Kiwis on hunt for ‘fabric’ of cosmos

Kiwi researchers have set out to shed light on one of the most enduring questions about our universe: what holds it together?

And no, Star Wars fans — it’s not the Force.

We know our Sun is one of billions of stars in our local galaxy, the Milky Way, which is itself one of billions of galaxies in the universe.

Less well understood, however, is the cosmic fabric that binds the universe.

“Astonishingly, the most promising answer [is the idea] galaxies have ‘ skeletons’ of dark matter outweighing all of the stars and gas inside them, but we have no idea what dark matter actually is,” University of Auckland cosmologist Professor Richard Easther said.

While it’s believed to make up 80 per cent of the mass of galaxies, dark matter has never been directly observed, and it doesn’t emit light or energy.

“What we do know is that dark matter isn’t made out of atoms, or any known fundamental particles — it must be something completely new,” Easther said.

“On top of that, simple dark matter theories are increasingly boxed in by experimen- tal results, so astrophysicists are looking at new ideas.”

One of the most exciting of those was what was called “ultralight dark matter”, where the dark matter behaved like a fluid, with pressure as well as a gravitational field.

“This additional complexity makes it harder to understand, but we are figuring out how to make detailed predictions for this model.”

In a new study, awarded a $910,000 Marsden Fund grant, Easther, Auckland postdoctoral researcher Shaun Hotchkiss and colleagues will use models to try to solve questions they’ve formulated.

“We solve equations describing large quantities of dark matter on a computer — a process something like modelling the atmosphere for a weather forecast.

“There are a lot of big computer simulations that model the origin and interaction of galaxies. The largest of them take weeks on some of the world’s biggest supercomputer facilities.”

One of the reasons ultralight dark matter had not received more attention was that its combination of pressure and gravity — and its complex interactions of small and large-scale behaviour — made it harder to model on a computer.

“So one of our key goals will be [developing] a powerful computer code for making these simulations work efficiently”.

Easther’s team aimed to focus on the dynamics of so-called “dwarf galaxies”, which could be composed of as few as 100 million stars, compared with the 200 to 400 billion within our Milky Way.

“We have a structured ‘ladder’ of problems that will allow us to build up the code slowly, and we plan to release it to the community so anyone can use it in their work,” he said.

“Dark matter is a huge deal for [astronomy and particle physics] and we are helping to solve the huge puzzle it represents — whether or not the ultralight dark matter hypothesis is right, putting it to the test will mark a big step forwards.”