Cosmology: the enigma of lithium is confirmed
Many of the predictions of the theory of primordial nucleosynthesis, one of the pillars of the Big Bang theory were successful. But disagreement remained between theoretical predictions and observations on the lithium abundance in the Milky Way. An international team of astronomers, measuring a similar distribution outside our galaxy, has confirmed that it is indeed a true cosmological puzzle.
Globular clusters are memories of galaxy formation and the state of matter when it is started. These sets are mostly populated by old stars of at least 10 billion years old and have reached a stage of development similar. Metal-poor, that is to say, in the jargon of astrophysicists heavier than hydrogen and helium elements, these stars must have formed at the same time and are contemporary with the oldest stars in the galaxies.
It is hoped there abundances of light measuring elements are isotopes of hydrogen lithium those before they were not significantly converted into heavier elements by nucleosynthesis. Detailed measurements of the abundances can therefore be used to constrain the theory of primordial nucleosynthesis describing the production of light elements minutes after the Big Bang.
A globular cluster around the Sagittarius dwarf galaxy
An international team of astronomers led by Alessio Mucciarelli (University of Bologna, Italy), with the participation of a CNRS researcher working in the laboratory galaxies, stars, physics and instrumentation of the Paris Observatory (Gepi), focused on determination of lithium abundance in old stars in the globular cluster Messier 54 (M54). Ever thought it was one of the many satellites of our Milky Way clusters, but was finally discovered in 1994, in fact, that is orbiting the Sagittarius dwarf galaxy. Giraffe spectrograph built Gepi equipment and VLT (Very Large Telescope) ESO since 2002 has evaluated the abundance of lithium in an environment located about 90,000 light years from our galaxy. The results of these observations were recently published in an article freely available on arXiv.
To new physics?
The observations were made in the giant stars of M54, but the theory of stellar evolution is used to connect the lithium abundance in these stars to dwarf in this cluster, which ultimately make comparisons with dwarf stars elderly in the Milky Way. In our galaxy, the abundance of lithium are measured three times lower than predicted by the theory of the Big Bang. However, this discrepancy could be the result of statistical fluctuations in the distribution of light elements that make our Milky Way a single local historic accident. On average, on a larger scale and in other galaxies, the distribution of lithium could then êtreresterait in good agreement with the theory of primordial nucleosynthesis.
But according to measurements made with M54, it would not be. The lithium abundances are very similar to those measured in our galaxy. Lithium is in the heart of a beautiful puzzle in cosmology itself.
How do I fix it? The primordial nucleosynthesis calculations are based on the well known and reproducible laboratory nuclear physics. It seems necessary to introduce new elements, either as regards the period of a few minutes after the Big Bang nucleosynthesis (for example, supersymmetric particles decay) or at the level of the physics of stars. Lithium may have been destroyed in the first generation of stars that little is still known or even during the evolution of stars on the main sequence standards.
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