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 Originally Posted by OngBonga
More on this. As I understand it, the photon is being "stretched" (for lack of a better word) by expanding spacetime. So as its frequency decreases, its wavelength increases proportionally.
So no energy is lost. Is this not the accepted model? It's like stretching an elastic band. Its mass remains constant, but its density does not.
Decreasing frequency is the same thing as increasing wavelength for any wave that moves at constant speed.
Since all photons move at c, this applies to all photons.
For photons
[wavelentgh] = c / [frequency]
or
[wavelength] * [frequency] = c
with c being appropriately the speed of light, which is a constant
The energy of a photon is
E_ph = h*[frequency] = h*c/[wavelength]
where h is Plank's constant (not the reduced constant)
Photons traveling through expanding spacetime have their wavelength increased (in the denominator), which, since they move at constant speed, means their frequency is reduced. All of which means the photons lose energy.
But here's the fucky part.
In a photon's non-inertial reference frame (frames moving at c are non-inertial), the universe is infinitely thin in its direction of travel and no time passes between when it is emitted and absorbed. Furthermore, the position of a photon is ill defined in any reference frame. The momentum of the photon is linked to its frequency as well, and since the momentum of the photon is fully defined, the position is fully undefined. Because of position momentum uncertainty.
So the sense of exactly what does it mean to say a photon is moving through expanding spacetime is not trivial.
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