How confident are we of the neutrino CMB energy density?

1. Jul 8, 2011

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)?

Last edited: Jul 8, 2011
2. Jul 8, 2011

Drakkith

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

3. Jul 8, 2011

bcrowell

Staff Emeritus
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

4. Jul 9, 2011

Chalnoth

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.

5. Jul 9, 2011

bcrowell

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