Cluster of galaxies, left, with visible ring of dark matter, right. Credit: NASA, ESA, MJ Jee and H. Ford (Johns Hopkins University)
Explorations in dark matter are advancing with new experimental techniques designed to detect axions, using cutting-edge technology and interdisciplinary collaboration to uncover the secrets of this elusive part of the cosmos.
There is a ghost haunting 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 properties are reflected in the name: dark matter.
Several experiments aim to reveal what it is made of, but despite decades of searching, scientists have been unable to do so. Now our new experiment, under construction at Yale University in the US offers a new tactic.
Dark matter has been present in the universe since the beginning of time and draws stars and galaxies together. Invisible and subtle, it does not appear to interact with light or any other form of matter. Actually it has to be something completely new.
The Standard Model of particle physics is incomplete, and this is a problem. We have to look for new fundamental particles. Surprisingly, the same shortcomings of the standard model provide valuable clues as to where they may be hiding.
The problem with the neutron
Let’s take the neutron for example. Together with the proton, it forms the atomic nucleus. Despite being generally neutral, the theory states that it is made up of three charged particles called quarks. Therefore, we would expect some parts of the neutron to be positively charged and others negatively. This would mean that the neutron would have what physicists call an electric dipole moment.
Yet many attempts to measure it have yielded the same result: it is too small to be detected. Another ghost. And we are not talking about instrumental imperfections, but about a parameter that must be smaller than one in ten billion. It’s so small that people wonder if it could be zero.
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) attempted to integrate theory and evidence.
They suggested that the parameter might not be zero. Rather, it is a dynamic quantity that slowly lost its charge and then evolved to zero Big bang. Theoretical calculations show that, if such an event were to occur, it must have left behind a large number of light, sneaky particles.
These were called ‘axions’, after a detergent brand, because they could ‘clear up’ the neutron problem. And more. If axions were created in the early universe, they have been hanging around ever since. Most importantly, their properties meet all expectations for dark matter. For these reasons, axions have become one of the favorite candidate particles for dark matter.
Axions would interact only weakly with other particles. However, this means that they would still interact with each other a little. The invisible axions could even transform into ordinary particles, including – ironically – photons, the essence of light. This can happen under certain conditions, such as in the presence of a magnetic field. This is a godsend for experimental physicists.
Experimental design
Many experiments attempt to evoke the axion spirit in the controlled environment of a laboratory. For example, some try to convert light into axions, and then back into light on the other side of a wall.
Currently, the most sensitive approach focuses on the dark matter halo that permeates the galaxy (and therefore Earth) with a device called a haloscope. It is a conductive cavity immersed in a strong magnetic field; the former captures the dark matter around us (assuming they are axions), while the latter triggers its conversion into light. The result is an electromagnetic signal that appears in the cavity and oscillates with a characteristic frequency depending on the axion mass.
The system works like a receiving radio. It must be properly adjusted to intercept the frequency we are interested in. In practice, the cavity dimensions are changed to allow for 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 laboratory. 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 choosing a radio station in a sea of white noise – a needle in a haystack – with an old radio that has to increase or decrease every time we turn the frequency knob.
Yet these are not the only challenges. Cosmology points to tens of gigahertz as the newest, promising frontier for axion searches. Because higher frequencies require smaller cavities, exploring that area would require cavities too small to pick up a meaningful amount of signal.
New experiments try 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 constituents: they are more than the sum of their parts. A cavity filled with conductive rods acquires a characteristic frequency as if it were a million times smaller, while the volume hardly changes. That’s exactly what we need. In addition, the rods offer a built-in, easily adjustable tuning system.
We are currently building the setup, which will be ready to collect 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 fuel progress that no mind can ever take away.
Written by Andrea Gallo Rosso, Postdoctoral Researcher in Physics, Stockholm University.
Adapted from an article originally published in The Conversation.