class_interface.py N_ur Definitions

[paulrogozenski]

The characterizations of ultra-relativistic degrees of freedom/massless neutrino density in CLASS differs from that of CAMB, where N_ur does not equal N_eff - number of massive neutrinos. Instead, N_ur takes on different values to maintain a neutrino temperature consistent with the CMB. The specific values are given in the CLASS explanatory file, linked here . Although these differences are slight, these values should be adjusted accordingly to reduce costly computation of slightly-hotter-than-expected neutrinos. Alternatively, the user can be able to set this value directly, allowing flexibility for parameterizing extended cosmological models

[joezuntz]

Hi @paulrogozenski . Thanks for this! I have never understood this topic properly. Could you recommend which parameters should be exposed to the user? Right now they set parameters called:

• nnu
• mnu
• num_massive_neutrinos
  and these are used to set the class parameters:
• N_ur = nnu - num_massive_neutrinos
• N_ncdm = nmassive
• m_ncdm = "mnu/num_massive_neutrinos" repeated nmassive` times.

Maybe it would be better to just drop trying to keep consistency with the camb parameters and expose the class parameter names directly? That would probably slow down people trying to do cross-checks, but maybe that's a good idea if they might get wrong answers if they don't check too carefully.