Deuterium is Big Bang nucleosynthesis's precision anchor. The primordial D/H ratio depends steeply on the baryon density, stars only destroy deuterium rather than make it, and the cleanest measurements, absorption systems of near-pristine gas backlit by distant quasars, deliver values at percent-level precision: the benchmark damped Lyman-alpha systems give D/H near 2.53 x 10^-5, in striking concordance with the CMB-derived baryon density (Cooke et al. 2014).
The strain lives in the ensemble rather than the best systems. Across the full set of measured absorbers, D/H values scatter beyond their formal uncertainties, with hints of variation across redshift that a single uniform nucleosynthesis event does not produce (Pettini and Cooke discussions of the measurement systematics). The candidate explanations pull in different directions: hydrogen interloper absorption masquerading as deuterium, modeling systematics in the velocity structure of the absorbers, genuine astration, stellar processing destroying deuterium in systems less pristine than assumed, or, most expensively, nucleosynthesis conditions that actually varied. The last option would be catastrophic for the standard framework, since one global BBN event permits exactly one primordial ratio.
The standing is a precision frontier where the mean is a triumph and the dispersion a nuisance not yet fully diagnosed; thirty-meter-class spectroscopy will multiply the clean-absorber census and settle how much scatter survives better data.