Tuesday, May 30, 2017

The Mystery of Cosmic Lithium

When most people think of lithium, they probably think of antidepressants, or possibly lithium batteries. But to a cosmologist, lithium is the misshapen puzzle piece -- the one that won't quite fit, no matter how hard you push it.

What does lithium mean to you?
Most of the elements were produced in the nuclear furnaces of stars or supernovae, but a handful were made in the first few minutes of the universe, when it was incredibly hot (billions of degrees) and dense. These big bang elements include helium, deuterium (a form of hydrogen with an extra neutron) and one isotope of lithium. Our theory, called "big bang nucleosynthesis," gives beautiful agreement with the observed helium and deuterium abundances, but it fails miserably for lithium -- the theory predicts about three times as much lithium from the big bang as we actually observe.

So what's going on here? As one of my high-school teachers used to say, "half-right equals all wrong." But the fact that big bang nucleosynthesis works so well for helium and deuterium suggests that maybe it just needs a few tweaks, instead of wholesale reworking. Roughly speaking, there are three likely alternatives:

1. Maybe the observations are misleading us. We can't measure the lithium abundance from the big bang directly, since matter from the early universe gets cycled through several generations of stars. Instead, we have to infer the primordial lithium abundance from present-day stars. So it's possible that some process inside of stars destroyed about 2/3 of the primordial lithium before we got to see it today.

2. Our predictions of element production in the early universe require a computer simulation of all of the nuclear reactions that take place when the temperature of the universe was a few billion degrees. Maybe something is missing from our nuclear physics calculations -- perhaps one of the reactions occurs at a faster or slower rate than our calculations suggest.

3. It's possible that something really weird is going on in the early universe, something that totally changes the production of lithium. One example would be a new elementary particle decaying in the early universe, with the products of the decay destroying the lithium. There's a precedent for "new and weird" as an explanation for astrophysical mysteries -- for many years the neutrinos detected from the sun fell short of the theoretical predictions. Many scientists believed that there must have been some error in our calculations of energy production in the sun. But it turned out that the neutrinos were oscillating from a type of neutrino that we could detect into a different type that we could not. So in this case, the boring explanation turned out to be wrong -- instead, the solar neutrinos were telling us something very new and important (and weird!) about the nature of the neutrinos themselves.

The lithium problem has been around for a while, and we still don't know what's going on. If I had to bet money, I'd put my money on option 1. But of course the most fun would be option 3.

1 comment:

Kathy said...

I'd no idea the missing neutrino mystery had been solved. Thanks for bringing it up.

In "Songs of Distant Earth," Clarke used the missing neutrinos to get the Sun to go nova :)

Good luck accounting for the missing lithium.