4 mins read 18 Jun 2020

Where Did All The Big Bang Lithium Go?

Surveys done using the world-class Anglo-Australian Telescope in NSW are helping astronomers to find out what has become of the lithium that’s been missing since the Big Bang.

Big Bang nucleosynthesis followed by the evolution of the early universe. Credit: Scientific American/Malcolm Godwin

If you were to name an astronomical survey after a well-known native Australian animal, galah might not be the first that comes to mind. These loud and often raucous birds are generally regarded by farmers as pests, and if an Australian calls you a galah, they are saying you a loud-mouthed idiot. But when your survey is based out of the Australian Astronomical Observatory and is called the Galactic Archaeology with HERMES survey, at least as an acronym, GALAH is a pretty good fit. And the data it has collected is helping researchers understand the composition of the early universe.

There is a well-known problem in cosmology known simply as the Lithium Problem. The Big Bang theory very well explains a lot of what we see in the universe now, such as the cosmic microwave background, the distribution of galaxies, and the expansion of space. It even predicts very closely the abundance of the two ubiquitous elements created before the universe was just five minutes old, hydrogen and helium. Tiny amounts of lithium were also created at this time in the process known as Big Bang nucleosynthesis, but there is a significant discrepancy in the amount that we measure in old stars, and the amount that theory predicts that we should be seeing. Therein lies the problem.

More Than Just Battery Fuel

Lithium is a fragile element that can be destroyed at relatively low temperatures in the insides of stars. But according to models, in old, low-mass stars composed mostly of hydrogen and helium, the lithium should just sit on the ‘surface’ rather than being forced inwards by convective currents. What astronomers have found though is a striking feature known as the ‘lithium dip’ that occurs in stars with surface temperatures between about 6,400 and 6,850 Kelvin. For stars that are warmer or cooler than this the depletion of lithium is not so pronounced, but coming up with an explanation that fits all of the observations is proving to be difficult. More data is needed.

That is where the GALAH survey comes in. GALAH has been exploring the chemistry of stars in the Milky Way with the goal of constructing a detailed physical picture of its formation and evolution. It uses HERMES, a high-resolution spectrometer on the Anglo-Australian Telescope (AAT), to record the chemical imprints of up to 400 stars at a time. The AAT has been operating since 1974 from the Siding Spring Observatory near Coonabarabran in NSW, and provides an opportunity for astronomers to observe parts of the night sky that cannot be seen from the northern hemisphere.

Where’s the Lithium?

Using a sample of over 100,000 stars from GALAH, an international team of researchers examined differences in the lithium abundance in stars both warmer and cooler than those in the ‘lithium dip’. Having such a large sample size with wide-ranging properties and chemistry meant that some reliable conclusions related to lithium abundances could be made. They found that it is in fact the stars warmer than the ‘lithium dip’ stars, those with temperatures up to 7,000 Kelvin, that are more likely to preserve the lithium abundance from the early universe that is predicted by the Big Bang theory. As well, stars in the three different temperature regimes, warm, ‘lithium dip’, and cool, had different lithium depletion mechanisms.

This knowledge will help to better define models of the Milky Way’s evolution - exactly the type of science the GALAH survey was designed to do. Understanding our existence is a gigantic puzzle, and every piece that is put into its proper place provides us with more clarity about ourselves and the cosmos. Australia is playing an important role in this journey of discovery, and with world-class facilities like the Anglo-Australian Telescope, we can expect it to continue doing so into the future.

Acknowledgment: The paper’s authors acknowledge the traditional owners of the land on which the Anglo-Australian Telescope stands, the Gamilaraary people, and pay their respects to elders past and present.