Astroparticle Physics

224 10 Big Bang Nucleosynthesisdeuterium spectroscopyhave never been part of stars. These produce absorptionspectra in light from even more distant quasars. The hydrogenLyman-α line at λ = 121.6 nm appears at veryhigh redshift (z ≥ 3) in the visible part of the spectrum.The corresponding line from deuterium has a small isotopicshift to shorter wavelengths. Comparison of the two componentsgives an estimate of the deuterium-to-hydrogen ratioD/H ̂= n d /n p . A recent measurement finds [19]n d /n p = (3.40 ± 0.25) × 10 −5 . (10.27)primordial elementabundancesMeasuring of the primordial abundance of 3 He turns outto be more difficult. There do not yet exist sufficiently reliablevalues for 3 He to test or constrain Big Bang Nucleosynthesis.Finally, the predicted abundances of light nuclei are confrontedwith measurements. The predictions depend, however,on the baryon density n b or, equivalently, η = n b /n γ .The predicted mass fraction of 4 He as well as the numbersrelative to hydrogen for D, 3 He, and 7 Li are shown as a functionof η in Fig. 10.5 [2].Fig. 10.5Predictions for the abundances of4 He,D,and 7 Li as a function ofthe baryon-to-photon ratio η. Y P isthe primordial 4 He mass fraction.Traditionally, the 4 He content ofthe universe is given as massfraction, while the other primordialelements are presented as numberfraction (see also the brokenvertical scale). The larger box for7 Li/H includes the systematicalerror added in quadrature to thestatistical errorThe deuterium fraction D/H decreases for increasing ηbecause a higher baryon density means that deuterium isprocessed more completely into helium. Since the resulting