One percent determination of the primordial deuterium abundance (1710.11129v3)

Abstract: We report a reanalysis of a near-pristine absorption system, located at a redshift z_abs=2.52564 toward the quasar Q1243+307, based on the combination of archival and new data obtained with the HIRES echelle spectrograph on the Keck telescope. This absorption system, which has an oxygen abundance [O/H]=-2.769+/-0.028 (~1/600 of the Solar abundance), is among the lowest metallicity systems currently known where a precise measurement of the deuterium abundance is afforded. Our detailed analysis of this system concludes, on the basis of eight D I absorption lines, that the deuterium abundance of this gas cloud is log_10(D/H) = -4.622+/-0.015, which is in very good agreement with the results previously reported by Kirkman et al. (2003), but with an improvement on the precision of this single measurement by a factor of ~3.5. Combining this new estimate with our previous sample of six high precision and homogeneously analyzed D/H measurements, we deduce that the primordial deuterium abundance is log_10(D/H)_P = -4.5974+/-0.0052 or, expressed as a linear quantity, (D/H)_P = (2.527+/-0.030)x10^-5; this value corresponds to a one percent determination of the primordial deuterium abundance. Combining our result with a BBN calculation that uses the latest nuclear physics input, we find that the baryon density derived from BBN agrees to within 2 sigma of the latest results from the Planck CMB data.

• The paper achieves a one percent measurement of the primordial deuterium-to-hydrogen ratio using refined HIRES spectroscopic data.
• The methodology analyzes eight deuterium absorption lines in a near-pristine gas cloud at redshift ~2.5, enhancing precision by a factor of 3.5 over previous measurements.
• The results align derived baryon densities from Big Bang Nucleosynthesis with Planck CMB data within 2-sigma, impacting cosmological and nuclear physics models.

Overview of One Percent Determination of the Primordial Deuterium Abundance

The paper "One percent determination of the primordial deuterium abundance," authored by Cooke, Pettini, and Steidel, meticulously examines the primordial deuterium-to-hydrogen ratio (D/H) in a gas cloud at a high redshift. Utilizing refined observational data from the W.M. Keck Observatory's HIRES echelle spectrograph, the authors reassess this ratio to determine the primordial deuterium abundance with a precision of one percent. This measurement plays an essential role in constraining Big Bang Nucleosynthesis (BBN) models and offers insights into the conditions of the early universe.

Summary of Methods and Results

The paper focuses on a near-pristine absorption system toward the quasar Q1243+307, located at a redshift of approximately 2.5. The absorption system exhibits a notably low metallicity with an oxygen abundance [O/H] = -2.769, or about 1/600th of the solar abundance. This feature, along with an analysis of eight deuterium absorption lines, allows for a precise deuterium measurement with a D/H ratio logged as $\log_{10}(D/H) = -4.622 \pm 0.015$. This result significantly enhances the precision by a factor of 3.5 compared to prior measurements referenced by Kirkman et al.

Integrating this result with previous precision D/H measurements analyzed uniformly, the authors calculate a primordial D/H ratio of $\log_{10}(D/H)_P = -4.5974 \pm 0.0052$—equivalently expressed as $10^5(D/H)_P = 2.527 \pm 0.030$. These measurements are conducive to a one percent determination of the primordial deuterium abundance.

Implications for Cosmology

The achieved precision on the deuterium abundance has profound implications for cosmology. When coupled with BBN theoretical models incorporating the latest nuclear physics inputs, this measurement yields a baryon density derived from BBN that is consistent with the value derived from Planck cosmic microwave background (CMB) data, albeit with a 2-sigma tension. Specifically, using BBN results, the baryon density is determined as $$100\,\Omega_{B,0}\,h^2(BBN) = 2.166 \pm 0.015 \pm 0.011$$. This finding shows the potential need for ramifications on the modeling of reactions such as the $^3$He+$d$ nuclear reaction rate.

Future Prospects

The high precision of these measurements paves the way for refining the constraints on cosmological models, especially concerning the effective number of neutrino species and baryon density in the universe. As a practical extension, it also highlights the capacity of modern spectrographs like ESPRESSO on the VLT to enhance the statistics of D/H measurements further. The data and approaches showcased in this paper stand as a stringent test of the Standard Model of cosmology and particle physics, with implications for both present and upcoming astrophysical measurements. The ongoing synergy between observational astrophysics and theoretical models continues to be pivotal in advancing our understanding of the universe's inception and its underlying physics.