The hunt for dark matter is about to get a lot cooler. Scientists are developing supercold quantum technology to hunt for the universe's most elusive and mysterious matter, which currently poses one of science's greatest mysteries.
Despite the fact that dark matter outnumbers the amount of ordinary matter in our universe by about six times, scientists don't know what it is. That's partly because no experiment devised by humanity has ever been able to detect it.
To tackle this conundrum, scientists from several universities in the UK have teamed up to build two of the most sensitive dark matter detectors ever built. Each experiment will hunt for a different hypothetical particle that could comprise dark matter. While they share some of the same qualities, the particles also have some radically different characteristics, requiring different detection techniques.
The equipment used in both experiments is so sensitive that its components must be cooled to within a thousandth of a degree above absolute zero, the theoretical and unattainable temperature at which all atomic motion would cease. This cooling must occur to prevent interference, or "noise," from the outside world from distorting the measurements.
Related: 'Immortal stars' could be feeding on dark matter at the heart of the Milky Way"We are using quantum technologies at extremely low temperatures to build the most sensitive detectors yet," team member Samuli Autti of Lancaster University said in a statement. "The aim is to observe this mysterious matter directly in the laboratory and solve one of the greatest mysteries in science."
How dark matter has left scientists out in the cold
Dark matter is a big problem for scientists because, despite making up about 80% to 85% of the universe, it remains invisible to us. This is because dark matter does not interact with light or "everyday" matter - and if it does, those interactions are rare or very weak. Or maybe both. We just don't know.
But because of these properties, scientists know that dark matter can't be made of electrons, protons, and neutrons - all parts of the baryon family of particles that make up everyday matter in things like stars, planets, moons, our bodies, ice, and the cat next door. All the "normal" stuff we can see.
The only reason we think dark matter exists at all is that this mysterious substance has mass. So it interacts with gravity. Dark matter can affect the dynamics of ordinary matter and light through that interaction, allowing its presence to be inferred.
Astronomer Vera Rubin discovered the presence of dark matter, previously suggested by scientist Fritz Zwicky, because she saw that some galaxies were spinning so fast that if their only gravitational influence came from visible, baryonic matter, they would fly apart. What scientists really want, however, is not an inference, but rather a positive detection of dark matter particles.
One of the hypothetical particles currently seen as the prime suspect for dark matter is the very light 'axion'. Scientists also suspect that dark matter could be composed of even heavier (as yet unknown) new particles with such weak interactions that we have not yet observed them.
Both axions and these unknown particles are expected to exhibit ultraweak interactions with matter, which could theoretically be detected with sensitive equipment. But two prime suspects means two studies and two experiments. This is necessary because current dark matter studies typically focus on particle masses between 5 and 1,000 times the mass of a hydrogen atom. That means that if dark matter particles are lighter, they might be missed.
The Quantum Enhanced Superfluid Technologies for Dark Matter and Cosmology (QUEST-DMC) experiment is designed to detect ordinary matter colliding with dark matter particles in the form of weakly interacting unknown new particles with masses ranging from 1% to a few times that of a hydrogen atom. QUEST-DMC uses superfluid helium-3, a light and stable isotope of helium with a nucleus of two protons and one neutron, cooled to a macroscopic quantum state to achieve record-breaking sensitivity in spotting ultraweak interactions.
However, QUEST-DMC would not be able to spot extremely light axions, which are thought to have masses billions of times lighter than a hydrogen atom. This also means that such axions would not be detectable through their interactions with ordinary matter particles.
But what they lack in mass, axions are thought to make up for in numbers, with these hypothetical particles being extremely abundant. That means it's better to search for this suspected dark matter using a different signature: the tiny electrical signal that's created when axions decay in a magnetic field.
If such a signal exists, detecting it would require stretching detectors to the maximum level of sensitivity allowed by the rules of quantum physics. The team hopes that their Quantum Sensors for the Hidden Sector(QSHS) quantum amplifier could do just that.
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If you're in the UK, the public can see both the QSHS and QUEST-DMC experiments at Lancaster University's Summer Science Exhibition. Visitors can also see how scientists infer the presence of dark matter in galaxies using a gyroscope-in-a-box that moves strangely due to unseen angular momentum.
In addition, the exhibition features a glowing dilution refrigerator to demonstrate the ultra-low temperatures needed for quantum technology. The model of the dark matter collision detector shows how our universe would behave if dark matter interacted with matter and light, just as everyday matter does.
The team's papers describing the QSHS and QUEST-DMC experiments have been published in the journal The European Physical Journal C and on the paper repository site arXiv.
