Monday, January 24

‘Waiting for a ghost’: the search for dark matter 1 km below an Australian city | Australia News


Dark matter is flowing through you, right now.

This mysterious and invisible material makes up more than 80% of the universe, an elusive network of particles that pass freely through matter. To observe it, you must get rid of all interference.

To study the matter of the universe, you must block the universe.

That’s what the Australian Research Council center of excellence for dark matter particle physics he is doing a kilometer underground, in a disused gold mine below the town of Stawell in the Victoria region.

Workers inside the dark matter laboratory
The Stawell Gold Mine, where a team of Australian scientists is investigating dark matter. Photograph: ARC Center of Excellence for Dark Matter Particle Physics

There, a cavern is being transformed into a laboratory for dark matter hunters, and is on track to be completed by the end of the year.

Swinburne University astronomer Alan Duffy, director of the institute for space industry and technology, describes dark matter as a “ghost.”

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

“It forms huge structures that span the universe, cosmic webs, and the galaxies we can see and live in lie along filaments of dark matter like morning dew on a cobweb in the backyard.

“As you read this, you will experience a few hundred million particles per second, of which maybe a couple actually collide with their atoms in the course of a day.”

The vast majority of the particles pass through you, and only a couple of the unfortunate ones bounce off one of your atoms.

This is why you need radio silence to detect them.

The center’s chief researcher, Phillip Urquijo, says that while more than 80% of the mass in the universe is made of dark matter, it is only visible through its interactions with other matter: its gravitational effect.

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

Inside the Lab for Dark Matter Hunters
The lab for dark matter hunters will be completed in the new year. Photograph: 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 cosmic rays. But wait, there is more.

“We put the experiment underground, as deep as we could in Australia, in one of the first working gold mines,” says Urquijo. “We can block the cosmic rays of the sun, but the rock and any material that we use in the construction of the experiment can contain natural radioactivity.”

The cavern is covered with a net and sprayed with a type of concrete. Then there is the search for pure metal.

Leftover splashes of radioactive dust from atomic bombs have infected metals since World War II, meaning that many people trying to detect dark matter have to obtain metals from before the bombs went off. From old shipwrecks, for example.

“When you make steel, you get a combination of new steel from iron ore and recycled iron. The steel produced during and since World War II … a large quantity of steel ended up with a component of radioactivity that was remnants of weapons tests, ”says Urquijo. “One option is to rescue long-sunken ships, ancient Roman ships and submarines that have been sitting on the ocean floor, where cosmic rays have not penetrated.”

That’s what a lot of dark matter hunters have had to do, but Urquijo says they’ve managed to get a purer version of steel, and Duffy says they’ve put in some safeguards.

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

The team created the purest crystals possible, sodium iodide crystals with lower levels of any kind of contamination than ever before.

Those pure crystals that will shine when a particle hits them are kept in copper tubes, inside a steel container, surrounded by a special liquid that will also shine when hit by a particle (it is called scintillating liquid).

“Now we are looking in two places for this flash of light,” he says.

“If the crystal shines, we look at the liquid around it, and if the liquid has also flashed, we know that it cannot be dark matter, because the chances of dark matter hitting the atoms twice are infinitesimally small.”

Inside the Lab for Dark Matter Hunters
The Stawell project is part of the Sodium Iodide Experiment with Active Background Rejection. Photograph: ARC Center of Excellence for Dark Matter Particle Physics

Think of those millions of particles flowing through you right now, and how rarely an atom is pinged. A different particle is likely to hit the water, then that pure, compact crystal. Two flashes. However, the cunning dark matter particle is so unlikely to hit something that there will only be a flash.

Astronomers have long theorized about dark matter.

For decades, dark matter hunters observed the behavior of galaxies, how light bends, and used space telescopes to track evidence of their existence. As that evidence grew, so did the understanding that dark matter is all around us, so it could be studied on Earth.

The Stawell project is part of the Background Active Rejection Sodium Iodide Experiment (Saber), which is trying to detect dark matter particles directly, rather than indirectly observing them through their effects. It is the southern hemisphere partner for a similar setup in Italy.

Duffy says that once we know more about dark matter, there will be technological consequences and spills, in the same way that learning to split the atom gave us nuclear medicine (and the undesirable weapons that have contaminated everything on the planet).

There is also the not insignificant role of dark matter in ensuring that galaxies, and by extension us, exist. There must be something out there that provides the gravity that allowed the galaxies to form, and still keeps them from moving apart.

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Thank you for your comments.

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

“It is a great question about our universe that we want to answer, it occurred in the early moments of our universe and is critical to our understanding of fundamental physics.”

So the dark matter hunters keep hunting, with the Hubble space telescope, the large hadron colliderand a laboratory a kilometer below the Victorian countryside in a former gold mine.

It is expected to be operational in the new year. So, says Duffy, “we expect a ghost.”


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