Can a Photon's Energy Approach Zero?

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jonmtkisco
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Is there any theoretical minimum energy value for a photon of near-infinite wavelength?

As the expansion of the universe continues redshifting the photons of free radiation (e.g., the remnants of the CMB), is it theoretically possible that the energy of individual photons could decrease to infinitely close to zero, or is there some sort of minimum cutoff value?

Jon
 

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Except, of course, that the real universe we live in has lots of free electrons (along with protons), albeit rather far apart (on average).

This means photons will have an interesting existence (shall we say) if their frequency is below the plasma frequency of the IGM ...
 
Calculate the energy of a photon with a wavelength of about 13.7 billion light years.