Converting invisible dark matter into visible light

Galaxy cluster, left, with a visible dark matter ring, right. Credits: NASA, ESA, MJ Jee and H. Ford (Johns Hopkins University)

Dark matter explorations are advancing with new experimental techniques designed to detect axions, leveraging advanced technology and interdisciplinary collaboration to uncover the secrets of this elusive component of the cosmos.

A ghost haunts our universe. This has been known in astronomy and cosmology for decades. Observations suggest that about 85% of all matter in the universe is mysterious and invisible. These two qualities are reflected in its name: dark matter.

Several experiments have aimed to reveal what it’s made of, but despite decades of research, scientists have come up short. Now our new experiment, under construction a Yale University in the US, is offering a new tactic.

Dark matter has been around the universe since the beginning of time, binding stars and galaxies together. Invisible and subtle, it doesn’t seem to interact with light or any other kind of matter. In fact, it has to be something completely new.

The Standard Model of particle physics is incomplete, and that’s a problem. We need to look for new fundamental particles. Amazingly, the very flaws in the standard model give precious clues as to where they might be hiding.

The problem with the neutron

Take the neutron, for example. It constitutes the atomic nucleus together with the proton. Despite being generally neutral, the theory states that it was made up of three charged constituent particles called quarks. Therefore, we would expect some parts of the neutron to be positively charged and others negatively charged; that would mean it had what physicists call an electric dipole moment.

However, many attempts to measure it have had the same result: it is too small to be detected. Another ghost. And we are not talking about instrumental deficiencies, but about 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 1970s, particle physicists Roberto Peccei and Helen Quinn (and later Frank Wilczek and Steven Weinberg) tried to reconcile the theory and the evidence.

They suggested that, perhaps, the parameter is not zero. Rather, it is a dynamic quantity that slowly lost its charge, evolving to zero, after the big bang. Theoretical calculations show that if such an event occurred, it should have left behind a multitude of light, stealthy particles.

These were called “axions” after a brand of detergent because they could “clear up” the neutron problem. And even more. If axions were created in the early universe, they have been around ever since. Most importantly, its properties tick all the boxes expected for dark matter. For these reasons, axions have become one of the preferred candidate particles for dark matter.

Axions would interact only weakly with other particles. However, this means that they would still interact somewhat. Invisible axions could even transform into ordinary particles, including, ironically, photons, the very essence of light. This can happen under particular circumstances, such as in the presence of a magnetic field. This is a godsend for experimental physicists.

Experimental design

Many experiments are trying to evoke the axion phantom in the controlled environment of a laboratory. Some aim to turn light into axions, for example, and then turn the axions back into light on the other side of a wall.

Currently, the most sensitive approach is aimed at the dark matter halo that permeates the galaxy (and consequently 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 axions), while the second induces the conversion into light. The result is an electromagnetic signal that appears inside the cavity, oscillating with a characteristic frequency depending on the mass of the axion.

The system works like a radio receiver. It must be adjusted appropriately to intercept the frequency we are interested in. Practically, the dimensions of the cavity are modified to suit different characteristic frequencies. If the axion and cavity frequencies don’t match, it’s like tuning a radio to the wrong channel.

The powerful magnet is moved to the Yale lab. Credit: Yale University

Unfortunately, the channel we are looking for cannot be predicted in advance. We have no choice but to scan all potential frequencies. It’s like picking out a radio station in a sea of ​​white noise – a needle in a haystack – with an old radio that has to get bigger or smaller every time we turn the frequency knob.

However, these are not the only challenges. Cosmology points to tens of gigahertz as the last and promising frontier for axion research. Since higher frequencies require smaller cavities, exploring this region would require cavities too small to capture a significant amount of signal.

New experiments are trying to find alternative paths. Our Axion Longitudinal Plasma Haloscope (Alpha) experiment uses a new cavity concept based on metamaterials.

Metamaterials are composite materials with global properties that differ from their components: they are more than the sum of their parts. A cavity full of conducting rods gets a characteristic frequency as if it were a million times smaller, even though its volume barely changes. This is exactly what we need. In addition, the rods provide an integrated and easy-to-adjust tuning system.

We are currently building the setup, which will be ready to take 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 elusive, Axions are fueling progress that no ghost will ever take away.

Written by Andrea Gallo Rosso, postdoctoral fellow in physics at Stockholm University.

Adapted from an article originally published on The Conversation.