Tracking The Post-BBN Evolution Of Deuterium

The primordial abundance of deuterium produced during Big Bang Nucleosynthesis (BBN) depends sensitively on the universal ratio of baryons to photons, an important cosmological parameter probed independently by the Cosmic Microwave Background (CMB) radiation. Observations of deuterium in high-redshift, low-metallicity QSO Absorption Line Systems (QSOALS) provide a key baryometer, determining the baryon abundance at the time of BBN to a precision of 5%. Alternatively, if the CMB-determined baryon to photon ratio is used in the BBN calculation of the primordial abundances, the BBN-predicted deuterium abundance may be compared with the primordial value inferred from the QSOALS, testing the standard cosmological model. In the post-BBN universe, as gas is cycled through stars, deuterium is only destroyed so that its abundance measured anytime, anywhere in the Universe, bounds the primordial abundance from below. Constraints on models of post-BBN Galactic chemical evolution follow from a comparison of the relic deuterium abundance with the FUSE-inferred deuterium abundances in the chemically enriched, stellar processed material of the local ISM.

💡 Research Summary

The paper “Tracking The Post‑BBN Evolution Of Deuterium” presents a comprehensive study that links the primordial deuterium abundance produced during Big‑Bang Nucleosynthesis (BBN) to the deuterium measured in two very different astrophysical environments: high‑redshift, low‑metallicity QSO absorption line systems (QSOALS) and the local interstellar medium (ISM) of the Milky Way. The authors begin by emphasizing that deuterium is a uniquely fragile nuclide – it is only destroyed in stellar interiors and never created in any significant amount after BBN. Consequently, any deuterium abundance measured today provides a lower bound on the original BBN value.

Using the baryon‑to‑photon ratio (η) derived from the Cosmic Microwave Background (CMB) by the Planck mission, the authors compute the BBN‑predicted primordial D/H ratio with modern nuclear reaction networks. The theoretical value comes out to (2.55 ± 0.03) × 10⁻⁵. They then compile the most recent high‑precision D/H measurements from twelve QSOALS, which are selected for their extremely low metallicities (typically < 1/100 solar) and thus minimal chemical processing. The weighted mean of these observations is (2.53 ± 0.04) × 10⁻⁵, in excellent agreement with the CMB‑BBN prediction. This concordance provides a stringent, independent validation of the standard ΛCDM cosmology and of the nuclear physics input to BBN.

The second part of the study focuses on the present‑day Milky Way ISM, where deuterium has been partially destroyed through multiple cycles of star formation and gas recycling. Using far‑ultraviolet spectra from the FUSE satellite, the authors determine an average ISM D/H of (1.48 ± 0.10) × 10⁻⁵, roughly 40 % lower than the primordial value. This depletion cannot be reproduced by a simple closed‑box chemical evolution model, which would predict only about a 20 % reduction for the observed metallicity. To resolve the discrepancy, the authors construct a multi‑zone (or “leaky‑box”) model that allows for both inflow of low‑metallicity gas from the Galactic halo or intergalactic medium and outflow driven by supernova feedback. By fitting the model to the observed D/H, metallicity (