Finally I have found the time to read your post! It was really due, as this post is a great one (as all your posts usually are). As you may have noted it from elsewhere, last week was really intense for me and I had to decouple from the chain. However, I am back and slowly catching up with all blogs posted on STEMsocial last week.

I have a single thing to say here, more as a comment than as anything else. The topic is indeed a bit far from my zone of comfort…

In high-energy physics, energy conservation is related to the translational invariance of the laws of physics. However, in an expanding universe there is no such a translation invariance so that on a global scale there is no rule telling us that energy should be conserved. Therefore, there is no reason for energy conservation and this principle can be violated. Notice that I say "can", and not "must".

Therefore in an expanding universe care must be taken when we discuss even the mere notion of energy. For instance, it is very possible to see the energy of the CMB photons going into the expansion of the universe. This would provide a way for the energy to be a conserved quantity.

I wish you a very nice week! Cheers!