By Andrew Grant
An underground experiment has imitated conditions from just after the Big Bang to produce the universe’s most confounding element, lithium. The experiment’s result reinforces what scientists call the lithium problem, a discrepancy between the amounts of the element thought to have been produced 13.8 billion years ago and the amounts observed in ancient stars. This discrepancy challenges theories about the universe’s earliest moments.
Carbon, oxygen and nearly all other naturally occurring elements were forged in the cores of stars. That’s not the case for lithium.
Scientists are confident that all of the universe’s lithium, as well as most of its helium and deuterium (heavy hydrogen), formed just minutes after the Big Bang, when the expanding cosmos cooled enough for protons and neutrons to bind into lightweight atomic nuclei. The theory that describes this primordial element production, called Big Bang nucleosynthesis, successfully predicts the abundances of deuterium and helium that astronomers observe in ancient stars.
Yet the theory does not successfully forecast the universe’s current lithium supply (SN: 9/8/12, p. 14). Stars contain one-quarter to one-half as much lithium-7 (which is made of three protons and four neutrons) as the theory predicts, and they contain perhaps 1,000 times more lithium-6 (three protons and three neutrons) than expected.
Before resorting to radical explanations for this discrepancy, however, scientists want to make sure their theory correctly accounts for how lithium formed in the early universe. So Alessandra Guglielmetti, a nuclear physicist at the University of Milan, and colleagues set out to re-create the universe’s production of lithium-6 in the lab.
Using the Laboratory for Underground Nuclear Astrophysics, or LUNA, located beneath Gran Sasso, a mountain in Italy, the researchers fired a beam of helium nuclei at a deuterium target. Unlike the sites of earlier, similar experiments, LUNA is shielded by about 1.4 kilometers of rock from above-ground particles, which can interfere with the experiment. LUNA can also probe energies equivalent to about a billion degrees Celsius, the temperature at which elements probably formed during Big Bang nucleosynthesis.
The researchers found that the experiment created nearly as much lithium-6 as theory predicts, and far less than is observed in ancient stars, they report in a paper that will soon appear in Physical Review Letters. “It’s a really beautiful measurement,” says Brian Fields, an astrophysicist at the University of Illinois at Urbana-Champaign. The result, when combined with similar findings from LUNA and other laboratories about the production of lithium-7, bolsters the Big Bang nucleosynthesis theory.
The finding also eliminates the possibility of a simple solution to the lithium problem. Now that they are more certain the foundations of their theory are correct, scientists must either find errors in measurements of lithium levels in space or come up with ideas for exotic early-universe processes that could account for the discrepancy, Fields says.
Astronomers are carefully studying old stars and inactive galaxies, which maintain much of the primordial hydrogen, helium and lithium from which they formed, to improve the precision of lithium measurements. Meanwhile, Fields and colleagues are exploring the possibility that dark matter interfered with lithium production. “There are ways of introducing mischief in the early universe,” he says.
