I'm reading the Wiki article below to say the invariant mass of photons in an expanding volume of space will decrease. I thought invariant mass of a photon was always zero and the energy of photon changed due to the expansion of space. So where did I go wrong?(adsbygoogle = window.adsbygoogle || []).push({});

, 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.

Quote from Wiki

General relativity

In general relativity, the total invariant mass of photons in an expanding volume of space will decrease

http://en.wikipedia.org/wiki/Conservation_of_mass#General_relativity

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# Invariant mass of a photon changes - from Wiki

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