AMA | September 2024

8171.477 - 8193.051 Sean Carroll

And that gives you a number that is not conserved in cosmology. That's not because dark energy is weird. It's not conserved if your universe has nothing in it but photons also. Every photon loses energy as space expands. What's really going on is that there's an interplay between the energy of the stuff, the photons or the dark energy or whatever, and the curvature of space-time.