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lesgourg / class_public

Public repository of the Cosmic Linear Anisotropy Solving System (master for the most recent version of the standard code; GW_CLASS to include Cosmic Gravitational Wave Background anisotropies; classnet branch for acceleration with neutral networks; ExoCLASS branch for exotic energy injection; class_matter branch for FFTlog)
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Growth factor as a function of k,i.e. D(k,z=100) #368

Open ravi398 opened 3 years ago

ravi398 commented 3 years ago

Dear sir I want to get growth factor as function of k is there any way to get this output from CLASS .? @ThomasTram ,sir would you be able to help? Thanking you regards Ravi kumar sharma

ThomasTram commented 3 years ago

Hi Ravi

I think the concept of a scale-dependent growth factor is confusing and redundant. What you are after is the transfer function for cdm, b, ncdm or some combination of these at redshift z=100. You get that by passing z_pk = 0.0, 100.0, and adjusting P_k_max_1/Mpc if you need to increase max k.

Cheers, Thomas

ravi398 commented 3 years ago

Thank you for reply. is there is way to get growth factor also in the same range of k as that of transfer function.?

regards Ravi kumar sharma

ravi398 commented 3 years ago

i am sorry if this is very syraightforward... i need the growth function or the quantity defined as f=dlog(D)/dlog(a) In the same range as tranfer functions which we get from output='mTk'(you explained in your comment). i need it for different m_ncdm values: growth f. vs k.

@ThomasTram , sir it would be helpful if you write a few lines explaining.

thanking you

regards Ravi sharma

ThomasTram commented 3 years ago

Hi Ravi. What you need is the transfer function of the velocity divergence, theta. It is related to the logarithmic derivative of the transfer function for delta through the continuity equation. (You also need the chain rule.) Depending on your use case,you also need to think about gauge. Anf if you are generating initial conditions for N-body simulations,there is a whole literature about this..

Cheers, Thomas

ravi398 commented 3 years ago

thank you sir yes i need it for initializing n body simulations ? may you suggest some reading or notes where i can find more information?

regards ravi sharma

ThomasTram commented 3 years ago

You can have a look at appendix A in this one, and also the discussion of initial conditions and N-body gauge here. Also have a look at the references in these papers.

Cheers, Thomas