I would like to know how hard it would be to implement the "gamma" parametrization of the growth rate (i.e. f = Ω

_{m}(z)

^{γ(z)}) inside CAMB/CosmoMC, in order to study the impact of a modified growth rate on the CMB power spectrum (through the ISW effect & lensing).

I understand that CAMB evolves the full linear perturbation theory equations, and that at high redshift there are effects from radiation and decaying modes, and the gamma approximation breaks down. There is also the effect of massive neutrinos which introduce a k dependence on the growth factor.

Supposing no massive neutrinos, could we somehow, at a certain late enough redshift, switch from the full perturbations equations to the gamma parametrization for the growth factor/rate ?

Thanks in advance !

Stephane