In his mid-thirties, Fritz Zwicky used Caltech’s Mount Wilson Observatory to measure the visible mass of the Coma Cluster, a large galaxy cluster close to Earth, and found an oddity. He uncovered that there was not enough visible mass to prevent the galaxies from escaping the gravitational pull of the cluster. This led him to propose the theory of dark matter. As defined by Stephen Hawking in A Brief History of Time: “Matter in galaxies, clusters, that cannot be observed directly but can be detected by its gravitational effect. As much as 90% of the mass of the universe may be in the form of dark matter”. The dark matter gravitational attraction acts as a “cosmic glue” to hold the universe together. It explains the large-scale structure of the universe and where the excess gravity in the universe could come from. Without dark matter, the motion of galaxies can’t be explained.
Following this discovery, scientists around the world began formulating theoretical models to explain the nature of dark matter. One popular framework is supersymmetry (SUSY). First proposed in 1971 by Wess and Zumino, SUSY could be a viable explanation of the composition of dark matter and is today one of the most relevant models. The SUSY framework is based on the idea that every known particle has a corresponding superparticle which has the same quantum numbers and mass, but a different spin. The fact that a superparticle has a different spin than the known particle could mean that it does not interact with the known particles in the same way as the other particles do, making it harder to detect. SUSY particles could constitute part of the dark matter.
However, one of the key challenges is to find evidence of supersymmetry. CERN is currently utilizing multiple approaches to search for evidence of SUSY particles, one involving the Large Hadron Collider (LHC). The LHC is the world’s largest (27km) and most powerful particle accelerator. LHC allows scientists to collide subatomic particles at extremely high energies. By doing so, these scientists are searching for the production of SUSY particles, such as squarks, gluinos, and neutralinos. The discovery of these particles would validate the SUSY model as it would provide strong evidence for the existence of new particles beyond the Standard Model of particle physics. Even though the experiments run in the LHC have been able to make ground-breaking discoveries such as the Higgs boson in 2012, as of today, these experiments have not been able yet to reveal SUSY particles and to validate the supersymmetry framework. This has led to many scientists calling for a fundamental reimagining of the laws of physics, and its influence on the modern world.
Four alternative theoretical models could explain dark matter: WIMPs, Weakly Interacting Massive Particles; Axions; Modified Gravity, and Extra Dimensions.
The WIMPs framework introduces new particles that have a much higher mass than known particles but interact only weakly with normal matter through gravitational forces. Even though this framework is quite popular, the WIMPs particles have not been observed yet by the many experiments underway. One of the most interesting is Lux-Zeplin (LZ). This research center is designed to search for evidence of WIMPs through direct detection. In this technique, scientists study the rare interactions of WIMPs with normal matter. The detector is the most sensitive in the world. The LZ researchers aim to take 1000 days’ worth of data over the next 3–5 years to measure potential direct detections.
A second is based on the Axions, a particle proposed in 1970 by Frank Wilczek. These particles are assumed to be extremely light and to have weak interactions with normal matter. The International Axion Observatory is currently conducting research using an axion Helioscope. It creates an intense magnetic field over a large area, which could help to detect axions. Similar to the experiments on WIMPs, no results have been able to demonstrate the existence of Axions.
A third interesting framework is the Modified Gravity model, which articulates that dark matter would not be made of new particles. Instead, this framework argues that gravity would behave differently on extremely large scales and could explain the mass of the universe with laws different than current ones. However, this framework is not widely accepted in the science community. Indeed, the modified gravity model has not been able to make accurate predictions that can be tested, nor successfully explain the gravitational influence of dark matter on the rotations of galaxies. Finally, this model often violates the equivalence principle of General Relativity.
A fourth hypothetical model is Extra Dimensions. This model proposes that dark matter is composed of heavy particles which are trapped in other dimensions. These particles in other dimensions would be hard to detect due to their weak interactions and would allow the dark matter to exist without conflicting with the rest of the universe. This model of Extra Dimensions has not been empirically confirmed either.
The true nature of dark matter remains a mystery. Despite multiple experiments across the world, dark matter has never been detected experimentally. However, the latest research on Supersymmetry seems to be the most promising. A recent LHC experiment has demonstrated the “missing momentum”, where the total net momentum measured after a collision is not zero, and the missing momentum could have been carried away by an undetected dark matter particle. This ground-breaking experiment at CERN could be a very promising step toward demonstrating the SUSY framework, and thus reinforcing the relevance of supersymmetry in understanding the composition of dark matter. Until then, however, academics will continue debating the true nature of dark matter, and with every paper, reimagine our world.
Written by Artus Oury
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