Baryon transfer function

The baryon transfer function retains acoustic oscillations generated while baryons belonged to the photon-baryon fluid. After recombination, baryons fall into cold-dark-matter potential wells, reducing the relative perturbation while leaving a small oscillatory imprint.

After recombination and when pressure is negligible, cold dark matter and baryons feel the same gravitational acceleration. Their density-contrast difference therefore obeysDuring matter domination $D=D_0+D_1{a}^{-1/2}$, whereas the growing total-matter mode is proportional to $a$, so ${\delta }_c/{\delta }_b\to 1$.