This Australian experiment is on the hunt for an elusive particle that could help unlock the mystery of dark matter.

Australian scientists are making strides toward solving one of the universe’s greatest mysteries: the nature of invisible “dark matter.”

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

ORGAN has put new limits on the possible characteristics of axions and has therefore helped narrow down your search. But before we get ahead of ourselves…

Let’s start with a story

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 a very dense, hot region.

Then the Big Bang happened and everything fell apart. The particles combined into atoms, which eventually clumped together to form stars, which exploded and created all kinds of exotic matter.

After a few billion years, Earth came along, finally filled with little things called humans. Great story, right? Turns out that’s not the whole story; it’s not even half.

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

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

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

When we look at the way things move in space, we find time and time again that we cannot explain our observations by considering only what we can see.

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

Therefore, there must be dark matter in these galaxies, which provides additional gravity and allows them to spin faster, without parts being flung out into space. We think that dark matter literally holds galaxies together.

So there must be an enormous amount of dark matter in the universe, attracting all the things we can see. It’s also passing through you, like some kind of cosmic ghost. You just can’t feel it.

How could we detect it?

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

Anyway, after the axion was proposed, scientists realized that the particle could also form dark matter under certain conditions. That is because axions are expected to have very weak interactions with regular matter, but still have some mass: the two necessary conditions for dark matter.

So how do you go about looking for axions?

Well, since dark matter is believed 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 turn into photons (particles of light) under the right conditions.

This is good news, because we are very good at detecting photons. And this is exactly what ORGAN does. Design the right conditions for axion-photon conversion and look for weak photon signals: tiny flashes of light generated by dark matter passing through the detector.

This type of experiment is called an axion haloscope and was first proposed in the 1980. There are a few in the world today, each slightly different in important ways.

Shining a light on dark matter

An axion is believed to convert to a photon in the presence of a strong magnetic field. In a typical haloscope, we generate this magnetic field using a large electromagnet called a “superconducting solenoid.”

Inside the magnetic field we place one or more hollow metal chambers, which are meant to trap the photons and bounce them around inside, making them easier to detect.

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

To prevent this, we have placed our resonator in a “dilution refrigerator”. This stylish refrigerator cools the experiment to cryogenic temperaturesaround −273°C, which greatly reduces noise.

The cooler the experiment, the better we can “hear” the faint photons produced during dark matter conversion.

Targeting massive regions

An axion of a certain mass will convert into a photon of a certain frequency or color. But since the mass of the axions is unknown, experiments must target their search to different regions, focusing on those where dark matter is thought to be most likely to exist.

If no dark matter signal is found, 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”, which is just a way of saying “we didn’t find any dark matter in this mass range, at this level of sensitivity”. This tells the rest of the dark matter research community to direct their searches elsewhere.

ORGAN is the most sensitive experiment in its specific frequency range. Its recent run detected no signs of dark matter. This result has set an important limit of exclusion on the possible characteristics of axions.

This is the first phase of a multi-year plan to search for axions. We are currently preparing the next experiment, which will be more sensitive and will target a new mass range not yet explored.

But why does dark matter matter?

Well, for one thing, 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 searched for things that, at the time, seemed to be strange physical phenomena beyond our comprehension. Dark matter is the same.

Consider all that humans have accomplished by understanding just one-sixth of the affair in the universe, and imagine what we could do if we unlocked the rest.

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