1. Department of Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden
2. Department of Physics and Astronomy, University of Århus, Ny Munkegade, 8000 Århus C, Denmark
3. GRAAL-UMR5024/ISTEEM (CNRS), Université Montpellier II, Place E. Bataillon, 34095 Montpellier, France
4. Institute of Astronomy, Russian Academy of Science, Pyatnitskaya 48, 119017 Moscow, Russia

Correspondence to: A. J. Korn¹ Correspondence and requests for materials should be addressed to A.J.K. (Email: akorn@astro.uu.se).

The measurement of the cosmic microwave background has strongly constrained the cosmological parameters of the Universe¹. When the measured density of baryons (ordinary matter) is combined with standard Big Bang nucleosynthesis calculations^{2, 3}, the amounts of hydrogen, helium and lithium produced shortly after the Big Bang can be predicted with unprecedented precision^{1, 4}. The predicted primordial lithium abundance is a factor of two to three higher than the value measured in the atmospheres of old stars^{5, 6}. With estimated errors of 10 to 25%, this cosmological lithium discrepancy seriously challenges our understanding of stellar physics, Big Bang nucleosynthesis or both. Certain modifications to nucleosynthesis have been proposed⁷, but found experimentally not to be viable⁸. Diffusion theory, however, predicts atmospheric abundances of stars to vary with time⁹, which offers a possible explanation of the discrepancy. Here we report spectroscopic observations of stars in the metal-poor globular cluster NGC 6397 that reveal trends of atmospheric abundance with evolutionary stage for various elements. These element-specific trends are reproduced by stellar-evolution models with diffusion and turbulent mixing¹⁰. We thus conclude that diffusion is predominantly responsible for the low apparent stellar lithium abundance in the atmospheres of old stars by transporting the lithium deep into the star.

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