Still eluding us (Image: Royal Observatory, Edinburgh/SPL)

MOMENTS after the big bang, the primordial soup of subatomic particles congealed into a few light elements, such as hydrogen, helium and lithium. But there’s a problem: models of the big bang indicate that there should be much more lithium than we see today. Are our theories wrong?

The latest search has turned up more lithium than ever before, but not enough to fit the models. That strongly hints that some unknown physics may have come into play right after the big bang.

Earlier research probed the atmospheres of stars near the edge of the Milky Way for lithium-7, the most common isotope. These stars are very old, and the amount of lithium they contain is related in a predictable way to the composition of the early universe. The observations focused on the stars’ outer layers because they don’t mix with material from the core, where lithium can be produced.


The amount of lithium-7 these studies found is about a third of what big bang models say it should be. One possible reason is that lithium is sinking into the stars’ interior and being destroyed by high temperatures.

Christopher Howk of the University of Notre Dame in Indiana and colleagues have expanded the search to gas clouds in a nearby satellite galaxy, the Small Magellanic Cloud. Using the Very Large Telescope (VLT) in northern Chile, they discovered nearly as much lithium in these clouds as big bang models predict (Nature, DOI: 10.1038/nature11407).

The shortfall remains significant, though. That’s because lithium is also produced by natural particle collisions and in supernovae. The new finding only deepens the mystery, says Howk. “Only if there is no change in the lithium abundance since the big bang is this problem solved,” he says.

Only if there is no change in the lithium abundance since the big bang is this problem solved

Other recent work also complicates things by hinting that the accretion disks of black holes can produce large amounts of lithium-7. Until recently, this idea had always been applied to the supermassive black holes found in the cores of galaxies. Now Miguel Pato of the Technical University of Munich, Germany, and Fabio Iocco of Stockholm University in Sweden have shown that even stellar-mass black holes can generate lithium as they swallow material from an orbiting companion star (arxiv.org/abs/1206.0736). Such systems, called microquasars, are very common, so their small yields of lithium can contribute significantly to the element’s abundance, says Pato.

All this suggests that the amount of lithium we see represents an even bigger shortfall from what should have been created after the big bang. Howk wonders whether exotic physics just after the big bang could be to blame. One speculation is that reactions involving dark matter – the enigmatic stuff thought to account for much of the universe’s mass – could have suppressed lithium production.

Howk and colleagues plan to test this idea at the VLT in November with more observations of the Small Magellanic Cloud.