Black hole theory deepens lithium crisis

Proposal identifies potential new source of element

The universe is lacking in lithium — and instead of solving what’s known as the “lithium problem,” a new study makes it even more complicated.

The work, published in the July 13 Physical Review Letters, suggests that some small black holes could be acting as lithium factories. The problem is, observed lithium levels are too low to accommodate such production. Those levels are also much lower than those predicted by otherwise robust theories describing how the first chemical elements were created just after the Big Bang.

“This makes the lithium problem worse,” says astrophysicist Brian Fields of the University of Illinois at Urbana-Champaign. “But it could point to more exotic goings-on in the Big Bang.”

Lithium, along with hydrogen and helium, is one of the few elements produced in the hot, energetic moments after the Big Bang. Unlike hydrogen and helium, observed lithium levels are three or four times lower than Big Bang physics predicts.

So scientists have been on the hunt for potential sources of lithium destruction.

But Fabio Iocco, a physicist at Sweden’s Stockholm University, has proposed the opposite: a potential lithium factory, in the form of relatively small black holes weighing in at an average of five solar masses. As some of these black holes drain material from a companion star, the sucked-in material forms a swirling, donut-shaped ring. Iocco calculated that the swirling mass ends up being hot enough — more than 10 billion degrees Celsius — to ignite nuclear fusion and synthesize large amounts of lithium.

According to Iocco’s calculations, even if just a tiny fraction of the Milky Way’s black holes munched matter in this fashion, the process would generate the same amount of lithium as the Big Bang. 

Fields is a coauthor on a second lithium study. In press at Nature, the study reports that interstellar lithium levels in a satellite galaxy, the Small Magellanic Cloud, mostly match theoretical predictions. It’s the first observation of interstellar lithium abundances outside the Milky Way, as traditionally scientists have measured the element’s abundance in the outer layers of stars in the Milky Way’s halo. These aging stars, born when the universe was young, should reflect the chemical abundances during that primordial period.

“It’s pretty straightforward to predict what abundance we should find, if we look at regions that probe the primordial lithium abundance,” says astronomer Chris Howk of University of Notre Dame in South Bend, Indiana, a coauthor on Fields’s study.

Looking between stars instead of inside them could help scientists determine whether some sort of unexpected stellar process — not fundamental Big Bang physics — is behind the lithium shortage. “This is exactly the kind of measurement we need,” Iocco says.

Though tantalizing, the two studies won’t be the final word on the matter, says astrophysicist Andreas Korn of Uppsala University in Sweden. “We’re still at a loss for a consistent theory,” he says.

Solving the lithium puzzle is possible if something is amiss with theories of lithium storage in stars, perhaps if stars process the element or shuffle it inward more quickly than expected.

Alternatively, it’s possible that exotic physics affected lithium levels immediately after the universe exploded into existence. This option points toward a role for primordial dark matter, particles that do not comprise “normal” observable stuff like protons, neutrons and electrons. As these particles decayed, they destroyed lithium atoms and depleted the universe’s supply, right from the start.

“At some level, we would dearly like this lithium problem to be real and pointing to something like this,” Fields says. “The data will tell us what’s going on.”

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