Why dark matter matters

“The problem, of course, is dark matter.”

These are the blunt words of the University of Adelaide’s Dr Irene Bolognino a Postdoctoral Research Fellow at the ARC Centre of Excellence in Dark Matter Particle Physics, based at the University of Adelaide.

She tries to understand how the world around us is made by capturing the vibrations of dark matter.

Dark matter may confound, but it matters. As well as sustaining the development of important detection technologies, it could find applications we haven’t yet considered.

Most of all, it matters because of its potentially paradigm-shifting significance in our understanding of the universe.

The primary candidate for dark matter is some new kind of elementary particle which hasn’t been discovered yet.

Bolognino is part of an ambitious project which spans the breadth of the Earth: SABRE (Sodium Iodide with Active Background Rejection Experiment). One half of the SABRE experiment is to stationed in Australia, making it one of the most sensitive, expensive and elaborate scientific endeavours ever undertaken in the country.

And Bolognino believes it has world-shaping potential.

“The project is, in my opinion, a very challenging and cutting-edge experiment, and it is here in Australia. It can change history and the study of dark matter.”

The highly sensitive experimental apparatus is cutting edge. The research and development around building the sensors may be used in other areas, Bolognino explains. “The technologies that we are applying to detect dark matter and keep the background as low as possible is very important. All the technology we are developing can be reused in different fields.”

Physics often has a reputation for being a bit intangible. But advances in our understanding of the universe often have direct – sometimes unintended – impacts on society.

These impacts can be in the form of new technologies. Developments in physics can change our perception of the world around us. So, why should we care about dark matter? Bolognino believes that understanding the universe is a key part of how we interact with it to improve ourselves.

“Dark matter is not reported in what we call the Standard Model. The Standard Model of particle physics is our bible. It reports all we know about all the particles, all the matter, everything,” she explains.

“The Standard Model of particle physics needs to change since there is experimental evidence of the existence of dark matter. And we will understand more deeply all the particle physics and all their interactions. It would be a big improvement in terms of particle physics, and all disciplines related.

“Knowledge about the particle interactions has proven very important, for example, in the medical field and has actually helped in developing medicines, some cancer therapies, for instance.”

It matters

“Dark matter is the majority of the matter in the universe,” says Bolognino.

“If we understand what it is made of, we will take an important step towards understanding our universe, our surrounding. Dark matter is not something that has nothing to do with the Earth. Millions of dark matter particles pass through the Earth every second.  It is not something we want to study far away, in another galaxy, for pure knowledge.

“The discovery of what most of the matter around us is made of, will require a change of the Standard Model of elementary particles known so far. Beyond the scientific perspective, the change of the Standard Model would have an impressive media interest: a yardstick can be represented by the importance that the media gave to the confirmation of the Higgs boson, in 2012 (awarded the Nobel prize in 2013), which was already foreseen by the Standard Model.”

The SABRE South experiment will also have a huge impact on Australia’s physics research according to Bolognino. “We will have high visibility. Australian research will benefit. Dark matter is, I would say, one of the most important problems. Australia is playing a leading role.”

Over 20 years ago, an experiment deep in an Italian mountain claimed to have yielded the first direct detection of dark matter.

But the results have never been confirmed.

“The SABRE project is, in my opinion, a very challenging and cutting-edge experiment, and it is here in Australia. It can change history and the study of dark matter.”

Irene Bolognino

The whole difficulty with this elusive substance – which should make up 85 percent of all matter in the universe – is the difficulty to detect it, since it interacts very weakly with ordinary matter.

Cosmological observations have for decades shown it should be there and an international team of physicists is now trying to settle accounts with dark matter.

Bolognino says the principle behind the experiment is “very simple”.

The Earth orbits the sun. So far, so good.

But the sun also moves through the Milky Way, and all the dark matter in our galaxy. Bolognino provides an analogy: “The effect is to have a wind – you can compare it to when you are riding a bicycle. If you ride a bicycle, you have the effect of having wind on your face, but actually the air is not moving. The effect is there because you are moving through the air and otherwise you wouldn’t notice it. The principle is the same.”

Due to its roughly circular orbit around the sun, for six months of the year, the Earth is moving in the same direction as the sun through the galaxy. For the other six months, we’re moving in the opposite direction.

As a result, any dark matter detection should show a modulation – a wave pattern. The peak is around June 2, and the trough around December 2.

The DAMA/LIBRA experiment in Italy during the early 2000s showed exactly this.

Case closed? Not quite…

Bolognino explains that the DAMA/LIBRA results conflict with other, more sensitive experiments which aim to find dark matter through different means. The modulation found by DAMA/LIBRA could be from a different source.

“They could be what we call a ‘seasonal effect’ – mainly muon cosmic rays coming from the space,” Bolognino explains. “Muons depend on humidity and temperature, so their flux is actually greater in summer and decreases in winter. We know that the seasonal dependence of muons shows a maximum around June 2. So, we need an experiment in another hemisphere.

“If what DAMA is detecting is due to muons, we expect to detect the same modulation but with the opposite phase.”

By having two detectors, one in each hemisphere, SABRE aims to confirm or reject DAMA’s claim to have seen dark matter. One of the detectors will be in the same lab as DAMA in the north.

The other, SABRE South, is a lot closer to home – at the Stawell underground physics laboratory. The lab is housed in an operational goldmine one kilometre below the Victorian country town of Stawell about about 235km north west of Melbourne.

“SABRE has a lot of potentiality,” Bolognino adds. “It is the only dark matter experiment located in the southern hemisphere because Stawell is the first underground physics laboratory in all the southern hemisphere.”

The detector has to be underground to suppress background radiation which would crowd out any potential dark matter signals.

What could SABRE South achieve?

SABRE South is set to begin muon detection in early 2023. Bolognino says that dark matter detection will begin in the final quarter of 2023. “In one year, we will be ready to start.”

“I think that we can make history because the other experiments currently running using sodium iodide crystals are not sensitive enough yet and they are all located in the northern hemisphere. So, our potential is very high,” she says.

So, when should we expect to be celebrating the results of the experiment?

“It is the only dark matter experiment located in the southern hemisphere because Stawell is the first underground physics laboratory in all the southern hemisphere.”

Irene Bolognino

“We expect to have the first results at the end of 2025,” Bolognino says. “I think that the best aspect. That’s why I love this experiment. I chose SABRE South. I wanted work here. I was working in SABRE North before and I asked to have a position to work on SABRE South because, whatever the result of SABRE South, it will be a success. There is no possibility to have a failure.

“In the case where we don’t detect anything, it would seem that it is not related to muons. If we see the modulation but with opposite phase, we can study muons. And if we did detect the same modulation as DAMA, wow, maybe we found dark matter.”