Posted: Jan 31, 2020 12:15 pm
newolder wrote:To clarify:...
But many people have just this reaction. It’s clear that cosmologists have not done a very good job of spreading the word about something that’s been well-understood since at least the 1920’s: energy is not conserved in general relativity. (With caveats to be explained below.)
The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance. A fancy way of saying “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved.
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More @ Energy Is Not Conserved by Sean Carroll
Finally, if "mass decays" but not "completely leaves the system" then the system (the universe) has changed, i.e. it's not constant.
No. The system has indeed changed, but it has its foundations upon energy that remains constant.