Absorption distance ($dX/dz=H_0(1+z{)}^2/H(z)$)

Absorption distance is a dimensionless path variable chosen so a non-evolving comoving absorber population with fixed proper cross-section has constant $d\mathcal{N}/dX$. It absorbs the geometrical factor in absorber incidence and comoving number density. A distribution per $dNdX$ and one per $dNdz$ therefore differ by $dX/dz$.