Dark-matter halo scaling at the current redshift. R200 is the radius
enclosing a mean density of 200 ρcrit(z), with
ρcrit(z) = 3H(z)2/(8πG), and
V200 = (GM200/R200)1/2 = 10 H(z) R200
is the circular velocity at R200. Both curves shift with redshift only
through H(z).