I am reading the book The Early Universe by Kolb and Turner and found the following:(adsbygoogle = window.adsbygoogle || []).push({});

"In the strictest mathematical sense the universe cannot be in thermal equilibrium, as the FRW cosmological model does not possess a time-like Killing vector".

It is just a matter of calculations to show that the Killing-equation does not have a

time-like solution in the Robertson-Walker metric, but I don't know how this implies anything

about thermal equilibrium. The books I have do not treat relativistic thermodynamics,

can someone explain this or point me to a good reference?

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# Thermal equilibrium in GR

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