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We’ll never find dark matter… without quantum tech

todayJuly 5, 2021 3

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Almost a century ago, Dutch astronomer Jacobus Kapteyn first proposed the existence of dark matter. He’d been studying the motion of stars in galaxies — a galaxy can be described, in rough terms, as a heap of stars, gas and dust rotating around a common center — and noticed that something was off. The stars in the outer layers of the galaxy were rotating much too fast to conform with the laws of gravity. Kapteyn’s hypothesis was that some invisible, massive stuff might be in and around the galaxy, making the outer stars reach the observed velocities.

From the 1960s to the ’80s, Vera Rubin, Kent Ford and Ken Freeman gathered more evidence in support of this hypothesis. They ultimately showed that most galaxies must contain six times as much invisible mass as they do visible stars, gas and dust.

Other observations in favor of dark matter followed, such as gravitational lensing and anisotropies in the cosmic microwave background. Gravitational lensing is a phenomenon in which light beams get bent around massive objects; the cosmic microwave background is an outer layer of the universe that would be quite homogeneous without dark matter but is rather lumpy in reality.

In the meantime, uncountable masterminds have devised theories and designed experiments to track down dark matter. Nevertheless, nobody has actually seen a dark matter particle as of now. That’s why, even today, senior scientists, as well as doctoral students like myself, are still conducting research on dark matter. And it still seems as though discovering dark matter is many centuries — maybe even millennia — away.

With recent advances in quantum computing, however, dark matter physics might experience a massive boost. The search for two types of dark matter — scientists don’t actually know whether they both exist but they’re trying to figure that out — might profit from quantum technology. The first type is the axion, whose existence might explain why the strong nuclear force doesn’t change if you flip a particle’s electric charge and parity. The other type is the dark photon. These particles would behave similarly to photons, the particles of light, except that dark photons aren’t light at all, of course.

Searching for axions

According to theory, axions should be wobbling through space and time at a particular frequency. The only problem is that theorists are unable to predict what that frequency might be. So, researchers are left to scan an enormous range of frequencies, one small band at a time.

Like an old radio receiver converts radio waves into sound, axion detectors convert axion waves into electromagnetic signals. This process gets more complicated, though, because axions oscillate at two different frequencies simultaneously.

You can picture this looking a little like a drunk person trying to get home from a party: They might take three steps to the right, then three steps to the left, then back to the right again. That’s one frequency, on the “left-right” spectrum. Because they also have massive hiccups, though, they might jump into the air at each HIC!, which occurs every four steps. That’s the second frequency, on the “up-down” spectrum.

Axions may be a little more sophisticated than drunk people, but they also have two frequencies, just like partygoers who have enjoyed a glass too many.

Mathematically, one can put these two frequencies together by quadratically adding them. That is, one multiplies the first frequency by itself, adds the second frequency multiplied by itself and then takes the square root.

In our drunkard’s example, three steps times themselves equal nine steps squared for the first frequency, four steps times themselves equal 16 steps squared for the second frequency, and together we get that the square root of nine steps squared plus 16 steps squared is five steps. This — in our example, five steps — is called electromagnetic field quadrature.

Credit: Author provided