Baryon catch-up approximation after recombination
= Baryon catch-up approximation after recombination
In a dark-matter-dominated growing mode with a constant relative offset and initially negligible baryon <density contrast>, $\Delta_B/\Delta_D=1-R_r/R=1-(1+z)/(1+z_r)$. Replacing this by $1-z/z_r$ requires high redshift. The initial relative velocity and finite baryon self-gravity alter transients, although the relative difference becomes small as the common growing mode increases.