Synopsis: Calibrating the cosmic clock

What are the uncertainties in determining how long ago nucleosynthesis of heavy elements occurred in astrophysical bodies?

Knowing when nucleosynthesis—the formation of new nuclei from existing nuclei—occurred in astrophysical sites can be crucial to our understanding of cosmology. One method to pin the process down in time is to compare the current abundance ratio of thorium to uranium (both of which have lifetimes of the order of the age of the universe) with calculations of this ratio at the time at which the nucleosynthesis that formed these elements took place. The assumption is that the nucleosynthesis itself happens over a time scale that is short compared to the time since it occurred.

In a paper published in Physical Review C, Zhongming Niu of Peking University and Baohua Sun and Jie Meng of Beihang University, both in China, present a study of the uncertainties in the calculation of the initial ratio of thorium to uranium. In particular, they focus on the importance of the models used to determine the nuclear masses and the nucleosynthesis processes themselves. Utilizing the abundances of uranium and thorium in the sun to restrict the models, Niu et al. are able to minimize the impact of the uncertainties. They find that the error due to the nuclear input alone is about $1.6$$2.2$ billion years (for reference, the age of the universe is about $14.6$ billion years). This estimate is lower, but not significantly so, than the observational uncertainties. In addition, they determine when nucleosynthesis occurred for three stars. New observations for other elements in stars and improved mass models could make a major impact on the thorium-uranium chronometer, and in general, studies of this kind help us learn what parts of the universe were undergoing the extreme conditions needed for nucleosynthesis to occur, and when. – William Gibbs

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