Science

This Australian experiment aims to find an elusive particle that could help solve the mystery of dark matter.

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This article was originally published in The Conversation. (will open in a new tab) The publication published an article in Expert Voices: Op-Ed & Insights on Space.com.

Ben McAllister (will open in a new tab) scientist and science communicator living between Perth and Melbourne. He holds positions at the University of Western Australia and Swinburne University.

Australian scientists are taking big steps towards unraveling one of the universe’s greatest mysteries: the nature of invisible “dark matter”.

The ORGAN experiment, Australia’s first major dark matter detector, recently completed a search for a hypothetical particle called the axion, a popular candidate among theories trying to explain dark matter.

ORGAN imposed new restrictions on the possible characteristics of axions and thus helped to narrow down their search. But before we get ahead of ourselves…

Let’s start with history

About 14 billion years ago, all the little pieces of matter—the fundamental particles that would later become you, the planet, and the galaxy—were compressed into one very dense, hot region.

Then the Big Bang happened and everything went to pieces. The particles coalesced into atoms, which eventually stuck together to form stars, which exploded and created all kinds of exotic matter.

After a few billion years, the Earth appeared, which eventually teemed with small creatures called humans. Cool story, right? Turns out that’s not the whole story; it’s not even half.

People, planets, stars and galaxies are made up of “ordinary matter”. But we know that ordinary matter makes up only one-sixth of all matter in the universe.

The rest is made up of what we call “dark matter”. Its name tells you almost everything we know about it. It doesn’t emit light (which is why we call it “dark”) and it has mass (which is why we call it “matter”).

If it’s invisible, how do we know it’s there?

When we observe how objects move in space, we find again and again that we cannot explain our observations if we consider only what we can see.

Rotating galaxies are a great example. Most galaxies rotate at a speed that cannot be explained by the gravitational pull of visible matter alone.

So there must be dark matter in these galaxies, which provides extra gravity and allows them to spin faster – without throwing parts into space. We think dark matter is literally holding galaxies together.

The Bullet Cluster is a massive cluster of galaxies that has been interpreted as strong evidence for the existence of dark matter. (Image credit: NASA)

This means that there must be a huge amount of dark matter in the Universe, attracting everything that we can see. It passes through you as well, like some kind of cosmic ghost. You just can’t feel it.

How could we find it?

Many scientists believe that dark matter may be made up of hypothetical particles called axions. Axions were originally proposed as part of a solution to another major problem in particle physics called the “strong CP problem” (which we could write an entire paper about).

Be that as it may, after the axion was proposed, scientists realized that, under certain conditions, the particle can also form dark matter. This is because axions are expected to have very weak interactions with ordinary matter, but still have some mass: two conditions required for dark matter.

So how do you feel about finding axions?

Well, since dark matter is thought to be all around us, we can build detectors right here on Earth. And fortunately, the theory that predicts axions also predicts that axions can transform into photons (particles of light) under the right conditions.

This is good news, because we are very good at detecting photons. This is exactly what ORGAN does. It creates the right conditions for converting axions into photons and looks for weak photon signals – small flashes of light generated by dark matter passing through the detector.

Such an experiment is called an axion haloscope and was first proposed in the 1980s. (will open in a new tab). There are several of them in the world today, and each one is slightly different in important respects.

The main detector of the ORGAN experiment. A small copper cylinder called a “resonance cavity” traps photons generated during the dark matter transformation. The cylinder is bolted to a “dilution cooler” that cools the experiment down to very low temperatures. (Image credit: author)

Shed light on dark matter

It is believed that the axion turns into a photon in the presence of a strong magnetic field. In a typical haloscope, we generate this magnetic field with a large electromagnet called a “superconducting solenoid.”

Inside the magnetic field, we place one or more hollow metal chambers that are designed to capture photons and cause them to bounce inside, making them easier to detect.

However, there is one catch. Anything that has a temperature constantly emits small, random flashes of light (which is why thermal imaging cameras work). These random spikes, or “noise,” make it difficult to detect the faint dark matter signals we’re looking for.

To get around this, we placed our resonator in a “dissolution refrigerator”. This fancy refrigerator cools the experiment down to cryogenic temperatures, around -273°C, which greatly reduces noise.

The colder the experiment, the better we will be able to “hear” the weak photons that arise during the transformation of dark matter.

Mass region targeting

An axion of a certain mass is converted into a photon of a certain frequency or color. But since the mass of the axions is unknown, experiments should aim their search at different regions, focusing on those where dark matter is considered more likely.

If no dark matter signal is detected, then either the experiment is not sensitive enough to hear the signal above the noise, or there is no dark matter in the corresponding axion mass region.

When this happens, we set an “exclusion limit” – it’s just a way of saying, “We haven’t found any dark matter in this mass range and at this sensitivity level.” This tells the rest of the dark matter research community to direct their search elsewhere.

ORGAN is the most sensitive experiment in the target frequency range. Its recent launch found no dark matter signals. This result established an important exclusion limit for the possible characteristics of axions. (will open in a new tab).

This is the first phase of a multi-year plan to find axions. We are currently preparing the next experiment, which will be more sensitive and target a new, as yet unexplored, mass range.

But why does dark matter matter?

Well, first of all, we know from history that when we invest in fundamental physics, we end up developing important technologies. For example, all modern computing is based on our understanding of quantum mechanics.

We would never have discovered electricity or radio waves if we hadn’t been doing things that at the time seemed like strange physical phenomena beyond our understanding. Dark matter is the same.

Think about what people have achieved by understanding only one-sixth of the matter in the universe, and imagine what we could do if we discovered everything else.

This article is republished from The Conversation (will open in a new tab) licensed under Creative Commons. Read original article (will open in a new tab).

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