By Dr Catriona Nguyen-Robertson MSRV, with Professor Alan Duffy
This article revisits a presentation to the Royal Society of Victoria on 12 September 2019 titled “Darkness Visible Down Under” by astrophysicist Professor Alan Duffy from Swinburne University.
“Beyond the shoreline exists this whole new world out there…how could you not want to explore it?” – Professor Alan Duffy.
If you look through the Hubble Space Telescope at a patch of sky the size of your thumb nail, you will find stars, and clouds of gas and dust, distributed across 6,000 individual galaxies. The universe is massive, but what we see is merely the tip of the iceberg. There is an invisible “Dark Universe” that outweighs everything that we can see five times over.
Dark energy makes up approximately 68% of the universe and dark matter makes up another 27%. Despite their dominance in the universe, we still are not sure of what they actually are. The rest – everything we can observe with all our instruments – makes up less than 5%.
Growing up in Northern Ireland, Alan Duffy would often press his face to the window to stare out at the stars. He has always been fascinated by the world around him. After reading Stephen Hawking’s A Brief History of Time, he was inspired to channel his curiosity into studying physics. Driven by his desire to want to know more, he undertook a PhD in astrophysics at the University of Manchester. Wanting to explore the unknown, he felt the pull of the Dark Universe. Now, as an astrophysicist at Swinburne University of Technology’s Centre for Astrophysics and Supercomputing, he has led a 15-year curiosity-driven research project into uncovering the nature of dark matter and the formation of galaxies, and the acceleration of the universe caused by dark energy.
Alan Duffy: “We have never known so little about our universe. Science is not done.”
For astrophysicists like Alan, it seems a nearly impossible task to study something currently undetectable, however, we can learn by studying their effects on other things. For example, while we cannot see the air, we can see its effect on the objects it moves, such as swaying tree branches in the wind. Similarly, dark matter is composed of particles that cannot be seen directly, but we know that it exists because of the effect it has on objects that we can see.
Seeing the invisible
When we observe the way things move in space, often, our observations do not make sense if we only consider what can be seen. Spinning galaxies that spin at great speeds over time are an excellent example. The Milky Way Galaxy, for example, is spinning, taking 250 million years to complete a full turn. But most galaxies spin, particularly their outermost reaches, at speeds that cannot be explained by the gravitational pull from visible matter alone.
When astronomers examined spiral galaxies in the 1970s, they expected to see stars in the centre of the galaxies moving faster than those at the outer edges. You would expect that stars at different distances from the centre of a galaxy would have different orbital speeds, just like Mercury orbits the Sun faster than planets further out in our solar system. Yet inner and outer stars of galaxies appeared to be travelling at the same pace. This hinted that there is more mass within the galaxy than meets the eye.
We do not know much about the vast, invisible clouds of dark matter, but whatever dark matter is, by providing extra mass to galaxies, it generates the extra gravity needed to hold them together. The galaxies are rotating at such high speeds that the gravity generated by their observable matter could not possibly be enough to hold them together – they should have torn themselves apart long ago. Dark matter must be providing extra gravity and holding them together so that they spin without parts being flung off into the cosmos. There must be an enormous amount of dark matter in the universe, pulling on all the things we see. And yet, invisible as it is, dark matter passes through our planet and passes through our bodies, unencumbered by the electric fields that give solid matter its apparent, well, solidness. Entirely imperceptible to us. It is like a cosmic ghost.
Alan investigates how dark matter helps galaxies form and keeps them intact. He uses supercomputers to simulate this process. But the more we learn about the visible parts of the universe, the bigger the gap becomes between what we predict about the unknown and what we see. He and other astrophysicists are hence constantly under pressure to gain a better understanding of dark matter and dark energy. Alan does this by testing his simulations of our galaxy’s formation and dark matter theories and comparing these to observations from telescopes to see if they match up.
Can we directly detect elusive dark matter?
Alan and his team are also devising ways of detecting dark matter in the laboratory. Located one kilometre underground, at the bottom of the Stawell Gold Mine, the Stawell Underground Physics Laboratory will facilitate experiments critical in the global search for dark matter.
SABRE South, the world’s first dark matter detector in the southern hemisphere, is led by the University of Melbourne’s Professor Elisabetta Barberio. It is one of a pair of experiments – SABRE South and SABRE North (in the northern hemisphere, under the Gran Sasso Mountain in Italy) – intended to distinguish true signals from possible local or seasonal influences. Essentially, the hope is that dark matter particles are detected in the same way at both sides. The SABRE experiment will be transported into the laboratory, with data collection expected to begin in 2024, a delayed start due to COVID.
Alan is a part of SABRE, attempting to detect dark matter. SABRE, short for Sodium Iodide with Active Background Rejection Experiment, uses sodium iodide crystals to search for proposed dark matter candidate particles. When dark matter particles, or indeed any particles, stream through the gold mine and collide with crystals, they will produce a flash of light (known as a scintillation) that will be picked up by sensitive detectors. Dark matter is thought to rarely interact with ‘normal’ matter, but occasionally, in a snooker-ball like collision with the nucleus of an atom, they send it recoiling – a reaction that ultimately produces the light that can then be observed.
Particle physicists believe that dark matter exists in the form of a particle – as does the ordinary matter with which we are so familiar (electrons, photons, etc.). Each of the particles in the Standard Model of Particle Physics – the building blocks of the visible universe – have their own properties that define what they are and how they interact with the other particles. The same is true for dark matter, however we do not know what the defining properties of these particles are.
This is what scientists hope to reveal. Alan and a global collaboration of researchers are turning the impossible into something possible. In their endeavour to learn more about our universe, they explore the unknown and the unobservable. There is more dark matter in the universe than all the gas, dust, planets, and stars combined. Consider everything humans have achieved with an understanding of only 5% of the matter in the universe. Imagine what we could do if we unlocked the rest.
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