New observations of the Small Magellanic Cloud have only heightened the mystery surrounding a decades-long cosmic conundrum.
The universe still has a lithium problem.
That’s the implication of a paper in this week’s Nature by researchers from the University of Notre Dame and the University of Illinois at Urbana-Champaign. Processes just after the Big Bang ought to have created certain amounts of elements such as hydrogen, helium, and lithium. Astronomers already knew the universe has far less of the isotope lithium-7 than it should — studies of the old stars that surround the Milky Way in a halo show that they have at most one-third the amount of lithium-7 predicted by careful models of what happened in the newborn universe. Some stars in the Milky Way’s disk do have more lithium-7, but such stars are generally thought to be younger than the halo stars and are therefore polluted by heavy elements made later in the universe’s history.
The lithium is a major thorn in astronomers’ sides, because other elements seem to match predictions just fine. Any process that could destroy the lithium would need to leave these other elements unscathed, and that’s a tricky proposition.
To dig into this problem, J. Christopher Howk (University of Notre Dame) and his colleagues measured the lithium outside the Milky Way. They pointed the high-resolution UVES spectrograph on the 8.2-meter Very Large Telescope at a massive, young star in the Small Magellanic Cloud, a dwarf galaxy orbiting the Milky Way. Light from the star passes through the gas and dust lying between the star and us and picks up the spectral fingerprints of the elements it encounters.
That interstellar material has about one-fourth the amount of heavy elements as the Sun. The value is at least 10 times higher than the heavy-element levels in many halo stars, but it’s a step in the right direction to studying primordial abundances, Howk says. The team detected an amount of lithium-7 that’s a bit lower than predicted but that could be consistent with the theory of big bang nucleosynthesis.
This might sound like a solution to the lithium problem, but it’s not. The amount of lithium should only go up with time as more stars create heavy elements and then spew those elements into space as the stars age and die. If the Small Magellanic Cloud now has about the same amount of lithium as big bang nucleosynthesis predicts, it had to begin with less.
“That worries us a bit,” Howk says. “There are a few ways to tune things to get out of having big bang nucleosynthesis be wrong, but they start to become a bit uncomfortable.”
Miguel Pato (Technical University of Munich, Germany) and Fabio Iocco (Oskar Klein Center for Cosmoparticle Physics, Sweden), who have worked on the lithium problem, note that an interesting point in the new paper is the hint Howk and his colleagues detected of another isotope of lithium, lithium-6. If that hint is the actual level of lithium-6 in the interstellar cloud the team studied (and Howk stresses that the data are not strong enough to make that claim), it would suggest that there’s about a thousand times more lithium-6 present than is predicted by big bang nucleosynthesis. Because the theory predicts how much lithium-6 there will be compared to lithium-7, the amount of lithium-6 could reveal the mechanism that made the lithium-7 and determine just how much of it originated in the universe’s first few hours. Howk says his team has more observations planned for this November to help pin down the amount of lithium-6 present.
Reference: J.C. Howk et al. "Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud." Nature, 6 September 2012. (Abstract available only — if I see a version of the full paper, I'll add the link.)