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5 mins read 07 Jan 2020

Finding Dark Matter below Victoria

Could 2020 be the year we finally learn what Dark Matter is? A new laboratory in Victoria is hoping to answer this question.

five workers stand inside a dark mine shaft tube. They are standing near a bright yellow vehicle and are all wearing high-viz clothing with helmets and lights on their helmets.
Inside the Stawell mine – these rocky walls will absorb the cosmic rays reducing the background noise to a dark matter detection. Credit: Pursuit – University of Melbourne.

A new laboratory, being built 1km below Victoria in an old gold mine is scheduled to soon become operational and its hoped it will change our view of the universe forever – revealing one of nature’s most mysterious and elusive materials: Dark Matter.

The Laboratory, buried deep within the Earth for additional shielding against the raining assault of cosmic-ray bombardment, will be the first dark matter detector in the southern hemisphere, joining an array of detectors above the equator.

The underground observatory called the Sodium Iodide with Active Background Rejection Experiment – or SABRE – features a range of Australian institutions in collaboration such as the Australian National University, Swinburne University of Technology, University of Adelaide, University of Melbourne and the Australian Nuclear Science and Technology Organisation (ANSTO). The Australian institutions also form part of a larger international team with universities and facilities from Italy and the USA.

The detector, being built in the now-retired Stawell gold mine (located in the western half of Victoria – roughly halfway between Melbourne and Adelaide), is one of two, twin-detectors – with the other being developed in Italy.

Dark Matter is an unknown substance theorised to exist in our universe, but as yet – never detected. It is unknown what the material is made from as it does not interact with normal matter through electromagnetic (EM) transaction. It cannot be seen, nor touched, nor felt – and the proof of its existence has been found through its indirect measurements – its gravitational effects on objects within our own universe.

Almost 100 years ago, a colourful scientist known as Fritz Zwicky observed clusters of galaxies measuring their rotational velocities about each other and found that the matter that could be seen – the light from the cluster – did not account enough for the speed of the galaxies. Zwicky called it ‘Dark Matter’ but the idea was not accepted by the science community for decades.

In the 1970s, pioneering work by Vera Rubin provided further strong evidence of this concept of this invisible mass – having studied the rotational velocities of stars in spiral galaxies, and finding that the matter that could be accounted for by measuring the light (EM spectrum) was only a portion of the matter present to generate the velocities of stars orbiting in the outer edges of these galaxies.

Today, Dark Matter is not only accepted it has been measured to account for approximately 85% of all the matter in our Universe, meaning the everything else that we can see, observe and interact with – stars, galaxies, planets, moons, humans – only make up 15% of the matter.

Detecting Dark Matter has been extremely hard, as the fundamental particles that make it up (and there have been a variety of proposals and ideas) do not interact with much with anything. It is on extremely rare occasions, that are random, that scientist believe some dark matter particles might interact with the nucleus of an atom of normal matter.

However, these interactions are similar to when a cosmic ray strikes a nucleus of an atom or even when radioactive decay interacts with materials. These two aspects generate billions of events that are considered background noise.

To get around this, scientists have decided to take their observatories deep underground and into old, refurbished mines – providing kilometres of Earth above the detectors to block any incoming cosmic rays from space.

In addition to this, the construction of the detectors – like SABRE – are made with advanced materials that are not exposed to or producing any form of radiation. Surprisingly, there are a large number of everyday objects that produce radiation – such as a banana. Whilst banana radiation is minute and non-threatening to our everyday lives, to these ultra-sensitive detectors, they flood the observatory with background radiation.

Illustration of the SABRE Dark Matter detector showing central cylinder with 8 horizontal mushroom like detectors, four to each side. Lines jumping in zig-zags simulate dark matter particles interacting within detector.
Illustration of the SABRE Dark Matter. Credit: Pursuit – University of Melbourne.

The SABRE detector’s purpose will be to replicate data produced in one of the nine existing northern hemisphere dark matter observatories – who have claimed that they have seen dark matter around the middle of the year, when Earth is moving at a different velocity as it does in December, with respect to the centre of the Milky Way Galaxy.

To counter this error, and to establish a baseline of whether or not there are seasonal variations in dark matter detections (summer vs. winter), the Stawell Detector is the first of its kind in the southern hemisphere – allowing the reduction of errors to be determined.

Should the same result be obtained – a peak in the data during the southern hemisphere summer vs. the winter, then scientists will need to re-think what the Italian results mean. However, if the data reflects the same peaks during the same time of year for each season – then this could reveal the first evidence for Dark Matter.

And if that evidence is found – science will be re-written forever.  

This article is based on a first publication in Pursuit. Read the original article.

Read more about the SABRE project here