About 80% of all the matter in the cosmos is of a form completely unknown to current physics. We call it dark matter, because as best we can tell it's…dark. Experiments around the world are attempting to capture a stray dark matter particle in hopes of understanding it, but so far they have turned up empty.
Recently, a team of theorists has proposed a new way to hunt for dark matter using weird "particles" called magnons, a name I did not just make up. These tiny ripples could lure even a fleeting, lightweight dark matter particle out of hiding, those theorists say. [The 11 Biggest Unanswered Questions About Dark Matter]
The dark matter puzzle
We know all sorts of things about dark matter, with the notable exception of what it is.
Even though we can't directly detect it, we see the evidence of dark matter as soon as we open up our telescopes to the wider universe. The first revelation, way back in the 1930s, came through observations of galaxy clusters, some of the largest structures in the universe. The galaxies that inhabited them were simply moving too quickly to be held together as a cluster. That's because the collective mass of the galaxies gives the gravitational glue that keeps the cluster together — the greater the mass, the stronger that glue. A super-strong glue can hold together even the fastest moving galaxies. Any faster and the cluster would simply rip itself apart.
But there the clusters were, existing, with galaxies buzzing around within them far faster than they should given the mass of the cluster. Something had enough gravitational grip to hold the clusters together, but that something was not emitting or interacting with light.
This mystery persisted unresolved through the decades, and in the 1970s astronomer Vera Rubin upped the ante in a big way through observations of stars within galaxies. Once again, things were moving too fast: Given their observed mass, the galaxies in our universe should've spun themselves apart billions of years ago. Something was holding them together. Something unseen. [11 Fascinating Facts About Our Milky Way Galaxy]
The story repeats all across the cosmos, both in time and space. From the earliest light from the Big Bang to the largest structures in the universe, something funky is out there.
Searching in the dark
So dark matter is very much there — we just can't find any other viable hypothesis to explain the tsunami of data in support of its existence. But what is it? Our best guess is that dark matter is some kind of new, exotic particle, hitherto unknown to physics. In this picture, dark matter floods every galaxy. In fact, the visible portion of a galaxy, as seen through stars and clouds of gas and dust, is just a tiny lighthouse set against a much larger, darker shore. Each galaxy sits within a large "halo" made up of zillions upon zillions of dark matter particles.
These dark matter particles are streaming through your room right now. They're streaming through you. A never-ending rain shower o' tiny, invisible dark matter particles. But you simply don't notice them. They don't interact with light or with charged particles. You are made of charged particles and you are very friendly with light; you are invisible to dark matter and dark matter is invisible to you. The only way we "see" dark matter is through the gravitational force; gravity notices every form of matter and energy in the universe, dark or not, so at the largest scales, we observe the influence of the combined mass of all these countless particles. But here in your room? Nothing.
Unless, we hope, there's some other way that dark matter interacts with us normal matter. It's possible that the dark matter particle, whatever the heck it is, also feels the weak nuclear force — which is responsible for radioactive decay — opening up a new window into this hidden realm. Imagine building a giant detector, just a big mass of whatever element you have handy. Dark matter particles stream through it, almost all of them completely harmlessly. But sometimes, with a rarity depending on the particular model of dark matter, the passing particle interacts with one of the atomic nuclei of the elements in the detector via the weak nuclear force, knocking it out of place and making the entire mass of the detector quiver.
Enter the magnon
This experimental setup works only if the dark matter particle is relatively heavy, giving it enough oomph to knock out a nucleus in one of those rare interactions. But so far, none of the dark matter detectors around the globe have seen any trace of an interaction, even after years and years of searching. As the experiments have ground along, the allowable properties of dark matter have slowly been ruled out. This isn't necessarily a bad thing; we simply don't know what dark matter is made of, so the more we know about what it isn't, the clearer the picture of what it could be.
But the lack of results can be a little bit worrying. The heaviest candidates for dark matter are getting ruled out, and if the mysterious particle is too light, it will never be seen in the detectors as they're set up right now. That is, unless there's another way that dark matter can talk to regular matter.
In a recent article published in the preprint online journal arXiv, physicists detail a proposed experimental setup that could spot a dark matter particle in the act of changing the spin of electrons (if, in fact, dark matter can do that). In this setup, dark matter can potentially be detected, even if the suspect particle is very light. It can do this by creating so-called magnons in the material.
Pretend you have a chunk of material at a temperature of absolute zero. All the spins — like tiny little bar magnets — of all the electrons in that matter will point in the same direction. As you slowly raise the temperature, some of the electrons will start to wake up, wiggle around and randomly point their spins in the opposite direction. The higher you raise the temperature, the more electrons wind up flipped — and each of those flips reduces the magnetic strength by just a little bit. Each of those flipped spins also causes a little ripple in the energy of the material, and those wiggles can be viewed as a quasiparticle, not a true particle, but something you can describe with math in that way. These quasiparticles are known as "magnons," probably because they're like tiny, cute little magnets.
So if you start off with a really cold material, and enough dark matter particles strike the material and flip some spins around, you'll observe magnons. Because of the sensitivity of the experiment and the nature of the interactions, this setup can detect a lightweight dark matter particle.
That is, if it exists.
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