Catching sight of dark matter

CHRIS HATZIS
Eavesdrop on Experts, a podcast about stories of inspiration and insights. It’s where expert types obsess, confess and profess. I’m Chris Hatzis, let’s eavesdrop on experts changing the world - one lecture, one experiment, one interview at a time.

There are phenomena in our universe that are almost beyond our realm of understanding – what lies in the dark space between the stars? How can we measure things that are invisible? What prevents the galaxies from expanding into oblivion? One explanation is the existence of dark matter, undetectable because it’s comprised of particles that don't absorb, reflect or emit light. The quest to understand dark matter requires innovative approaches.

Elisabetta Barberio is Professor of Physics at the University of Melbourne and the Director of the ARC Centre of Excellence for Dark Matter Particle Physics. Elisabetta is leading groundbreaking Australian research in the hunt for dark matter and it’s all happening 1000 metres underground in Stawell, Victoria, with the construction of the Stawell Underground Physics Laboratory, or SUPL. The SUPL will allow researchers, led by founding SUPL Director Professor Elisabetta Barberio, to search for dark matter, the so-far undiscovered elementary particle that perhaps makes up a quarter of the universe.

Professor Elisabetta Barberio sat down for a Zoom chat about her work with Dr Andi Horvath.

ANDI HORVATH

Elisabetta, what is dark matter?



ELISABETTA BARBERIO
As far we can see in the universe, stars, galaxies and dust is all made of atoms and atoms are made nuclei and electrons. This is the ordinary matter that makes up Earth and us. However, when we look at the cosmos and we look at the galaxy and the rotation of the galaxy, we realise that in the cosmos, there must be more matter than what we can see. This matter that we cannot see but must be there because gravity tells us it's there, is the dark matter. What is the composition of this mysterious substance, we don't know. One of the main, the biggest challenges in fundamental physics of this century is really understanding the nature of dark matter.

ANDI HORVATH

How much dark matter makes up the universe? If there's normal matter and there are other particles, which we've identified that make up the universe, how much is dark matter the composition of the universe?



ELISABETTA BARBERIO

If we discuss about how much matter there is in the universe, dark matter is about 85 per cent of the matter in the universe. If we consider also energy because from Einstein we know that energy is equal to mass times the speed of light squared, at that point, the universe is composed of 25 per cent dark matter, five per cent ordinary matter, the matter we are made of, and the remaining is all what we call dark energy, that is even more mysterious than dark matter.

ANDI HORVATH
Okay. So, there's a lot of particles, a lot of energy, but you're focussing on dark matter. Tell me about the dark matter particle? How is it different to say, cosmic rays or radiation?

ELISABETTA BARBERIO
The dark matter particle, from the name dark matter is called like that because it doesn't emit light. So, if you have a telescope, you cannot see. But it does not only not emit light in the visible spectrum, it doesn't emit any electromagnetic radiation, so radio waves, infrared, ultraviolet. No matter which kind of astronomical instrument you try to look at the sky, you cannot see. The particularity of dark matter is that it doesn't interact very much, if at all, with ordinary matter. For this reason, we really don't know what it is. We have hypotheses on what it is based on the fact that it's keeping the galaxy together, based on the fact that we know that if you rewind back in time, the universe gets smaller and smaller and smaller until all the energy and mass of the universe goes in a single point, called a singularity, and this is the Big Bang. If we look at the history of the universe, we know that galaxies have been formed at a certain time in the history of the universe, and they've been formed where there were pockets of this dark matter. So, we know that dark matter is there.

We know that in our galaxy, the majority of the material in our galaxy is dark matter. As Earth goes around the galaxy, it goes in this flow of dark matter. From these astronomical observations, we know that dark matter is massive, as in mass, otherwise it would not be feeling gravity. We know that it is going quite slow, so that means much slower than the speed of light. This is another characteristic, so that means that it's quite massive, it's not very light. It weighs much more than an electron. Then we know that - exactly - it does not interact with anything. This is what we know of dark matter. It's a particle with these characteristics. Of the particles that we've discovered up to now, none of them have these characteristics.

ANDI HORVATH
Okay. So, dark matter is the particle that holds the universe together. Is it travelling through my body right now? Is it like radiation? Can it go through or bounce off some of the atoms in my body?

ELISABETTA BARBERIO

Dark matter passes almost through everything. If you think about our galaxy, you can think about our galaxy and then you can think that our galaxy is immersed in a cloud of dark matter. As we travel with our solar system around the centre of the galaxy, we are passing through this cloud of dark matter, and this dark matter passes through our body, through Earth, interacting with it very, very little. I would say we have millions of dark matter particles passing through our bodies every day, continuously. At the moment, as we are talking, there is this dark matter wind that passes through us. Now, dark matter, you can consider that is not moving and Earth is moving through the dark matter. So, there is this apparent wind of dark matter thrown at us that is about - there is wind flow with a velocity of about 800 kilometres an hour, it's quite strong wind. Then if we look at the millions of particles of dark matter that pass through us every day, maybe a handful of them interact with some of the nuclei that we have in our bodies.

ANDI HORVATH
Okay. So, it's theoretically - or hypothetically, it's there, but we haven't actually observed the particle. Can you give us some examples of particle physics history, where we've theorised about a particle and then found it?

ELISABETTA BARBERIO
Well, of the recent years in particle physics, for example the Higgs boson. We know that at a certain point we needed to understand how from the Big Bang builds up with energy, mass originated, mass of the fundamental particle. One of the theories involved the existence of a new particle called the Higgs boson. Higgs was the guy that proposed the particle. For 40 years, particle physicists looked for this Higgs boson. Eventually, it was discovered in 2011 and announced in 2012 in Geneva, using the Large Hadron Collider data. This was something that was predicted, but we needed to chase for more than 40 years.

ANDI HORVATH
I'm sure that one of the most common questions you get is why do we need to know these particles are there?



ELISABETTA BARBERIO
Well, as I said, it's the majority of the material of the universe. At the end of the day, we know only five per cent of the universe. We have a theory that regulate fundamental particle enforces, then we don't know anything else. We are in an island that is only five per cent of the universe, and then there is all this other part of the universe that is completely unknown. For example, dark matter could be very complex. Some people think it's a single particle that makes up so much of the universe. Some other people think that the dark universe is like a universe that is as complex as our universe but is interacting so little with ours that we cannot see. It's quite interesting. It's just exploring the most unknown.

ANDI HORVATH
So, particular physics helps our understanding of nature and the galaxy itself, its basic understandings. I know one of the questions that physicists often don't like, but how can we use this? How have we used knowledge of particle physics in the past?

ELISABETTA BARBERIO

Fundamental physics is interesting. There are two ways you can use knowledge. One is just time. If you think about the beginning of the 20th century, there was the quantum revolution. So, people understood that the atom and nucleus didn't work with the classical physics that had been studied up until there. They needed to introduce a new way of describing physics, and that work is called the quantum physics. If you ask in the 30s, 1920s, 1930s, what do you do with this quantum physics? People thought that it was just a curiosity, just how we discussed now about dark matter. But if you think about it, this quantum physics is the one that brought nuclear physics, particle physics. Now we are thinking about quantum computing, quantum engineering is becoming part of everyday life. Our phone, our computers, everything is based on that. So, fundamental physics, at the beginning, the first time you encounter, it's just a curiosity, understanding more about the universe. Then, years later, it's becoming one of the tools of everyday life.

The other part is that to explore these small particles, to explore this universe, we need to create instruments that don't exist because the problem has never been encountered before. So, we need to invent new technology. This new technology is really providing rupture in technological development that are quite important. For example, the fact that you can now miniaturise in your phone the memory, so you can store a lot of information. You know, most of our mobile phones are as powerful, if not more powerful, than the computers that brought the men on the moon. It stems from the fact that in some experiments in particle physics, we needed to miniaturise our electronics because they didn't have the space. This is something that has been given to society.

Another thing that is quite big that changed completely society, is that to communicate at CERN with all the people around the world for these experiments, like the LHC experiment, 3000 people. It happened that someone invented the web. Not that there didn't exist the internet - a way to navigate the internet and to share information. It was very easy to use. That was the web revolution that - now we're having a podcast. It's part of that revolution. So, that is the way that the technology will be used - what we discover in the end, many years from now or less years from now, depending on what we discover, will have a technological application. On the other hand, there is always technology that we invent to chase these unknown particles.

ANDI HORVATH
True. We often lose track of how much knowledge is accumulated and recontextualised in other research laboratories to make the things we have. I think there's a famous quote of someone - was it someone commenting to Faraday about electricity saying, well, that's great, but what do we do with it? So, there you go. Now, talking about detecting dark matter, what is the photosensitive paper that you're going to use to detect dark matter? Because we can't detect it yet. So, what are the various technologies that are going to help us detect dark matter?

ELISABETTA BARBERIO
We need to find atoms, substance in which when they interact with the - dark matter interacts with the substance, either they produce light, or we can put them at very low temperatures, so very cold, almost at the absolute zero, so that when dark matter interacts with the substance, they get a little bit hotter because there is energy released, and we can measure this difference in temperature. We need to choose this kind of material, that either emits light or we can measure the temperature. We need substances that are very, very pure because dark matter interacts so rarely with other matter that any radioactivity that we have in the material can mimic dark matter. What we are doing is using - developing materials that are ultra-pure, like having so little radioactivity that even our body is too radioactive to be close to this substance.

Then we place this substance underground with instrumentation that can read either the temperature or the light, and embed this substance that could be salt, like sodium iodide salt crystals that produce light or xenon, liquid xenon that also emit light when hit by dark matter, or even cooled down - the sodium iodide crystal almost at the absolute zero and see how much they increase the temperature when dark matter interacts with them, and place them underground. So, the noise that comes from cosmic rays and bombards us all the time in the surface, the dark matter signal does not disturb them. Then we wait.

ANDI HORVATH
Wow. That sounds amazing. So, you're using these very purified crystals or materials, or they're at very low temperature, and even our human bodies give off too much radiation that they will react to it. That's amazing. That's very sensitive detection. Now, you mentioned the laboratory is underground and you've been involved in building an underground laboratory. Has it been built? It's about a kilometre underground in Stawell, Victoria, Australia.



ELISABETTA BARBERIO

So, to catch dark matter that is all around us, in the galaxy, we need to go deep underground because we don't want all these cosmic rays that reach us every day from the sun, the stars and the galaxy to spoil the signal of dark matter. Just to give you an idea, we have billions of cosmic rays reaching our bodies every day, a million dark matter particles reaching our body, but while almost all the cosmic rays interact with our atoms, only a handful of dark matter particles interact with our atoms. Obviously, if you want to do an experiment, you need to go where there are no cosmic rays, so we need to go deep underground. Our very sensitive materials are placed deep underground. When we speak deep underground, we're speaking about eight kilometres or more.

Since we want to do this first dark matter experiment in the southern hemisphere, that will be done in Australia, we needed to build an underground site to locate our detector. We found the perfect site in Stawell, nearby the Grampians because there was a mine. There is a mine, there's a gold mine that is one of the deepest mines in Australia. They offered to host us underground, one kilometre underground. We just finished the excavation a few months ago of the cavern where there will be fitted the laboratory where our dark matter experiment will go. We are one kilometre underground. To reach this laboratory, we need to drive down in the mine for 15 kilometres and it takes about 20 minutes because you cannot speed in the mine. When we reach there, at the moment there is just - it looks like a big hole in the ground, but with time, we will fit a full high-tech laboratory, where our experiment will go.

You can also see, there are also experiments - all these kinds of experiments to catch dark matter are underground, so going there, our detector will be able to see dark matter because the cosmic ray cannot reach one kilometre underground.



ANDI HORVATH
That is very exciting that there's an underground laboratory that's going to be looking for the matter that holds together the universe. When do experiments start?

ELISABETTA BARBERIO
We are building the experiments, so if people want to see part of our experiments, it's in Wantirna, in a shed in Wantirna. Well, it's a laboratory in Wantirna, a big laboratory, this was a big - the external shell of our experiment, and will be brought down next year, down in the underground laboratory. The underground laboratory needs to be fitted now and made - rendering the walls, and making the floor, putting all the service and so on. So, we expect that in about one year from now, this laboratory will be ready, so we can start bringing down our experiment. It's called the SABRE experiment, and start commissioning our experiment and build it.

ANDI HORVATH
This is of great importance to the Australian and Commonwealth state governments. Is there other collaborators that are also working with the Australian physicists that are going to be searching for evidence of dark matter?

ELISABETTA BARBERIO
Yes. So, this experiment that we are building is involving four universities in Australia, the University of Melbourne, Swinburne, ANU and the University of Adelaide. There is also a federal agency, that is ANSTO. But there is also international collaborators. The strongest collaborators and the larger group comes from Italy, from the Istituto Nazionale di Fisica Nucleare, and that is the funding agency for particle and nuclear physics in Italy. There is a group from Rome, there is a group from Milan, and there is also that comes from another underground laboratory that is under the mountain in Italy, in central Italy, called the Laboratori Nazionali del Gran Sasso. We have also Americans. We have Princeton University and also the Bell Labs, so this is a large collaboration. Now, this experiment that we are putting down in the Stawell laboratory that we call Stawell Underground Physics Lab, SUPL, will be the first of its kind all over the world. It will be the first dual experiment in which there will be two parts of the experiment, one located in Australia and one located in Italy under the Gran Sasso.

The idea is that we have another experiment under the Gran Sasso in Italy that is - over 20 years, there is a signal that could be interpreted as a dark matter particle signal. This is huge and this is quite important. However, no other experiment has been able to replicate the signal. There are many hypotheses why, most probably because nobody could be able to replicate the purity of the crystal - of the sodium iodide crystal of this experiment. This experiment here, this same experiment, will test this signal of dark matter that we have in Gran Sasso, and it will be essential to test this signal in Gran Sasso, that we have two experiments, one in Gran Sasso and one in Australia.

ANDI HORVATH
Elisabetta, this is very exciting. So, in the next year or so, fingers crossed that we have the right purified crystal and the right conditions in which to detect dark matter, the hypothesised dark matter will be observable, which will be an exciting point in particle physics. I hope one day I'll be interviewing you with some good news from the physics laboratory that says, we've found the stuff that holds together the universe. What led you into this area of particle physics? Because it's such an invisible world to deal with.

ELISABETTA BARBERIO

I'm very curious, and I always wanted to know how things worked and what is in our universe. There is nothing more interesting than digging in fundamental physics, how things work, what is the ultimate component of all the material that we are made of, and also the material that the universe is made of and we don't know what it is. There's been always this curiosity in me to know what it is. I always felt like I was - I was like a kid under a Christmas tree waiting to open the presents. It was the same way I felt like when we started looking for the Higgs and the Large Hadron Collider. I really felt like a kid. So, there is the present and you want to open the present to see what is inside. So, that's my curiosity.

ANDI HORVATH

Elisabetta, what misconceptions do people have about particle physics?



ELISABETTA BARBERIO
Because we describe very small objects, really on a quantum scale, I think it may be seen as very abstract. That we are speaking about these objects, these particles, these forces that are out there, and are very esoteric. In reality, they are the forces and the particles that make us up. So, it may be seen like fundamental science has very little to do with us. In reality, it is fundamental science that explains how things work at the end of the day. It is also fundamental science that, to be done, you need to invent new technology to see this particular particle, that we have been using quite a bit in our everyday life. I just mentioned before the miniaturisation of electronics and the web.

ANDI HORVATH

Being a science fiction fan, I'm often hearing the word antimatter. Is antimatter a thing?



ELISABETTA BARBERIO
Yes, it is. It's - the Big Bang, at the beginning. There was an equal - when energy became matter, there was an equal amount of matter and antimatter. Antimatter is matter with the opposite sign. So, the antielectron is an electron that is charged positive instead of negative.

ANDI HORVATH
Oh, so how is it different to dark matter?



ELISABETTA BARBERIO
Okay, antimatter is our matter, is the opposite of our matter. So, it's an electron that, instead of being charged with a negative charge, that is a positive charge, and the universe of matter. Antimatter disappeared, most of it, soon after the Big Bang. This is another mystery why and so on and how. This is another story that involves neutrinos. Dark matter was, at the same time, in this big universe, was also coming there. Dark matter was also produced at the Big Bang. Since we don't what that matter is, we don't know - the production - we don't know anything about it.

ANDI HORVATH
No. All we know is that dark matter matters, and good luck for looking for it.

ELISABETTA BARBERIO

Yes.

ANDI HORVATH
What would you like us to think about when we're contemplating the bigger picture, being such a small person in such a big universe?

ELISABETTA BARBERIO
Well, this is the science that is trying to put everything together, to explore this huge unknown that is around us, trying to make sense of where we are and our position in the cosmos. See, dark matter has been a little bit - the discovery of dark matter and dark energy has been like the Copernican Revolution of our century. Before Copernicus, people believed that Earth was the centre of the universe. Then we thought, oh, the sun is the centre of the universe. Then as time passed, oh, there is the galaxy. Then you realise that there is no centre of the universe. Now we know that what we believe, everything that we know, everything, all our technology, it's just a tiny bit of this huge universe, of which we know very little. There is still a lot to learn.

ANDI HORVATH

Professor Elisabetta Barberio, thank you.

ELISABETTA BARBERIO
Thank you very much.

CHRIS HATZIS
Thank you to Elisabetta Barberio, Professor of Physics at the University of Melbourne and the Director of the ARC Centre of Excellence for Dark Matter Particle Physics. And thanks to our reporter Dr Andi Horvath.

Eavesdrop on Experts - stories of inspiration and insights - was made possible by the University of Melbourne. This episode was recorded on July 22, 2020. You’ll find a full transcript on the Pursuit website. Production, audio engineering and editing by me, Chris Hatzis. Co-production - Silvi Vann-Wall and Dr Andi Horvath. Eavesdrop on Experts is licensed under Creative Commons, Copyright 2020, The University of Melbourne. If you enjoyed this episode, review us on Apple Podcasts and check out the rest of the Eavesdrop episodes in our archive. I’m Chris Hatzis. Join us again next time for another Eavesdrop on Experts.