General Relativity
In general relativity, the total invariant mass of photons in an expanding volume of space will decrease, due to the red shift of such an expansion (see Mass in general relativity). The conservation of both mass and energy therefore depends on various corrections made to energy in the theory, due to the changing gravitational potential energy of such systems.
Read more about this topic: Conservation Of Mass, Exceptions or Caveats To Mass/matter Conservation
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