It's interesting to note that, when you look at the universe as a whole, energy is not conserved. (It also isn't time- translation-symmetric.) It's been such a

It's interesting to note that, when you look at the universe as a whole, energy is not conserved. (It also isn't time- translation-symmetric.) It's been such a successful concept that you'll even find people denying that that can be the case. I'm one of them, somewhat -- I suspect it implies that we're missing part of the system, hence the universe as we know it isn't closed.