I've seen posters on a number of forums, including this one, telling us that conservation of energy doesn't apply to cosmic microwave background radiation. This is of some concern because, for example, if the temperature of space doesn't follow the First Law, then how can I believe anyone talking about what should happen to the "entropy of the universe" in regard to the Second? Sorry if this is a basic point, but it seems crucial to have some confidence about it.
Now clearly the photons have the same energy if you accelerate to the frame in which they were emitted to receive them, but you can only do that in one direction. Another issue is whether the number of photons associated with CMB could have increased over time, and if so how. (Unruh...?)
One way to look at it is to calculate the total energy of the CMB relative to other matter in the cosmos - but I don't know if that is a valid approach nor what past values of this number were.
More appealing is the argument that the light is simply stretched, with the energy dispersed over a larger area. Problem in my mind is that the Stefan-Boltzman law scales per the fourth power (so an 1100^4 reduction in intensity), but the volume would seem to scale as the third power (so an 1100^3 increase in volume). But I'm not sure the peaks on the black-body curve are the right part to compare...
Question: Is there a less-difficult way (not requiring a full understanding of GR and cosmology) to clear up these issues and explain how (if) conservation of energy applies to CMB?
