A new, precise measurement of the primordial abundance of Deuterium

TL;DR

This study presents a precise measurement of the primordial deuterium abundance using a metal-poor damped Lyman alpha system at $z=3.04984$, analyzed with a dedicated spectral-fitting pipeline that simultaneously models H I, D I, and metal lines while rigorously characterizing random and systematic errors. The result, $\log({\rm D I/H I}) = -4.596 \pm 0.009$, corresponds to $(D/H)_{\rm p} = (2.535 \pm 0.05)\times10^{-5}$ and implies $100\Omega_{\rm b,0}h^2({\rm BBN}) = 2.23\pm0.09$, in excellent agreement with the CMB value $100\Omega_{\rm b,0}h^2({\rm CMB}) = 2.22\pm0.042$. This concordance suggests no need for non-standard physics to reconcile BBN with the CMB, emphasizing the importance of homogeneous, self-consistent analyses of D/H_p measurements across DLAs. The work also highlights the heterogeneity of previous D/H_p data and advocates assembling a uniform dataset to improve constraints on BBN nuclear reactions and subsequent light-element processing. Planck-era CMB data are expected to further tighten the cosmic baryon density, while continued D/H_p measurements will remain crucial for understanding primordial nucleosynthesis and early-Universe physics.

Abstract

The metal-poor damped Lyman alpha (DLA) system at z = 3.04984 in the QSO SDSSJ1419+0829 has near-ideal properties for an accurate determination of the primordial abundance of deuterium, (D/H)_p. We have analysed a high-quality spectrum of this object with software specifically designed to deduce the best fitting value of D/H and to assess comprehensively the random and systematic errors affecting this determination. We find (D/H)_DLA = (2.535 +/-0.05) x 10^(-5), which in turn implies Omega_b h^2 = 0.0223 +/- 0.0009, in very good agreement with Omega_b h^2 (CMB) = 0.0222 +/- 0.0004 deduced from the angular power spectrum of the cosmic microwave background. If the value in this DLA is indeed the true (D/H)_p produced by Big-Bang nucleosynthesis (BBN), there may be no need to invoke non-standard physics nor early astration of D to bring together Omega_b h^2 (BBN) and Omega_b h^2 (CMB). The scatter between most of the reported values of (D/H)_p in the literature may be due largely to unaccounted systematic errors and biases. Further progress in this area will require a homogeneous set of data comparable to those reported here and analysed in a self-consistent manner. Such an endeavour, while observationally demanding, has the potential of improving our understanding of BBN physics, including the relevant nuclear reactions, and the subsequent processing of 4He and 7Li through stars.

Figures (7)

• Figure 1: Selected metal lines in the $z_{\rm abs} = 3.04984$ DLA in the QSO SDSS J1419$+$0829, reproduced from Cooke et al. (2011). In each panel, the black histogram is the observed spectrum and the red continuous line is the theoretical line profile fitted to the data. Vertical tick marks above the spectrum indicate the velocities of the two absorption components, with parameters listed in section \ref{['sec:obs']}. The $y$-axis scale is residual intensity. The normalized quasar continuum and zero level are shown by the blue long-dashed and green dashed lines, respectively.
• Figure 2: Portion of the UVES spectrum of the QSO SDSS J1419$+$0829 (black), together with the model fit (red). The $1 \sigma$ error spectrum is shown in blue (near the zero level). Vertical dash lines mark the positions of QSO spectral features, as indicated. Light blue labels denote emission lines at $z_{\rm em} = 2.98576$, green labels emission lines at $z_{\rm em} = 3.04224$, and red labels emission lines at $z_{\rm abs} = 3.04954$.
• Figure 3: The Ly$\alpha$ region in J1419$+$0829. Top panel: Observed spectrum in black and best-fitting model QSO spectrum in red. Middle panel: The normalized spectrum, obtained by dividing the observed spectrum by the model spectrum, is shown in black together with the best fitting damped Ly$\alpha$ absorption profile (see section \ref{['sec:abs']}) in red. The neutral hydrogen column density is $\log N{\rm (H\,\textsc{i})/cm^{-2}} = 20.391 \pm 0.008$. Bottom panel: Expanded central portion of the middle panel shown in the rest frame of the $z_{\rm abs} = 3.04984$ DLA. In all three panels the $1 \sigma$ error spectrum is shown in blue.
• Figure 4: Lyman series lines in the $z_{\rm abs} = 3.04984$ DLA. Transitions to higher energy levels than Ly14 are too closely spaced in wavelength to be resolved. The black histogram is the observed spectrum, while the red continuous line is the model fit to the absorption features (see section \ref{['sec:abs']}). Vertical tick marks above the spectrum indicate the three absorption components contributing to the H i (red) and D i (green) absorption. For completeness, we also show (dashed red line) the model H i and D i absorption superposed on blended lines which were not used to constrain the model parameters. Dotted red lines are used for fits to features partly blended with D i and H i Lyman lines. In all panels, the $y$-axis scale is residual intensity.
• Figure 5: Histograms showing the distribution of values of $\log {\rm D\,\textsc{i}/H\,\textsc{i}}$ in 2000 Monte Carlo random realisations of the spectrum of J1419$+$0829, as described in section \ref{['sec:errors']}.