There’s an Abundance of Lithium-6 in the Universe, and Scientists Don’t Know Why

The difference between Lithium-6 and Lithium-7 might not seem like much, but it is poking holes in our understanding of how atoms formed during the Big Bang.

Technically, they’re off by a single neutron: 6Li contains three neutron and 7Li contains four. Compared to theoretical calculations, experiments show that the universe contains way too much 6Li and too little 7Li. Using basic principles of physics, scientists can compute how much of each type of lithium should have been created during the Big Bang. But, the theory doesn’t match the data—each time they look to the sky, physicists observe a different balance of lithium than expected.

Cosmologists have looked to the small and large Magellanic Clouds, shown here, for measurements of 6Li and 7Li to compare against theoretical models.

Scientists have good reason to believe their calculations are correct. They accurately predict the observed quantities of many other elements with startling accuracy.

A recent experiment published in Physical Review Letters tried to replicate the conditions of the Big Bang to prove that more lithium was produced than scientists originally calculated. The lithium would come from a fusion of a tritium atom and helium ion, which could only be replicated with a giant laser.

Explaining the experimental set up, here’s Chris Lee writing for Ars Technica:

The laser is used to compress a capsule containing tritium (a hydrogen atom with two neutrons couch surfing in the nucleus) and 3He (a helium atom with a missing neutron). The compression and shockwave are so fast that the heavy nuclei have very little time to accelerate, so the result is a cold, dense plasma. This plasma has a temperature and density that is pretty much what cosmologists think was present during Big Bang nucleosynthesis. Thus, tritium and helium ions can fuse to form 6Li at rates that correspond to exactly those that should have been present during the Big Bang.

But the experiment didn’t produce enough 6Li to account for all that’s observed—that is, the new results support the existing perplexing calculations. The disconnect between theoretical and experimental values of lithium in the universe cannot be explained, and that’s a big problem for our understanding of basic physics.

Fortunately, there’s one possible explanation for the discrepancy that hasn’t been ruled out yet—lithium is created and destroyed inside stars. It’s possible that scientists accurately predicted how much 6Li was produced during the Big Bang, and the rest was created later in stars. But for now, lithium is challenging our understanding of the Big Bang, big time.