general relativity - Einstein Field Equations in other space-time dimensions than 3+1?

This question is apparently quite simple but I can't seem to find an answer to it, so I was hopping anyone could clarify me.

Are the Einstein field equations (EFE) only valid for a 3+1 dimensional space-time?

I've read somewhere, which I can't remember or find, that there were problems with the EFE in a 2+1 dimensions...Why would that be?

What about 1+1?

Answer

There is nothing "wrong" with the Einstein field equations in $2+1$ as indicated by the comments, but they do have interesting, significantly restricted behavior in $2+1$ dimensions.

For example, the Wikipedia page referred to by Olof in the comments says that in $2+1$, every vacuum solution is locally either $\mathbb R^{2,1}$, $\mathrm{AdS_3}$, or $\mathrm{dS}_3$. Here's why. In $d+1$ with $d\neq 1$, the vacuum field equations (those with $T_{\mu\nu} =0$) can be manipulated to show that

$$R_{\mu\nu} = \frac{R}{d+1}g_{\mu\nu}$$ On the other hand, one can show (see Weinberg Gravitation and Cosmology eq. 6.7.6) that in $2+1$, the Riemann tensor satisfies $$R_{\lambda\mu\nu\kappa} = g_{\lambda\nu} R_{\mu\kappa} - g_{\lambda\kappa}R_{\mu\nu} - g_{\mu\nu}R_{\lambda\kappa} + g_{\mu\kappa} R_{\lambda\nu} - \frac{1}{2}(g_{\lambda\nu}g_{\mu\kappa} - g_{\lambda\kappa}g_{\mu\nu})R$$ and combining these results gives $$R_{\lambda\mu\nu\kappa} = \frac{1}{6}(g_{\lambda\nu}g_{\mu\kappa}-g_{\lambda\kappa}g_{\mu\nu})R$$ which is precisely the Riemann tensor for a maximally symmetric spacetime in $2+1$ which gives the result.

Notice that this behavior is in stark contrast to the vacuum behavior in $3+1$. For example, take the vacuum region outside of a spherically symmetric massive body in $3+1$ (like a black hole). This region is not flat, but in $2+1$ with vanishing cosmological constant any vacuum region outside of a massive body would be. Pretty strange.