Expansion, redshift, energy loss

[zaybu]

Maybe this question has already been asked, anyway, here it goes. The universe is expanding and according to relativity, the radiation is redshifted, meaning there is a loss of energy. If energy is conserved, where does that loss of energy go?

 

[nicksauce]

Newtonian point of view: Since the radiation has a pressure, it does PdV work in expanding the universe, which exactly makes up for the loss of energy.

GR point of view: Since the FRW metric does not have a time-like Killing vector, there is no reason to expect energy to be conserved.