Da big bang

[Viking]

Now as it is a photon it only travels at the speed of light. It doesn't slow down. Therefore if it loses energy it will have to change its wavelength/frequency. So as it loses energy it redshifts.

According to a wiki article, the speed of light in water is 0.75 c. I'm not sure about the validity of that velocity, but the fact that the speed of light depends on the medium is the cause of Cherenkov radiation. Photons do not have any mass; such that the velocity would not really matter for the energy?



Since space is not a perfect vacuum, one would assume that even in the so called vacuum of space; light does not travel at a constant speed?

Of course, none of this appear to have any relevance to the debate at hand (a debate which is a bit beyond me).