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Tensor Einstein Equations

The Einstein equations tell us that the tensor metric source is governed by
\ddot H + 2{\dot a \over a} \dot H +
k^2 H = 8\pi G a^2 [
p_f^{\vphantom{{(2)}}} \pi^{{(2)}}_f + \pi^{{(2)}}_s]\, ,\end{displaymath} (60)
where note that the photon contribution is $\pi^{(2)}_\gamma = {8 \over 5}\Theta_2^{(2)}$.

Wayne Hu