Previously we were writing that the internal energy transformed as a^{3(\gamma-1)}, which is incorrect. The pressures were also written to transform as a^{3\gamma}. In this MR I have rectified this to be correct, as
As we store P' and want the conversion factor to physical, we need it to be a^{-3\gamma}.
As for the internal energy, we can have a similar argument
where we need the factor for u to be a^{-3(\gamma - 1)}.