A robust upper limit on N_eff from BBN, circa 2011

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.

5 pages, 1 table, 1 figure. Minor improvements and extensions in the analysis, clarifications and reference added, conclusions slightly strengthened. Matches version published in Phys. Lett. B

References in corpus (7)

• Primordial Nucleosynthesis: from precision cosmology to fundamental physics
• Cosmology seeking friendship with sterile neutrinos
• Big Bang Nucleosynthesis as a Probe of New Physics
• The primordial abundance of 4He: a self-consistent empirical analysis of systematic effects in a large sample of low-metallicity HII regions
• Using BBN in cosmological parameter extraction from CMB: a forecast for Planck
• Dark Radiation Emerging After Big Bang Nucleosynthesis?
• The path to metallicity: synthesis of CNO elements in standard BBN