A robust upper limit on N_eff from BBN, circa 2011

TL;DR

This paper establishes a conservative upper limit on the effective number of neutrinos using primordial nucleosynthesis constraints, emphasizing minimal assumptions about helium abundance and challenging previous indications of extra neutrino species.

Contribution

It provides a robust bound on N_eff from BBN data with weak astrophysical assumptions, independent of CMB and neutron lifetime measurements.

Findings

Delta Neff <= 1 at 95% C.L.

Primordial nucleosynthesis alone does not favor extra neutrinos.

The reported 2 sigma hint is likely driven by CMB data or questionable helium measurements.

Abstract

We derive here a robust bound on the effective number of neutrinos from constraints on primordial nucleosynthesis yields of deuterium and helium. In particular, our results are based on very weak assumptions on the astrophysical determination of the helium abundance, namely that the minimum effect of stellar processing is to keep constant (rather than increase, as expected) the helium content of a low-metallicity gas. Using the results of a recent analysis of extragalactic HII regions as upper limit, we find that Delta Neff<= 1 at 95 % C.L., quite independently of measurements on the baryon density from cosmic microwave background anisotropy data and of the neutron lifetime input. In our approach, we also find that primordial nucleosynthesis alone has no significant preference for an effective number of neutrinos larger than the standard value. The ~2 sigma hint sometimes reported in the literature is thus driven by CMB data alone and/or is the result of a questionable regression protocol to infer a measurement of primordial helium abundance.