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Today in physics we were looking at how the energy of a photon is the product of Planck's constant and the frequency of the photon, therefore the lower the frequency, the lower the energy of the photon.

The Cosmic Microwave Background, which I understand is a literal stretching of space over time, therefore increasing the wavelength of the radiation emitted during the big bang, being electromagnetic waves (photons), which will of course decrease the frequency.

So, if over time an individual photon's frequency has decreased, so has its energy... Where has the excess energy gone?

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marked as duplicate by anna v, Qmechanic Sep 14 '18 at 0:44

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Remember that the energy of a photon is a frame variant quantity. If the emitter at the time of emission is moving towards the detector then it will be higher and if it is moving away then the energy will be lower. At no point did it lose any energy, but the detected energy is different.

The radiation of the CMB comes from a surface called the surface of last scattering. This surface is very old and very far away, and in cosmology things that were very far away are all moving away from us. Therefore there is a Doppler-like redshift. Nowhere along the path did the photon lose energy, but if you “parallel transported” the velocity of the surface of last scattering to here and now you would find that it is moving very rapidly away from us.

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