I am having trouble understanding the sterile neutrino analysis performed in CosmoMC March 2013 (Planck 2013 results).

More specifically:

In CMB_Cls_simple.f90 what is the physical significance of the comparison: If Ω

_{ν}h

^{2}> Ω

_{ν}h

^{2}

_{sterile}(or else ...)?

CMB_Cls_simple.f90

Code: Select allp>

```
if (CMB%omnuh2>CMB%omnuh2_sterile) then
neff_massive_standard = num_massive_neutrinos*default_nnu/3
P%Num_Nu_Massive = num_massive_neutrinos
P%Nu_mass_eigenstates=P%Nu_mass_eigenstates+1
if (CMB%nnu > neff_massive_standard) then
P%Num_Nu_Massless = CMB%nnu - neff_massive_standard
else
P%Num_Nu_Massless = 0
neff_massive_standard=CMB%nnu
end if
P%Nu_mass_degeneracies(P%Nu_mass_eigenstates) = neff_massive_standard
P%Nu_mass_fractions(P%Nu_mass_eigenstates) = (CMB%omnuh2-CMB%omnuh2_sterile)/CMB%omnuh2
else
neff_massive_standard=0
end if
```

_{ν}h

^{2}> Ω

_{ν}h

^{2}

_{sterile}seems to be valid all the time when a sterile neutrino is included, since in Params_CMB.f90, Ω

_{ν}h

^{2}

_{sterile}has been added to Ω

_{ν}h

^{2}

Params_CMB.f90:

Code: Select allp>

`CMB%omnuh2 = CMB%omnuh2 + CMB%omnuh2_sterile`

I would really appreciate some help to better understand the physics behind this.

Thank you,

Ana