 Solution

Use units $c=1$ throughout. Set $\chi =|\mathbf{x}|$, so the spatial Euclidean metric is $d{\mathbf{x}}^2=d{\chi }^2+{\chi }^{2}d{\Omega }^2$. On either hemisphere of the embedded three-sphere, the constraint gives $u^2=R^2-{\chi }^2$ and henceIntroduce the dimensionless comoving coordinate $r=\chi /R$. ThenThis is the positively curved Friedmann-Lemaître-Robertson-Walker metric, with $a(t)$ the physical curvature radius. An equally valid convention retains $r=\chi$, takes $a=b$, and sets $K=R^{-2}$; only the coordinate normalization differs. The chart with $r\le 1$ describes a hemisphere, not the entire closed slice. Writing $r=\sin \psi$ gives $d{\ell }^2=R^2(d{\psi }^2+{\sin }^2\psi d{\Omega }^2)$, with $0\le \psi \le \pi$, displaying the full spherical spatial slices of a closed FLRW universe.