> What’s more, there is an energy associated with any given volume of the universe. If that volume increases, the inescapable conclusion is that this energy must increase as well. And yet physicists generally think that energy creation is forbidden.
It's important to remember that energy conservation is not an axiom of basic physics. It is a consequence of Noether's theorem and temporal symmetry in the laws of physics. In the framework of GR, this means that any static metric will have some conserved quantity that corresponds to energy (though, in general, it will not be the same as the Newtonian energy). But if the metric is not static (as in the case of an expanding universe), there is no need for energy to be conserved.
It's important to remember that energy conservation is not an axiom of basic physics. It is a consequence of Noether's theorem and temporal symmetry in the laws of physics. In the framework of GR, this means that any static metric will have some conserved quantity that corresponds to energy (though, in general, it will not be the same as the Newtonian energy). But if the metric is not static (as in the case of an expanding universe), there is no need for energy to be conserved.