Curvature radius of an open FLRW universe ($r_c=c/[H_0\sqrt{1-{\Omega }_m}]$)

Normalize the present scale factor to one. In a dust-only open cosmology with no cosmological constant, the Friedmann equation gives $-k{c}^2=H_0^2(1-{\Omega }_m)$. The present comoving and physical curvature radii are therefore $r_c=c/[H_0\sqrt{1-{\Omega }_m}]$. At another epoch the physical curvature radius is $a{r}_c$.

// Target: cosmic-microwave-background-anisotropy.bigb