by A ghost is haunting our universe. This has been known in astronomy and cosmology for many years. Observations suggest that about 85% of all matter in the universe is mysterious and invisible. Both of these qualities are reflected in its name: dark matter.
Many experiments have aimed to reveal what it is made of, but despite years of searching, scientists have come up short. Now our new experiment, being built at Yale University in the US, is offering a new tactic.
Dark matter has surrounded the universe since the beginning of time, pulling the stars and galaxies together. Invisible and subtle, it does not seem to interact with light or any other matter. In fact, it must be something completely new.
The standard model of particle physics is incomplete, and this is a problem. We have to look for new elementary particles. Surprisingly, the same flaws of the standard model provide valuable clues as to where they may be hiding.
The trouble with the neutron
Let’s take the neutron, for example. It makes up the atomic nucleus together with the proton. Despite being neutral overall, the theory says it was made up of three charged constituents called quarks. Because of this, we would expect some parts of the neutron to be positively charged and others negatively – this would mean that the physicist had an electric dipole moment.
However, many attempts to measure it have yielded the same result: it is too small to notice. Another ghost. And we are not talking about the inadequacy of tools, but a parameter that must be less than one part in ten billion. It is so small that people wonder if it could be zero at all.
In physics, however, the mathematical zero is always a strong statement. In the late 70s, particle physicists Roberto Peccei and Helen Quinn (and later, Frank Wilczek and Steven Weinberg) tried to reconcile theory and evidence.
They suggested, perhaps, that the parameter is not zero. Rather it is a dynamical quantity that slowly lost its charge, changing to zero, after the Big Bang. Theoretical calculations show that, if such an event were to occur, it must have left behind a multitude of light, sneaky particles.
These were called “axions” after a brand of detergent because they could “clean up” the neutron problem. And even more. If axes were created early in the universe, they’ve been hanging around ever since. Most importantly, their properties check all the expected boxes for dark matter. For these reasons, axons are now one of the most favored dark matter particles.
Axes would interact only weakly with other particles. However, this means they would still interact quite a bit. The invisible axes could even transform into ordinary particles, including – ironically – photons, the essence of light. This could happen in certain circumstances, for example in the presence of a magnetic field. Hello this is for experimental physicists.
Experimental design
Many experiments are trying to evoke the axial ghost in a controlled laboratory environment. Some aim to convert light into axes, for example, and then convert axes back to light on the other side of a wall.
At present, the most sensitive approach focuses on the mass of dark matter passing through the galaxy (and therefore, the Earth) with a device called a haloscope. It is a conductive cavity immersed in a strong magnetic field; the first captures the dark matter around us (assuming it’s axes), while the second triggers the change to light. The result is an electromagnetic signal visible inside the cavity, oscillating with a characteristic frequency depending on the axial mass.
The system works like a radio receiver. It must be properly adjusted to intercept the frequency we are interested in. In practice, the dimensions of the cavity are changed to accommodate different characteristic frequencies. If the frequencies of the axial and cavity do not match, it is just like tuning a radio to the wrong channel.
Unfortunately, the channel we are looking for cannot be predicted in advance. We have no choice but to scan all possible frequencies. It’s like picking a radio station in a sea of white noise – a needle in a haystack – with an old radio that needs to get bigger or smaller every time we turn the frequency knob.
However, those are not the only challenges. Cosmology points to tens of gigahertz as the latest promising frontier for action research. Because higher frequencies require smaller cavities, too small a cavity would be required to capture a meaningful amount of signal to probe that region.
New experiments are trying to find other ways. Our Axion (Alpha) Longitudinal Plasma Haloscope experiment uses a new cavity concept based on metamaterials.
Metamaterials are composite materials with global properties that differ from their constituents – they are greater than the sum of their parts. A cavity filled with conducting rods acquires a characteristic frequency as if it were a million times smaller, and its volume hardly changes. That’s exactly what we need. In addition, the rods provide a built-in, easy-to-adjust tuning system.
We are currently building the setup, which will be ready to accept data in a few years. The technology is promising. Its development is the result of collaboration between solid-state physicists, electrical engineers, particle physicists and even mathematicians.
Despite being so obscure, axions are fueling progress that no ghost will ever take away.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Andrea Gallo Rosso is a member of the ALPHA collaboration. He receives funding from the Swedish Research Council.
