- #1
jstrunk
- 55
- 2
1. I can't understand one step in the derivation of the Einstein tensor from the Bianchi identity.I have looked in a lot of books and all over the internet and everyone glosses over the same point as if its obvious, but it isn't obvious to me.
2. Below is the entire derivation. It seems to be the usual one that most authors use but I have spelled it out in more detail. The problem is in the third term going from step 9 to step 10. I haven't seen any justification for it. All the identities I have to work with involve Riemann tensors with one or zero upper indexes, none have two upper indexes. I tried to work it out explicitly by expanding the Riemann and Ricci tensors in terms of metrics but that quickly became a morass. I don't think that is how the authors are doing it. I think they are using some
identity or technique that I don't know.
3.
[tex]
\[
\begin{array}{l}
R_{\alpha \beta \mu \nu ;\lambda } + R_{\alpha \beta \lambda \mu ;\nu } + R_{\alpha \beta \nu \lambda ;\mu } = 0 \\
\left[ {g^{\alpha \mu } R_{\alpha \beta \mu \nu } } \right]_{;\lambda } + \left[ {g^{\alpha \mu } R_{\alpha \beta \lambda \mu } } \right]_{;\nu } + \left[ {g^{\alpha \mu } R_{\alpha \beta \nu \lambda } } \right]_{;\mu } = 0 \\
\left[ {R_{\beta \mu \nu }^\mu } \right]_{;\lambda } + \left[ {R_{\beta \lambda \mu }^\mu } \right]_{;\nu } + \left[ {R_{\beta \nu \lambda }^\mu } \right]_{;\mu } = 0 \\
R_{\beta \mu \nu ;\lambda }^\mu + R_{\beta \lambda \mu ;\nu }^\mu + R_{\beta \nu \lambda ;\mu }^\mu = 0 \\
R_{\beta \mu \nu ;\lambda }^\mu - R_{\beta \mu \lambda ;\nu }^\mu + R_{\beta \nu \lambda ;\mu }^\mu = 0 \\
g^{\beta \nu } R_{\beta \nu ;\lambda } - g^{\beta \nu } R_{\beta \lambda ;\nu } + g^{\beta \nu } R_{{\rm{ }}\beta \nu \lambda ;\mu }^\mu = 0 \\
\left[ {g^{\beta \nu } R_{\beta \nu } } \right]_{;\lambda } - \left[ {g^{\beta \nu } R_{\beta \lambda } } \right]_{;\nu } + \left[ {g^{\beta \nu } R_{{\rm{ }}\beta \nu \lambda }^\mu } \right]_{;\mu } = 0 \\
\left[ R \right]_{;\lambda } - \left[ {R_{{\rm{ }}\lambda }^\nu } \right]_{;\nu } + \left[ {R_{{\rm{ }}\nu \lambda }^{\mu \nu } } \right]_{;\mu } = 0 \\
R_{;\lambda } - R_{{\rm{ }}\lambda }^\nu _{;\nu } + R_{{\rm{ }}\nu \lambda }^{\mu \nu } _{;\mu } = 0 \\
R_{;\lambda } - R_{{\rm{ }}\lambda }^\nu _{;\nu } - R_{{\rm{ }}\lambda }^\mu _{;\mu } = 0 \\
R_{;\lambda } - R_{{\rm{ }}\lambda }^\mu _{;\mu } - R_{{\rm{ }}\lambda }^\mu _{;\mu } = 0 \\
R_{;\lambda } - 2R_{{\rm{ }}\lambda }^\mu _{;\mu } = 0 \\
2R_{{\rm{ }}\lambda }^\mu _{;\mu } - R_{;\lambda } = 0 \\
Define{\rm{ }}G^{\alpha \beta } \equiv g^{\alpha \nu } \left[ {R_{{\rm{ }}\nu }^\beta - \frac{1}{2}\delta _{{\rm{ }}\nu }^\beta R} \right] = R^{\alpha \beta } - \frac{1}{2}g^{\alpha \beta } R = G^{\beta \alpha } \\
G_{{\rm{ }};\beta }^{\alpha \beta } = \left[ {2R_{{\rm{ }}\lambda }^\mu - \delta _{{\rm{ }}\lambda }^\mu R} \right]_{;\beta } = 0 \\
\end{array}
\]
[/tex]
2. Below is the entire derivation. It seems to be the usual one that most authors use but I have spelled it out in more detail. The problem is in the third term going from step 9 to step 10. I haven't seen any justification for it. All the identities I have to work with involve Riemann tensors with one or zero upper indexes, none have two upper indexes. I tried to work it out explicitly by expanding the Riemann and Ricci tensors in terms of metrics but that quickly became a morass. I don't think that is how the authors are doing it. I think they are using some
identity or technique that I don't know.
3.
[tex]
\[
\begin{array}{l}
R_{\alpha \beta \mu \nu ;\lambda } + R_{\alpha \beta \lambda \mu ;\nu } + R_{\alpha \beta \nu \lambda ;\mu } = 0 \\
\left[ {g^{\alpha \mu } R_{\alpha \beta \mu \nu } } \right]_{;\lambda } + \left[ {g^{\alpha \mu } R_{\alpha \beta \lambda \mu } } \right]_{;\nu } + \left[ {g^{\alpha \mu } R_{\alpha \beta \nu \lambda } } \right]_{;\mu } = 0 \\
\left[ {R_{\beta \mu \nu }^\mu } \right]_{;\lambda } + \left[ {R_{\beta \lambda \mu }^\mu } \right]_{;\nu } + \left[ {R_{\beta \nu \lambda }^\mu } \right]_{;\mu } = 0 \\
R_{\beta \mu \nu ;\lambda }^\mu + R_{\beta \lambda \mu ;\nu }^\mu + R_{\beta \nu \lambda ;\mu }^\mu = 0 \\
R_{\beta \mu \nu ;\lambda }^\mu - R_{\beta \mu \lambda ;\nu }^\mu + R_{\beta \nu \lambda ;\mu }^\mu = 0 \\
g^{\beta \nu } R_{\beta \nu ;\lambda } - g^{\beta \nu } R_{\beta \lambda ;\nu } + g^{\beta \nu } R_{{\rm{ }}\beta \nu \lambda ;\mu }^\mu = 0 \\
\left[ {g^{\beta \nu } R_{\beta \nu } } \right]_{;\lambda } - \left[ {g^{\beta \nu } R_{\beta \lambda } } \right]_{;\nu } + \left[ {g^{\beta \nu } R_{{\rm{ }}\beta \nu \lambda }^\mu } \right]_{;\mu } = 0 \\
\left[ R \right]_{;\lambda } - \left[ {R_{{\rm{ }}\lambda }^\nu } \right]_{;\nu } + \left[ {R_{{\rm{ }}\nu \lambda }^{\mu \nu } } \right]_{;\mu } = 0 \\
R_{;\lambda } - R_{{\rm{ }}\lambda }^\nu _{;\nu } + R_{{\rm{ }}\nu \lambda }^{\mu \nu } _{;\mu } = 0 \\
R_{;\lambda } - R_{{\rm{ }}\lambda }^\nu _{;\nu } - R_{{\rm{ }}\lambda }^\mu _{;\mu } = 0 \\
R_{;\lambda } - R_{{\rm{ }}\lambda }^\mu _{;\mu } - R_{{\rm{ }}\lambda }^\mu _{;\mu } = 0 \\
R_{;\lambda } - 2R_{{\rm{ }}\lambda }^\mu _{;\mu } = 0 \\
2R_{{\rm{ }}\lambda }^\mu _{;\mu } - R_{;\lambda } = 0 \\
Define{\rm{ }}G^{\alpha \beta } \equiv g^{\alpha \nu } \left[ {R_{{\rm{ }}\nu }^\beta - \frac{1}{2}\delta _{{\rm{ }}\nu }^\beta R} \right] = R^{\alpha \beta } - \frac{1}{2}g^{\alpha \beta } R = G^{\beta \alpha } \\
G_{{\rm{ }};\beta }^{\alpha \beta } = \left[ {2R_{{\rm{ }}\lambda }^\mu - \delta _{{\rm{ }}\lambda }^\mu R} \right]_{;\beta } = 0 \\
\end{array}
\]
[/tex]