I just want to point out that the fact that the universe is expanding does not mean that it breaks time symmetry in a way that is relevant for Noether’s theorem. Systems can change over time and conserve energy. Furthermore, Noether’s theorem describes local physics, and local physics is unchanged by general relativity. Also, if you consider the energy of spacetime itself, then energy is conserved in the universe. There is a caveat that in GR the energy of spacetime curvature can’t be localized in the way energy usually is. This makes some people uncomfortable, and they choose to say energy is not conserved for the universe (seemingly ignoring that spacetime carries energy as in gravitational waves). You can just as well say that energy is conserved and that the energy of spacetime is only well defined for the universe as a whole.
ozymandian___ t1_jd0eij6 wrote
Reply to comment by BlueParrotfish in If the universe is constantly expanding, how is the law of conservation of energy obeyed? by Pristine_Pop_7818
I just want to point out that the fact that the universe is expanding does not mean that it breaks time symmetry in a way that is relevant for Noether’s theorem. Systems can change over time and conserve energy. Furthermore, Noether’s theorem describes local physics, and local physics is unchanged by general relativity. Also, if you consider the energy of spacetime itself, then energy is conserved in the universe. There is a caveat that in GR the energy of spacetime curvature can’t be localized in the way energy usually is. This makes some people uncomfortable, and they choose to say energy is not conserved for the universe (seemingly ignoring that spacetime carries energy as in gravitational waves). You can just as well say that energy is conserved and that the energy of spacetime is only well defined for the universe as a whole.