The search for dark matter is about to reach new limits: Scientists are developing ultracold quantum techniques to search for the universe’s most elusive and mysterious substance, which currently poses one of science’s greatest mysteries.
Although dark matter exists in the universe at roughly six times the amount of regular matter, scientists don’t know what it is, at least in part because human-designed experiments have been unable to detect it.
To tackle this challenge, scientists from multiple universities across the UK have teamed up to build two of the most sensitive dark matter detectors ever conceived. Each experiment searches for different hypothetical particles that could make up dark matter. While these particles have some common properties, they also have fundamentally different properties, which require different detection techniques.
The equipment used in both experiments is so sensitive that its components must be cooled to a thousandth of a degree above absolute zero – the theoretical, unreachable temperature at which all atomic movement stops. This cooling is necessary to prevent interference, or “noise”, from the outside world from distorting the measurements.
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“We are using quantum techniques at cryogenic temperatures to build the most sensitive detector ever,” said team member Samri Aouti from Lancaster University. It said in a statement“The goal is to observe this mysterious material directly in the lab and solve one of the greatest mysteries in science.”
Why dark matter has given scientists the cold shoulder
Dark matter poses a big problem for scientists because it makes up about 80% to 85% of the universe, yet is virtually invisible to us. This is because dark matter doesn’t interact with light or “everyday” matter, and if it does, its interactions are rare or very weak, or both. We don’t know.
But because of these properties, scientists know that dark matter cannot be made up of electrons, protons, and neutrons, which are all part of the family of heavy particles that make up the “ordinary” matter we see: stars, planets, the moon, our bodies, ice cream, and your neighbor’s cat.
In fact, the only reason dark matter is thought to exist is because this mysterious substance has mass, which is why it interacts with gravity. Through these interactions, dark matter can affect the dynamics of ordinary matter and light, allowing us to infer its existence.
Astronomer Vera Rubin discovered the existence of dark matter, which scientist Fritz Zwicky had previously theorized, when she observed that some galaxies rotate so fast that if their gravitational influence came only from visible baryonic matter, they would be torn apart. But what scientists really want is not speculation, but rather the definitive detection of dark matter particles.
One of the hypothetical particles currently being put forward as a prime candidate for dark matter is the very light “axion.” Scientists also theorize that dark matter could be made up of new, more massive (as yet unknown) particles that have yet to be observed because their interactions are so weak.
Both the axion and these unknown particles interact with matter so weakly that they could theoretically be detected with instruments sensitive enough. But having two prime suspects means two investigations and two experiments are needed. This is necessary because current searches for dark matter typically focus on particles with masses between 5 and 1,000 times the mass of a hydrogen atom, meaning that if dark matter particles are lighter than this, they could be missed.
The Quantum Enhanced Superfluid Technology for Dark Matter and Cosmology (QUEST-DMC) experiment has been devised to detect normal matter colliding with dark matter particles in the form of unknown new weakly interacting particles with masses ranging from 1% to several times that of a hydrogen atom. QUEST-DMC cools superfluid helium-3, a light and stable isotope of helium with a nucleus of two protons and one neutron, into a macroscopic quantum state, enabling record-breaking sensitivity to detect extremely weak interactions.
But QUEST-DMC will not be able to find the extremely light axions, which are theorized to have masses billions of times that of hydrogen atoms, which also means that such axions cannot be detected by interactions with ordinary matter particles.
But what axions lack in mass, they are thought to make up for in numbers, suggesting that these virtual particles are quite abundant, meaning that the hunt for these dark matter suspects is better done using a different signature: tiny electrical signals produced by axions’ decays in magnetic fields.
If such a signal exists, detecting it would require extending detectors to the highest level of sensitivity allowed by the laws of quantum physics. The team believes that a quantum sensor for the hidden sector could be (QSHS) Quantum Amplifiers have the ability to do just that.
If you’re in the UK, you can follow the QSHS and QUEST-DMC experiments here: Lancaster University Summer Science FairVisitors can also see how scientists can infer the presence of dark matter in galaxies using boxed gyroscopes, which move strangely due to invisible angular momentum.
Additionally, the exhibit will feature a light-up dilution refrigerator demonstrating the ultra-low temperatures needed for quantum technologies, while a model of a dark matter particle collision detector shows how the universe would behave if dark matter interacted with matter and light in the same way as everyday matter.
The team’s paper detailing the QSHS and QUEST-DMC experiments has been published in The European Physical Journal C and on the paper repository site. arXiv.
