The business end of the University of Rochester’s Omega laser. (credit: Lawrence Livermore National Lab)

Last month, we reported on a small but enduring mystery in cosmology: why is there so much of one isotope of lithium around? Both ⁶Li and ⁷Li should have been produced when the first atoms formed after the Big Bang. But how much of them should have been made.

The question comes down to basic nuclear physics. When two hydrogen atoms collide under pressure, what is the probability that they will make helium? That sort of physics also applies to collisions between other elements, some of which produce lithium. It is an astonishing achievement that cosmologists can, from basic physics, predict the relative fractions of hydrogen and helium produced in the Big Bang. It is just as astonishing that we can look back in time and measure these fractions and know that cosmologists have it almost exactly right.

Almost. These calculations fall flat when it comes to lithium. They suggest there should be much more ⁷Li than we observe in the Universe and a lot less ⁶Li. Does that mean the estimates are wrong, or is there a real discrepancy? New experiments indicate that when it comes to ⁶Li, the problem seems to be with the Universe and not our calculations.