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Stéphane Ilic
Joined: 11 Feb 2014 Posts: 15 Affiliation: IRAP Toulouse

Posted: October 20 2016 


Hello all,
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 

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Jason Dossett
Joined: 19 Mar 2010 Posts: 94 Affiliation: The University of Texas at Dallas

Posted: October 24 2016 


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: http://isit.gr  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: http://aliojjati.github.io/MGCAMB/mgcamb.html  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: http://eftcamb.org/  Uses an effective field theory approach to modifying growth. This is nice because it does not assume the quasistatic approximation used in the previous two modified codes. 

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Stéphane Ilic
Joined: 11 Feb 2014 Posts: 15 Affiliation: IRAP Toulouse

Posted: October 25 2016 


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 !
S. 

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