From the big bang to the present - has the mass + energy of the universe changed ? I understand that there could be dark matter etc. etc. but as far as current Physics goes - is mass + energy essentially constant as the universe expands ?
Probably not. Kinetic energy gets damped out by something called “cosmological drag”: Basically, if you’re moving at some speed, eventually you’ll end up in a region of the Universe where that speed is at rest. Meanwhile, the dark energy content of the Universe is continually increasing, since it has (or at least, appears to have) a constant energy density, but the volume of the Universe is increasing. One could contrive a universe where these two effects canceled each other out, but that wouldn’t be our Universe.
And no, this is not a violation of the Law of Conservation of Energy. That’s a local law, and need not apply globally.
Thanks Chronos for the reply. Since dark energy is a part of our universe and since it is continually added - is it fair to say that the parts of our universe did not start at the big bang (vis-a-vis the newly added dark energy) ?