Implementing Interacting ncdm Species

[robbieellis12]

Hi,
I am a student new to cosmology and am trying to test myself by implementing a new species in the code. I have spent a few days reading the documentation/lecture notes/papers but am hitting a few roadblocks though I believe my goal is quite simple.

I am trying to implement a sterile neutrino that can be produced via neutrino self interactions (described in this paper)). Essentially I wish to have 3 massive neutrinos as well as one extra heavier one with Boltzmann equation df_s/dt=c(E,T)f_v_a where f_s is the sterile neutrino distribution function and f_v_a is the distribution function for active neutrinos.

What I have done is entered 4 ncdm species with the main change being that in perturbations.c I change the Boltzmann hierarchy for the last species by adding a term like c(E,t)(y1+y2+y3) to dy for one of the species (in pertubations_derivs). I believe this should work but there’s two things bothering me:

when entering the mass of the sterile neutrino in ncdm (40.3keV) it blows up the total mass density so that omega_lamba becomes something like -999 —which we don’t want. I want to essentially vary the mass and interaction stregnth to yield a final dark matter density that is consistent with what we observe but the budget equation being enforced prevents me from doing this. I’m not sure if I can just remove the budget constraint and run several trials varying my parameters until I get a sensible result. It seems like the code is predicting that the ncdm density will be much larger than it will be due to the modifications I have made.

I also realize I have not changed the zero order equation. I have changed the pertubations but it doesn’t seem like I have changed how the zeroth order Boltzmann equation only involving f^(0) evolves . I haven’t seen any reference to where this part is computed.

Overall I wish to know if my approach of implementing this by modifying the code relevant to ncdm makes sense and if anyone has any suggestions for overcoming the issues with the fixed budget and zeroth order Boltzmann equation. I apologize for any ignorance on my part, this is all very new to me and I would greatly appreciate help from anyone more knowledgeable than me!