A decades-old clash between modern cosmology and stellar observations may have just gotten worse.
A paper set to appear in Physical Review Letters later this month might add to a problem that’s had astronomers baffled for 30 years: the universe doesn’t have enough lithium.
As the third element in the periodic table, lithium is one of a few elements that have an abundance closely tied to processes just after the Big Bang. Detailed models of big bang nucleosynthesis predict certain levels of these elements, such as hydrogen and helium, and for the most part these models closely match what observers see in the cosmos.
But 30 years ago, Monique and François Spite (Paris Observatory) reported that the isotope lithium-7 was far rarer in old, metal-poor stars in the Milky Way’s halo than it should be. These stars formed in our galaxy’s early days, back when its chemical makeup more or less matched what existed after the universe’s birth. Relatively cool and with poor mixing between surface and interior, such stars should have lithium-7 levels in keeping with primordial abundances.
Yet these stars have at most one-third the amount of lithium-7 predicted. Even lower levels are found in the most primitive stars — stars with very low levels of heavy elements, which weren’t created by big bang nucleosynthesis. This upper limit became known as “the lithium problem.”
Astronomers have devised various solutions to explain the missing lithium, but nothing’s really worked. Any process that could deplete the lithium would need to happen in stars of various temperatures and compositions and without messing up the abundances of the other elements, François Spite says.
Now, Fabio Iocco (Oskar Klein Center for Cosmoparticle Physics, Sweden) and Miguel Pato (Munich Technical University, Germany) have added another potential hurdle: black holes.
Recent work suggests that in the early galaxy there may have been a fair number of “microquasars,” stellar-mass black holes yanking material off a stellar companion and shooting jets of superhot plasma into space. Iocco and Pato looked at the conditions in the hot accretion disks around these black holes, where temperatures can reach tens to hundreds of billions kelvin. Such temperatures jump over those where lithium-7 is merely disrupted (around 2.5 million K) and up to the point where the helium reactions that create lithium happen, Iocco says.
The duo found that, even if only 1% of the Milky Way’s microquasars produced temperatures hot enough to create lithium-7, the amount created would rival that expected from the universe’s first few hours.
So, the question remains: where is all the lithium?