MadSci Network: Astronomy |
Interesting question! The answer lies in the fact that Newton's laws fail with respect to the Universe. In general relativity, it is impossible to define a "total energy" of the Universe, so it is impossible to define what "conservation of energy" means for the Universe. This is why the cosmological redshift of photons does not violate conservation of energy. Also, there is no kinetic energy associated with the expansion of the universe. These issues apply both to a normal expanding universe as well as an accelerating one.
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