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CAMB/CosmoMC : Modified growth

Posted: October 20 2016
by St├ęphane Ilic
Hello all,

I would like to know how hard it would be to implement the "gamma" parametrization of the growth rate (i.e. f = \Omega_m(z)^{\gamma(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 !


CAMB/CosmoMC : Modified growth

Posted: October 24 2016
by Jason Dossett
I would suggest against implementing the gamma parameterization if trying to get its effect on the CMB spectrum. Gamma is not entirely suitable for looking for modifications to gravity and comparing with the CMB.

There are a few different codes available to look at modified growth parameters and how they affect the CMB.

1. ISiTGR: - This code uses a few parameterizations for the modified gravity parameters so that you can have different choices of how they vary in time and scale.

2. MGCAMB: - This is similar to ISiTGR but has slightly different ways of parameterizing the MG parameters and does include an input to vary gamma, but as i discussed earlier, using gamma will not be as accurate or as realistic as using the other MG parameters.

3. EFTCAMB: - Uses an effective field theory approach to modifying growth. This is nice because it does not assume the quasi-static approximation used in the previous two modified codes.

CAMB/CosmoMC : Modified growth

Posted: October 25 2016
by St├ęphane Ilic
Hello Jason and thank you for your answer !

I was aware of two of the three codes you mentioned, but thank you for the summary !

I'll check them out and see if they suit my needs.

Cheers !


Re: CAMB/CosmoMC : Modified growth

Posted: April 13 2022
by Minh Nguyen
Sorry for digging up this topic, but here we're thinking of doing something similar. I'm unsure of how this can be potentially a bad idea. Could someone clarify/elaborate a bit more?