Cosmological baryon density derived from the deuterium abundance at redshift z = 3.57

Abstract

The primordial ratio of the number of deuterium to hydrogen nuclei (D/H) created in big bang nucleosynthesis is the most sensitive measure of the cosmological baryon to photon ratio and the cosmological density of baryons Ω b (1-5). In the interstellar medium (ISM) of our Galaxy D/H = 1.6 ± 0.1 × 10 −5 (6), which places a strict lower limit on the primordial abundance, because stars reduce the proportion of D in the ISM. Quasar absorption systems (QAS) should give definitive measurements of the primordial D because they can sample metal-poor gas at early epochs where the destruction of D should be negligible. A probable measurement of a high abundance ratio was reported in one QAS: D/H ≈ 24 × 10 −5 (7,8). Here we report a measurement of low D/H = 2.3 ×10 −5 , in another QAS, which is consistent with D/H in the ISM, with models of Galactic chemical evolution, (9,10), and with earlier data (7,8) provided that D line is strongly contaminated. We have internal consistency checks, and believe that this is the first direct measurement of a primordial abundance ratio. It provides the most accurate measurement of the baryon density: a high value of Ω b = 0.05, assuming a Hubble constant H 0 = 70 km s −1 Mpc −1 .Deuterium is rarely seen in QAS because the associated hydrogen is usually distributed over a wide range of velocities and its absorption covers deuterium. We selected QSO 1937-1009 because we found that the absorption system at z = 3.572 showed unusually weak metal lines in low resolution spectra, which we took to indicate that the gas was distributed in one or two narrow velocity components (11). We obtained optical spectra of QSO 1937-1009 (z em = 3.78, V ≃ 17.5) with the HIRES echelle spectrograph (12) on the W. M. Keck 10-m telescope. The high quality spectra make this amongst the best characterized QAS, and show strong absorption at the expected position of the D Lyα line and weak but highly significant absorption at D Lyβ .The weak metal lines in the system, C II, C IV, Si II, and Si IV, appear to have the same profiles, which are adequately described by two components separated by 15 km s −1 .

Varying leptonic chemical potentials and spatial variation of primordial deuterium at high z

“…Recently several groups [1][2][3][4][5][6] have reported measurements of the deuterium abundance in Lyman-limit absorption line systems with red-shifts 0.48 < z < 3.5 on the line of sight to quasars; these are believed to give essentially the primordial value.…”

“…The model predicts a large mass fraction of 4 He (35-60%) and 7 Li (up to 10 −9 ) in deuteriumrich regions. Because of lepton family symmetry, the angular variations of cosmic microwave background radiation can be sufficiently small although still observable in future measurements.Recently several groups [1][2][3][4][5][6] have reported measurements of the deuterium abundance in Lyman-limit absorption line systems with red-shifts 0.48 < z < 3.5 on the line of sight to quasars; these are believed to give essentially the primordial value.Surprisingly some groups have claimed a high value, D/H ≈ 2 · 10 −4 on the basis of ground-based data taken with the Keck telescope, but this result is now thought to be due to various errors [3] and the best value available from two "clean" systems is 3 · 10 −5 [5]. However, Webb et al [6] report a high deuterium abundance,…”

Massive decaying τ neutrino and Big Bang nucleosynthesis

“…On the other hand, if the low D abundances, D/H = (3.39 ± 0.25) × 10 −5 [4,5], are adopted, there is a discrepancy between the standard BBN theory and the observational measurements. This leads to the similar problem which was pointed out by Hata et al (1995) [6] though they used the data for D and 3 He in solar neighborhood and a simple chemical evolution model to infer the primordial abundances.…”

“…For example, since neutrino is one of the dominant components of the energy density in the universe at the BBN epoch, the change of neutrino energy density affects the Hubble expansion rate and alters the freeze out time of neutron to proton ratio. Then the predicted light element abundances, especially 4 He in this case, are sensitively changed. More precisely we measure the primordial component of light element abundances, more strongly we constrain on such neutrino properties.…”

“…They found that theoretical predictions agree with the observational data if the number of neutrino species N ν is less than 2.6. The point is that the prediction of the standard (N ν = 3) BBN for 4 He is too large if the photon-baryon ratio η is chosen to fit the D data ( or the prediction of D is too large if η is determined by 4 He data). In this case, we should look for non-standard scenarios.…”