Thursday , September 29 2022

‘Waiting for a ghost’: the search for dark matter 1 km below an Australian town | australia news


Dark matter is flowing through you right now.

This mysterious, invisible matter makes up more than 80% of the universe, an elusive web of particles that freely pass through matter. To observe this, you need to get rid of all interference.

To study things in the universe, you have to keep the universe out.

The Australian Research Council’s Center of Excellence for Dark Matter Particle Physics is doing a mile underground in a disused section of a gold mine below the town of Stawell in Victoria.

Workers in the dark matter lab
Stawell’s goldmine, where a team of Australian scientists is investigating dark matter. Photo: ARC Center of Excellence for Dark Matter Particle Physics

There, a cave is being turned into a lab for dark matter hunters and is on track to be finished by the end of the year.

Swinburne University astronomer Alan Duffy, director of the Institute for Space Technology and Industry, describes dark matter as a “ghost.”

“Dark matter is a cloud that surrounds us… it flies through us, through solid walls, from Earth,” he says, comparing it to the wind that is only seen by its effects.

“Massive structures spanning the universe form cosmic webs, and the galaxies we can see and live in stretch along filaments of dark matter like morning dew in a backyard spider’s web.

“As you read this, you will experience several hundred million particles per second, perhaps a few of which will actually collide with your atoms over the course of a day.”

The vast majority of particles pass through you, with only a few unlucky ones looking at one of your atoms.

That’s why you need radio silence to detect them.

The center’s principal investigator, Phillip Urquijo, says that more than 80% of the mass of the universe is made up of dark matter, but that this is only visible through its interaction with other matter, namely the effect of gravity.

And observing this means blocking interference from other particles. Radiation from the sun and the radioactive decay of ordinary matter and radioactive particles that have infected metals since the first atomic bombs were launched (more on that later) distorts this picture.

Inside the lab for dark matter hunters
The lab for dark matter hunters will be completed in the new year. Photo: ARC Center of Excellence for Dark Matter Particle Physics

In the lab, there are 1,000 meters of rock between the researchers and the surface, slabs of rock to block out cosmic rays. But wait – there’s more.

“We screwed up our experiment as much as we could in one of the first working gold mines in Australia,” says Urquijo. “We can block the sun’s cosmic rays, but the rock and any material we used in the construction of the experiment may contain naturally occurring radioactivity.”

The cave is closed with a net and some kind of concrete is sprayed on it. Then there is the quest for pure metal.

Sprinkles of radioactive dust from atomic bombs have been contaminated with metals since the second world war, meaning that many people trying to detect dark matter have to procure the metals before the bombs are detonated. For example, from old shipwrecks.

“When you produce steel, you have a combination of new steel and recycled iron from iron ore. Steel produced during and since WWII… many steels have resulted in a component of radioactivity left over from weapons tests,” says Urquijo. “One option is to salvage long-ago ships, ancient Roman ships, and sunken submarines sitting at the bottom of the ocean, unpenetrated by cosmic rays.”

That’s what many dark matter hunters have had to do—but Urquijo says they’ve managed to procure a purer version of the steel, and Duffy says they’ve taken some safety precautions.

Duffy, who is also principal investigator on the project, says they have established a “veto system” unlike other projects that have salvaged lead ingots from sunken 2,000-year-old Roman galleys.

The team created the purest crystals possible, sodium iodide crystals with lower levels of all kinds of contamination than had been produced before.

These pure crystals, which will glow when a particle hits them, are kept in copper tubes in a steel vessel (this is called the scintillator fluid), surrounded by a special liquid that will glow when struck by a particle.

“Now we’re looking at two places for this flash of light,” he says.

“If the crystal glows, we look at the fluid around it, and if the liquid also glowed, we know it couldn’t be dark matter, because the probability of dark matter hitting atoms twice is infinitely small.”

Inside the lab for dark matter hunters
The Stawell project is part of the Sodium Iodide with Active Background Rejection Experiment. Photo: ARC Center of Excellence for Dark Matter Particle Physics

Think of the millions of particles passing through you right now and how rarely an atom pings. A different particle is likely to hit the water, then that pure, compact crystal. Two flashes. Yet the odds of the cunning dark matter particle hitting anything is so low that there will only be a flash.

Astronomers have long theorized about dark matter.

For decades, dark matter hunters have looked at the behavior of galaxies, how light is bent, and have used space telescopes to monitor for evidence of its existence. As this evidence grew, so did the fact that dark matter was all around us and could be studied on Earth.

The Stawell project is part of the Sodium Iodide with the Active Background Denial Experiment (Sabre), which seeks to detect dark matter particles directly rather than indirectly observing them through their effects. Southern hemisphere partner of a similar installation in Italy.

As we learn more about dark matter, Duffy says, there will be technological byproducts and emanations – just as learning how to split the atom gives us nuclear medicine (and the unwanted weapons that have so polluted everything on the planet).

Also, dark matter plays a non-trivial role in enabling galaxies – and by extension ours – to exist. There must be something that provides the gravity that allows galaxies to form and yet keeps them from flying apart.

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“You can create a universe in the supercomputer – that’s what I do,” he says. “If you don’t put dark matter there, there isn’t enough gravity for galaxies to form. We owe our existence to dark matter.

“This is a big question about our universe that we want to answer, and it’s critical to our understanding of fundamental physics that was produced in our early universe.”

So dark matter hunters continue to hunt with the Hubble space telescope, the large hadron collider, and a laboratory in a gold mine a kilometer below the Victorian countryside.

It is expected to be operational in the new year. Then Duffy says, “We’re expecting a ghost.”

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