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Is Energy Conserved When Photons Redshift In Our Expanding Universe?

21 Aug 2019, 14:01 UTC
Is Energy Conserved When Photons Redshift In Our Expanding Universe?
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When a light source moves in a particular direction, the light gets blueshifted along the direction of motion and redshifted against the direction of motion. This Doppler redshift is superimposed atop, and independent of, any cosmological redshift that’s due to the expanding Universe. If the Universe were contracting, there would be a cosmological blueshift instead. (WIKIMEDIA COMMONS USER BREWS OHARE)When the Universe expands, photons redshift to longer wavelengths and lower energies. So where does that energy go?Imagine the ultimate version of a toy Universe: it’s expanding, it’s full of material, and through it all, there’s one photon ⁠ — or quantum of light ⁠ — that we keep track of and forbid from interacting with any other particle. The photon, at any given time, will have all the properties you expect a quantum of electromagnetic radiation to have, including a direction of propagation, a polarization for its electric and magnetic fields, and a wavelength that dictates how much energy is inherent to that photon.Well, as photons travel throughout the expanding Universe, they experience the effects of that expansion, which stretch it to longer wavelengths. Longer wavelengths imply a decreased energy, and a decrease in energy implies that either energy is ...

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