Astronomers may have solved the embarrassing problem of why the universe has so much less lithium than predicted by standard cosmology. By observing a set of stars in a distant globular cluster, Andreas Korn of Uppsala University in Sweden and colleagues in Denmark, France and Russia have concluded that the lithium diffuses over time into the stars' hot interiors, where it is then burnt up (Nature 442 657). Apart from confirming that our understanding of the Big Bang is correct, the discovery is also vindication for theoretical astrophysicists, who had predicted this diffusion effect all along.
Lithium — together with hydrogen and helium — is one of very few elements to have been synthesised in the Big Bang. However, experimental observations have shown that the amount of lithium in the atmospheres of the universe’s very oldest stars is about one third of the value predicted by recent analyses of the fluctuations in the cosmic microwave background. Researchers have therefore not been sure what is wrong — theory or observation.
Korn and co-workers may now have an answer to this question. Using a spectrometer on the European Southern Observatory’s Very Large Telescope in Chile, they studied 18 stars in a distant globular cluster called NGC 6397, which formed roughly a few hundred million years after the Big Bang. Globular clusters are useful to study this problem because the stars are all the same age and had the same initial chemical composition, but are at different stages of evolution.
By comparing their observations with theoretical models of how nuclei behave in the atmospheres of stars, the researchers conclude that lithium diffuses into the interiors of stars over time, where it is burnt up at temperatures of over 2.5 million Kelvin. Calcium, iron and other “metallic” nuclei, in contrast, can survive the trip through the stellar interior.
The researchers estimate that these stars originally contained 78% more lithium as we observe today. In other words, the initial amount of lithium agrees with predictions from Big Bang nucleosynthesis. “This finding restores confidence in standard big-bang nucleosynthesis, quantitative spectroscopy and sophisticated stellar evolution models,” the authors say.
Although Korn is happy that the work resolves what he calls “one of the most distressing discrepancies we had in connection with the Big Bang theory”, he warns that researchers will have to be more careful when interpreting the spectra of old, unevolved stars “because the elemental abundances they show us are not eternal, but a function of time”. He also says that the onus is now on theorists to fully explain why the lithium behaves in this way.
