Dark matter is one of the most enduring and enigmatic mysteries in modern physics. What dark matter actually is has remained elusive, with several hypothetical particles such as weakly interacting massive particles (WIMPs) and axions proposed by scientists.
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Research in recent years has demonstrated that axions could be more of a viable candidate than WIMPS. Theoretical studies on symmetry laws suggest that if these hypothetical particles exist in large enough quantities, they would behave similarly to dark matter.1
What are Axions?
The axion has its roots in the Standard Model of particle physics. This hypothetical particle was proposed to solve a key question in particle physics: the strong charge parity (CP) problem in quantum chromodynamics (QCD.) QCD is not a CP symmetric theory, unlike quantum electrodynamics (QED.)
Axions are characterized by their extremely low mass, weak interactions, and the potential to form a Bose-Einstein condensate.2 These properties of axions would help answer a central problem in particle physics: why neutrons do not possess an electric dipole moment, despite being composed of quarks. This fundamental imbalance would be canceled out by axions.2
Axions are much lighter than WIMPs. Some scientists theorize that these particles could be as light as neutrinos. Several experiments have searched for WIMPs (which were one of the earliest candidates for dark matter) but with no success: as a result, other potential candidates have emerged in the quest to understand dark matter, in particular axions.4
Why Axions Could Be Dark Matter
Dark matter was first inferred in 1933 to answer a conundrum in physics: visible matter could not account for all the observed mass in the Universe. Much of the matter was “missing” and could not be seen. In the 1970s, the existence of dark matter was confirmed by Vera Rubin and W. Kent Ford.
Axions are a potentially more robust candidate for dark matter than WIMPs for a few reasons. Firstly, recent research suggests that they could explain the observed gravitational effects caused by the existence of dark matter without contradicting existing models of particle physics.5
Axions could also have been produced in the early universe in sufficiently large enough quantities. This would explain the prevalence of dark matter in the universe today and why it far outweighs the amount of visible matter. Finally, these hypothetical particles fit well with current observations of the physical universe such as cosmic structure formation and galactic rotation curves.2
Recent Research and Experimental Evidence
Much research has been conducted into confirming the existence of axions and their potential as dark matter candidates.
The Axion Dark Matter eXperiment (ADMX) is an experiment that aims to detect axions by converting them into microwave photons, using an axion holoscope. Strong magnetic fields are employed to make this possible. The experiment is a collaboration between the Fermilab, the University of Washington, UC Berkley, and others.
The Broadband Reflector Experiment for Axion Detection (BREAD) aims to narrow down the search for axions by taking a broadband approach and better identifying the expected characteristics of these theoretical dark matter particles. CAST (CERN Axion Solar Telescope) is searching for axions from the Sun.
A new experiment from the University of Oxford and various partners is searching for Axions using the European XFEL Facility. In this experiment, extremely fast flashes of X-rays are shone through thin blocks of germanium with titanium sheets between them to convert photons into axions.
Only axions are able to the cross through the blocks to the detector due to the presence of the titanium sheets. These particles are then converted back into photons which are thus detected, confirming their existence in a “light-shining-through-walls” experiment.3
Other experiments are aiming to detect axion-like particle hints from pulsars and neutron stars. Additionally, axions could be detected indirectly via radio and X-ray anomalies. Furthermore, axion miniclusters and their role in cosmic structure are the focus of some current studies. New theoretical constraints from quantum field theory could also prove beneficial for axion and dark matter research.
Challenges and Uncertainties
The road toward confirming the existence of axions and whether they are viable dark matter candidates is challenging for particle physicists. Axions have, thus far, not been detected directly, with only indirect hints as to their existence. More sensitive experiments will be required to confirm or rule out axions in the expected mass range.
Furthermore, there are alternative explanations such as modified gravity theories5 that could, if verified, pose fundamental problems for the study of dark matter itself. It's possible that dark matter may no longer be the leading explanation for why most of the universe's matter seems to be missing.
In Summary
In summary, whilst axions are one of the most promising dark matter candidates, definitive proof is still missing, and alternative theories could emerge in the coming years to explain this enigmatic mystery in modern physics.
Regardless of the outcome of research in this area, the search for axions is helping to answer some fundamental questions about physics and our universe.
Further Reading and More Information
- Sutter, P (2021) WIMPS vs. Axions: What is Dark Matter? [online] Universe Today. Available at: https://www.universetoday.com/articles/wimps-vs-axions-what-is-dark-matter (Accessed on 09 March 2025)
- Spergel, D (1996) Axions [online] Princeton University. Available at: https://www.astro.princeton.edu/~dns/MAP/Bahcall/node16.html (Accessed on 08 March 2025
- University of Oxford (2025) New X-ray experiment could solve major physics puzzles [online] EurekAlert! Available at: https://www.eurekalert.org/news-releases/1074045 (Accessed on 08 March 2025)
- Timmer, J (2023) No WIMPs! Heavy particles don’t explain gravitational lensing oddities [online] ArsTechnica. Available at: https://arstechnica.com/science/2023/04/gravitational-lensing-may-point-to-lighter-dark-matter-candidate/ (Accessed on 08 March 2025)
- McGaugh, S.S & Lelli, F (2016) Radial Acceleration Relation in Rotationally Supported Galaxies Phys. Rev. Lett. 117, 201101 [online] APS. Available at: https://doi.org/10.1103/PhysRevLett.117.201101 (Accessed on 08 March 2025)
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