Written by El Hebert '24
Edited by Alyssa Steinbaum '23
At the moment of the Big Bang, the universe, and all the energy it would ever have, blasted into being. The first few minutes saw the birth of matter as atomic nuclei congealed - hydrogen and helium, the lightest elements, the stuff of stars. And then came lithium, element three. That’s where the trouble began.
Our universe still holds remnants of its ancient past. By peering out at distant objects born billions of years ago, scientists can measure the elements that made them. They’ve found that hydrogen and helium abounded in the early universe, in proportions near-identical to those predicted by our current Big Bang model - so far, so good. But to measure the early lithium supply, they must look closer to home.
In the fuzzy outer halo of the Milky Way, the galaxy’s oldest known stars burn quietly on. These faint lights hail from a time before heavier elements accumulated in the region. Nearly forty years ago, François and Monique Spite at the Paris Observatory noted that halo stars fail to destroy lithium by nuclear fusion. They are time capsules of lithium abundance in the newborn universe - and they don’t have nearly enough .
The most up-to-date Big Bang atomic synthesis models predict that the universe began as a high-energy stew of particles and light. As these particles interacted, some of them - protons and neutrons - formed the first atomic nuclei. Two reactions in particular created lithium: one building off of lighter hydrogen, and one ripping components from heavier beryllium. Using the remarkably accurate values obtained for the early abundance of the lighter elements, physicists can calculate how much lithium should have arisen. When they turned to the halo stars to finally check their predictions, however, the numbers simply didn’t line up: halo star measurements show that the newborn universe was missing nearly two-thirds of its expected lithium . So where did it all go? Did it ever even form in the first place?
Astronomers, particle physicists, mathematicians, and cosmologists have struggled to explain the Lithium Problem since it was first discovered. Some of their work adjusts the Big Bang nucleosynthesis model itself, chipping away at the gap by correcting uncertainties in our predictions.
Earlier this year, researchers from the University of Tokyo examined one of the lithium-producing processes more closely than ever before. Their reaction - the decay of beryllium and a neutron into lithium and a proton - is notoriously unruly due to the instability of the ingredients. So instead, the team used the “Trojan Horse” method: they “smuggled” the neutron into their particle beam, concealed inside a hydrogen nucleus. With this method, they made the most precise measurements of lithium yields yet, and they found their calculations to be 10% lower than model predictions. These results indicate that at least part of the Lithium Problem may be solved by updating our models. Reaction rates need to be corrected, and completely new reactions might remain undiscovered [3, 4].
On the other hand, maybe our lithium abundance measurements were completely wrong in the first place. Halo stars might not be the perfect time capsules we thought they were. Other possibilities exist, though. For example, dense stellar corpses called white dwarfs were recently found to contain traces of early lithium, probably from asteroids crashing into their atmospheres. If halo stars do burn lithium in nuclear reactions after all, those preserved rocky remnants would still be safe. Measuring them just might reveal that the problem was of our own making all along .
But if these corrections still fail to account for the lithium gap, future scientists will face a reckoning. Explaining lithium could require a departure from our accepted Standard Model of particle physics. Even now, exotic solutions crop up with names like dark matter decay and supersymmetric interaction. Is spacetime itself lumpy and nonuniform like Rocky Road ice cream? We don’t know yet, but such a scenario could explain a few cosmic mysteries, the Lithium Problem among them .
The universe is big, strange, old, and full of unknowns. A gap between theory and result at such a fundamental level is an uncomfortable thing, but it’s also an opportunity. The Lithium Problem warned us that something in our model doesn’t add up, and now it’s our turn to rebuild it, getting steadily closer to the truth at the beginning of everything.
 Spite F, Spite M. Abundance of lithium in unevolved halo stars and old disk stars-Interpretation and consequences. Astronomy and Astrophysics. 1982 Nov [cited 2021 Nov 6];115:357-66. Available from: https://ui.adsabs.harvard.edu/abs/1982A%26A...115..357S.
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 Hayakawa S, La Cognata M, Lamia L, Yamaguchi H, Kahl D, Abe K, Shimizu H, Yang L, Beliuskina O, Cha SM, Chae KY. Constraining the Primordial Lithium Abundance: New Cross Section Measurement of the 7Be+ n Reactions Updates the Total 7Be Destruction Rate. The Astrophysical Journal Letters. 2021 Jul 1 [cited 2021 Nov 6];915(1):L13. DOI: 10.3847/2041-8213/ac061f.
 University of Tokyo. Closing the gap on the missing lithium: Researchers account for some of the lithium missing from our universe. ScienceDaily. 2021, July 1 [cited 2021 Nov 6]. Available from: www.sciencedaily.com/releases/2021/07/210701112629.htm.
 Kaiser BC, Clemens JC, Blouin S, Dufour P, Hegedus RJ, Reding JS, Bédard A. Lithium pollution of a white dwarf records the accretion of an extrasolar planetesimal. Science. 2021 Jan 8 [cited 2021 Nov 6];371(6525):168-72. DOI: 10.1126/science.abd1714.
[Image Citation] A glimpse of the ancient universe, through the Hubble Deep Field. NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI), hosted on Wikimedia Commons.