potential energy between, for example, seperated charges or masses when the space between them expands. I know the effect is likely not measurable, but energy can't just be appearing, right?
    Last edited: Apr 1, 2017
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  3. Apr 1, 2017 #2


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    @John Morrell

    Just a note ... you are posting a bunch of questions that don't really belong in the general physics section
    you should be posting these in the cosmology sub-section under the astronomy section :smile:

    I have asked for this one to be moved

  4. Apr 2, 2017 #3


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    Energy is not conserved in an expanding universe.
  5. Apr 2, 2017 #4
    Okay, mind slightly blown. Also thanks for telling me about that catagory; somehow I didn't see it when I posted.
  6. Apr 2, 2017 #5


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    GR does not provide an unambiguous definition of energy, so it is theoretically justified to exempt photons from the principle of energy conservation. There is, however, another way of looking at things. If you assume expansion constitutes a form of work, it would be reasonable to suspect the missing photon energy helps power it. By the same token should the universe ever collapse, it would be reasonable to expect photons to become blue shifted, thus recovering the energy loss due to expansion. In a quantum theory of gravity, a less ambiguous definition of energy may emerge, and help resolve the issue of energy conservation. So, yes, photons lose energy via redshift, but, it to conclude that it simply vanishes is not yet warranted.
  7. Apr 3, 2017 #6
    I don't mean this as a personal theory, but is it not possible that the dark energy causing the expansion of the universe is what makes up the energy difference caused by the redshifting of photons?
  8. Apr 3, 2017 #7
    Does the CMB have less energy now than when it was emitted, or is the same energy just spread out over a larger space and presents as redshift?

    Not sure if that makes sense...
  9. Apr 3, 2017 #8


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  10. Apr 5, 2017 #9


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    There is a way to make this match up with the math (by exploiting a similarity with the Newtonian equations that describe a freely falling object in a gravitational field, and thereby defining a "gravitational potential energy" for the universe), but AFAIK it only works for a closed, matter-dominated universe with zero cosmological constant/dark energy.

    Since AFAIK the mathematical similarity I referred to above does not work for photons (more precisely, it doesn't work for a radiation dominated universe), I don't think this can be justified by the math. And even if it could be, as above, I don't think it would work for a nonzero cosmological constant/dark energy.

    I don't think so. See above.
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