The primordial Helium abundance and the baryonic content of the Universe
Trinh Xuan Thuan
Astronomy Department, University of Virginia
Helium is the second most abundant element in the Universe. Produced mainly in the first few hundred seconds of the Universe, its abundance has increased at most by 10%-20% during subsequent stellar nucleosynthesis, making possible a precise determination of its value. Such a determination is of special interest because standard Big Bang nucleosynthesis theory predicts the primordial abundance of Helium-4 relative to hydrogen (and of the other light elements Deuterium, Helium-3 and Lithium-7) to be a function of a single cosmological parameter, the baryon-to-photon ratio, given the number of relativistic neutrino species and the neutron lifetime.
I shall discuss how we can use the emission-line spectra of low-luminosity metal-deficient blue compact dwarf galaxies to derive a precise primordial helium mass fraction Y_p= 0.2452+-0.0015, which implies a baryon-to-photon number ratio of 4.7 x 10**(-10). I shall discuss the consistency of this measurement of the baryonic content of the universe with those obtained from Deuterium and Cosmic Microwave Background measurements, and how this fits in the overall dark matter and dark energy content of the Universe.