Cosmological lithium puzzle comes closer to being solved

Lithium is unique in the cosmos. It’s one of the only elements thought to have at least three cosmic origins: the Big Bang, nuclear reactions in stars, and cosmic ray collisions. Scientists have used observations of ancient stars to guess how much lithium-7 was present soon after the Big Bang. But, weirdly, they’ve found a discrepancy between the observations and model predictions; a relatively large amount of the isotope seems to be missing. Determining the reason for the difference could help researchers better understand the formation and chemical evolution of the universe, either by revealing weaknesses in the models or how stellar observations are interpreted—or by finding the missing lithium.

Cosmic rays provide clues to lithium’s origins. The rays are known to contain large quantities of the element’s two stable isotopes: lithium-6 and lithium-7. Some researchers have suggested that a portion of the lithium-7 could be the missing amount created by the Big Bang. An international team of scientists has now put that hypothesis to rest with precision measurements of both isotopes (Phys. Rev. Lett. 2025, DOI: 10.1103/PhysRevLett.134.201001).

This is the first experimental result of its kind, says Vitali Choutko, a physicist at the Massachusetts Institute of Technology who coordinated data analysis for the new work. This is the first magnetic spectrometer coupled with a precision velocity detector in space. No other experiments have been able to measure high-energy cosmic rays with the same exactness.

The measurements were made possible by the Alpha Magnetic Spectrometer (AMS) on the International Space Station. Using the AMS, the team measured 2 million lithium nuclei over about 12 years. Those data were shared with three research groups, each tasked with determining the ratio of lithium-6 to lithium-7 in cosmic rays across a range of energies, Choutko says. The teams all came to the same conclusion: the amount of each isotope in the cosmic rays was about the same and remained constant across the high-energy range.

If large amounts of lithium-7 were in the interstellar medium, the AMS researchers would have seen different results, says Katharina Lodders, a cosmochemist at Washington University in St. Louis who was not involved in the new work. Effectively, the analysis “eliminates one possibility of where the ‘missing’ ⁷Li could be hiding,” Lodders writes in an email. “So the cosmological lithium problem remains.”

The result doesn’t surprise Brian Fields, a theoretical astrophysicist at the University of Illinois Urbana-Champaign. Because of other interstellar observations, he’s skeptical of the theory that a large amount of lithium is missing in the universe. Instead, he thinks the discrepancy comes from incorrect assumptions in cosmic models. But “theorist opinions are a dime a dozen. Data is the final arbiter of what’s going on,” Fields says. And the AMS data “is just super gorgeous.”