## Friday, 4 December 2020

### Robertson-Walker metrics

In section 8.2 we meet what Carroll calls the Robertson-Walker metrics:$${ds}^2=-{dt}^2+R^2\left(t\right)\left[\frac{{d\bar{r}}^2}{1-k{\bar{r}}^2}+{\bar{r}}^2{d\Omega}^2\right]=-{dt}^2+a^2\left(t\right)\left[\frac{{\rm dr}^2}{1-\kappa r^2}+r^2{d\Omega}^2\right]$$The second version is Carroll's preferred form - 'flouting' conventional wisdom.

Four people found the equations in various teams. They are Alexander Friedmann (Russian), Georges Lemaître (Belgian), Howard Robertson (USian) and Arthur Walker (British) and the metrics are often named after one or some or all. Carroll favours the English speakers.

As usual I check Carroll's equations and (not so usual) have three minor complaints.
1) When he gives the Christoffel symbols at his equation 8.44 the third line is$$\Gamma_{01}^1=\Gamma_{02}^2\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \Gamma_{03}^3=\frac{\dot{a}}{a}$$there should be an $=$ sign between $\Gamma_{02}^2$ and $\Gamma_{03}^3$!
2) Just after equation 8.35 he says that a flat 3-manifold could be described by 'a more complicated manifold such as the three-torus $S^1\times S^1\times S^1$'. How can a torus, something generated by circles, be flat? I think he should have said a flat thee-torus. It can be generate from three circles but not as simply as the usual doughnut which is not flat.
3) Before that, just after equation 8.29, he says $k$ sets the curvature and therefore the size of the spatial surfaces. One might think that $k=0$ sets the size as infinite (flat curvature) but the aforementioned flat three-torus is a counter example with $k=0$ and finite size. So $k=0$ does not set the size as you might have expected. Similar arguments apply when $k<0$.

I learned a few other things too: I suspected that if the metric is diagonal then the Ricci tensor must be diagonal. Not true! But on the plus side I found Win's supercharged formula for calculating Ricci tensor components in that case. It avoids the need to calculate Christoffel and Riemann components. I also learned a bit more about smooth isometric embedding.

I also noticed something slightly mysterious that Carroll does not draw to our attention: In the closed universe case (finite size?) the radial coordinate $\bar{r}$ is constrained by ${\bar{r}}^2<1$. So the universe might be even more closed than we thought.

Read it all at Commentary 8.2 Robertson-Walker Metrics.pdf (7 pages + 5 of calculations)