How confident are we of the neutrino CMB energy density?

[BillSaltLake]

From an entropy argument it is concluded that the CMB neutrino temperature is (4/11)^1/3 of the CMB photon temperature. This assumes massless neutrinos. Although neutrino contribution to the present energy density is therefore very small, it was significant must earlier, such as at the time of the Last Scattering Surface. How reliable is this conclusion? Is there a different analysis that would yield a different result, so that for example ⁷Li nucleosynthesis might be reconciled (although I'm not sure that there is an adjustment of the neutrino density that could fix the Li problem)?

 

[Drakkith]

From what I have read, neutrinos have a small but non-zero mass.
I'm not sure how that effects everything though.

 

[bcrowell]

I hadn't heard of the 7Li problem until now. Here is a paper that may help others who, unlike me, aren't familiar with it: http://arxiv.org/abs/0808.2818

 

[Chalnoth]

From an entropy argument it is concluded that the CMB neutrino temperature is (4/11)^1/3 of the CMB photon temperature. This assumes massless neutrinos. Although neutrino contribution to the present energy density is therefore very small, it was significant must earlier, such as at the time of the Last Scattering Surface. How reliable is this conclusion? Is there a different analysis that would yield a different result, so that for example ⁷Li nucleosynthesis might be reconciled (although I'm not sure that there is an adjustment of the neutrino density that could fix the Li problem)?

Well, neutrinos have long been taken into account in CMB observations, so I really doubt it.

But in any event, the 7Li problem is way, way down at the tail of primordial nucleosynthesis where it is very hard to measure. At present we just can't say for sure whether the problem appears as a result of our lack of understanding of stellar physics, or whether it's our lack of understanding of some of the precise details of big bang nucleosynthesis.

 

[bcrowell]

Here's a paper that just came out today arxiv.org/abs/1107.1117 that talks about the 7Li problem.