Absorption distance
= Absorption distance
{title2=$dX/dz=H_0(1+z)^2/H(z)$}
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 $dN\,dX$ and one per $dN\,dz$ therefore differ by $dX/dz$.