A star cluster some 80,000 light-years from Earth looks mysteriously deficient in the element lithium, just like nearby stars, astronomers reported on Wednesday.
That curious deficiency suggests that astrophysicists either don't fully understand the big bang, they suggest, or else don't fully understand the way that stars work. (See interactive story "Cosmic Questions.")
The discovery, slated for publication in an upcoming issue of Monthly Notices of the Royal Astronomical Society, doesn't threaten the basic idea of the big bang—that is, that the cosmos began in a state of enormous heat and density, then expanded and evolved into the universe of stars and galaxies we see today. (Related: "Big Bang Discovery Opens Door to the 'Multiverse.'")
"The most radical solution to the problem is that the big bang theory is incomplete. But less radical solutions haven't yet solved the problem," says theoretical physicist Brian Fields of the University of Illinois at Urbana-Champaign, who was not involved in the observations.
Still, the mismatch between theory and observations is a sign that something isn't right. According to basic cosmological theory, the universe served as a nuclear reactor during its first few minutes of existence, forging the three lightest chemical elements: hydrogen, helium, and lithium. Every element heavier than these, including oxygen, nitrogen, carbon, silicon, and the rest, was created either in the cores of stars or in powerful supernova explosions.
Based on their understanding of those very earliest nuclear reactions, theorists can predict how much of the three light elements should have been produced, says Joel Primack, a physicist at the University of California, Santa Cruz, who was also not part of the new observations. In general, those predictions are spot-on. "It's one of the great successes in cosmology," he says.
As long as you leave out lithium, that is.
"With lithium," Primack says, "the predictions are about three times higher than what we actually see in stars."
This shortfall of lithium was uncovered in 1982, when the husband-and-wife astronomy team of François and Monique Spite first measured it. "So many people have gone back and re-measured it since then and gotten the same answer," says Primack, "that it's regarded as being well confirmed."
But nobody had measured stellar lithium in stars beyond the Milky Way, and that's what Alessio Mucciarelli of the University of Bologna and his colleagues set out to do with the new study. "We said, 'OK, if the problem is the same in another galaxy, it would confirm that this is a universal problem, not just local,'" Mucciarelli says.
They chose a globular cluster of stars known as Messier 54, part of a small galaxy known as the Sagittarius dwarf galaxy, which is being slowly torn apart by the gravity of the Milky Way. "We honestly weren't sure it was even feasible," he said, "because these stars are so faint."
In the end, it took 30 hours of observation with the European Southern Observatory's Very Large Telescope in Chile, one of the world's most powerful, to get the evidence. Stars outside the Milky Way have the same lithium deficit as those within.
One possible explanation for the lithium shortfall could be that stars originally contained much more lithium than they do now, and that the element was somehow destroyed in nuclear reactions. "It's not a crazy idea," says Fields, "but it's hard to work out in detail."
A more likely explanation, according to both Fields and Primack, is that there was some sort of energy release in the first few minutes after the big bang that suppressed the production of lithium, and which isn't being accounted for. If that's true, the now-missing lithium may have started out in another form as cosmological detritus. That flotsam, in turn, may have decayed into "dark matter"—a kind of invisible stuff thought to make up most of the matter in the universe today.
If that's true, says Primack, "the lithium problem might not be telling us something boring about stars. It might be telling us something fundamental about dark matter."
Given that the nature of dark matter is one of the major outstanding mysteries of cosmology, that could be a very big deal indeed.
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