The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background1. The predicted2,3,4 primordial 7Li abundance is four times that measured in the atmospheres of Galactic halo stars5,6,7. This discrepancy could be caused by modification of surface lithium abundances during the stars’ lifetimes8 or by physics beyond the Standard Model that affects early nucleosynthesis9,10. The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium11 that is not susceptible to the in situ modifications that may affect stellar atmospheres. Here we report observations of interstellar 7Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun’s metallicity. The present-day 7Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN.
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We thank the European Southern Observatory for granting us time for this project as part of proposal 382.B-0556. We also thank A. Fox and H. Sana for discussions about the UVES data and A. Korn, P. Molaro, T. Prodanovic, D. Romano, and D. Welty for input on the project that improved the paper.
The authors declare no competing financial interests.
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Howk, J., Lehner, N., Fields, B. et al. Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud. Nature 489, 121–123 (2012). https://doi.org/10.1038/nature11407
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