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 many of them should have been made?
The question boils down to basic nuclear physics. If two hydrogen atoms collide under pressure, what is the probability that they will make helium? That kind of physics also applies to collisions between other elements, some of which produce lithium. It is an astonishing feat that cosmologists, from basic physics, can predict the relative fractions of hydrogen and helium produced in the Big Bang. It’s just as amazing 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 a lot more 7Li than we observe in the universe and much less 6Li. Does this 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 with our calculations.
Lithium lingers suspiciously
Part of the problem is that lithium can also be produced and burned in stars, so it’s more difficult to determine how much of what we see represents the primordial fraction. And there are more ways to make heavier elements, making our calculations more uncertain. For now, no one is quite sure if there is a problem or not.
Finding out if our calculations are correct is where a huge amount of money and a big laser can come in handy. You see, fusing two elements together is actually quite trivial. You take element one, strip the electrons from it to create ions, accelerate the ions like crazy and fire them at a solid target of a second element. As long as the energy is high enough, you get a small fraction of the fusion products, plus a lot of gamma rays and other high-energy radiation products.
The nice thing about these experiments is that you know the density of the target, and you know how many accelerated ions hit the target during your experiment. After that, a simple series of experiments will tell you how many new atoms have been produced. That, in turn, tells us how likely each of the possible nuclear reactions is.
Unfortunately, these kinds of experiments aren’t very helpful in understanding the production of the first elements (a process called Big Bang nucleosynthesis). The energies involved in the accelerators are about a factor of 10 or more higher than expected during Big Bang nucleosynthesis, so the rates we get from these experiments are probably not very representative. Combined with theoretical uncertainties about the expected amount of lithium, no one was really sure whether we should be concerned about lithium or not.
You can start worrying now
This is where our friendly but deadly laser comes into play. OMEGA’s laser facility has a laser that produces 17 kilojoules of energy in 600 ps (10-12s) – for those keeping track at home, that’s 28 TW of power. (Don’t be fooled by the large numbers, as a single power cable to your home can deliver 17 kilojoules in about two seconds.)
The laser is used to compress a capsule containing tritium (a hydrogen atom with two neutrons surfing in the nucleus) and 3He (a helium atom with a missing neutron). The compression and shock wave 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 cosmologists believe were present during the Big Bang nucleosynthesis. For example, tritium and helium ions can fuse to form 6Li at speeds exactly matching those that should have been present during the Big Bang.
However, measuring the fusion products isn’t as easy as in previous experiments, in part because you’re not actually collecting reaction products. This type of fusion combustion can also produce other products that may lead to an underestimation of lithium production. In this case, the researchers used gamma-ray detectors to look for gamma rays with energies corresponding to fusion events specific to lithium production. That leaves you with a mess of background measurements and detector efficiencies to deal with. Taking all the junk into account, this measurement is slightly less accurate than the experiments performed at higher energies.
Still, the researchers show that this particular path to 6Li is too slow to account for the amount we observe in early stars. Excitingly, this was the last trail yet to be fully explored, I think. The other main routes to lithium are better studied and their rates are more precisely defined. So if our estimates of the fraction of primordial lithium are correct, standard physics cannot explain the total amount of lithium we observe today.
With a single but. That is, however, that we cannot be absolutely sure that we are correctly estimating the fraction of primordial lithium. Stars can produce lithium, and accounting for that is difficult. But the indications are that there’s a real problem here, and this is one of the most likely places where we can say that the Standard Model of physics doesn’t work.
Physical assessment letters2016, DOI: 10.1103/PhysRevLett.117.035002