Astroparticle Physics

10.6 Detailed BBN 223Fig. 10.4Evolution of the mass and numberfractions of primordial elements.4 He is given as a mass fractionwhile the other elements arepresented as number fractionsatively small amounts of heavier elements have been producedthrough stellar burning of hydrogen. A recent surveyof data [2] concludes for the primordial 4 He mass fractionY P = 0.238 ± 0.002 ± 0.005 , (10.25)where the first error is statistical and the second reflects systematicuncertainties. In contrast to the helium-4 content ofthe universe which is traditionally given as a mass fraction,the abundances of the other primordial elements are presentedas number fractions, e.g., n7 Li /n p ≡ n7 Li /n H for 7 Li.The best determinations of the 7 Li abundance come fromhot metal-poor stars from the galactic halo. As with 4 He, oneextrapolates to zero metallicity to find the primordial value.Recent data [2] give a lithium-to-hydrogen ratio of7 Li abundancemetal-poor starsn7 Li /n H = 1.23 × 10 −10 . (10.26)The systematic uncertainty on this value is quite large, however,corresponding to the range from about 1 to 2 × 10 −10 .Although deuterium is produced by the first reaction inhydrogen-burning stars through pp → de + ν e , it is quicklyprocessed further into heavier nuclei. Essentially, no netdeuterium production takes place in stars and any presentwould be quickly fused into helium. So to measure the primordialdeuterium abundance, one needs to find gas cloudsat high redshift, hence far away and far back in time, thatprimordial deuterium