For years, scientists have been aware that the visible Universe is constructed upon a foundation of dark matter. Yet, the true nature of dark matter remains a mystery. Most of the evidence collected so far has pointed towards WIMPs (Weakly Interacting Massive Particles) as the main building blocks of dark matter. Despite this, the search for WIMPs has proven fruitless, leading researchers to consider other potential candidates.
Among these alternatives is the axion, a lightweight force-carrying particle initially suggested to address a separate issue in physics. Though axions have properties that align with dark matter, they have not received as much attention as WIMPs. However, a recent study posits that certain features of a gravitational lens, largely produced by dark matter, might be better explained by axions’ properties.
So what exactly is an axion? In simple terms, it is an extremely lightweight, spinless particle that functions as a force carrier. Axions were initially proposed to preserve the conservation of charge parity in quantum chromodynamics – the theory describing the behavior of the strong force holding protons and neutrons together. Researchers conducted experiments to test axions’ compatibility with other theoretical models and sought ways to detect them. Nevertheless, axions have remained one of many possible solutions to an unresolved issue.
Axions have caught the attention of scientists investigating dark matter. However, dark matter’s behavior seemed better suited to a heavier particle like WIMPs. In contrast, axions could be as light as almost massless neutrinos. Previous searches for axions ruled out many heavier masses, further complicating the matter. As the search for WIMPs continues to yield disappointing results, axions may be gaining ground in the race for dark matter alternatives. Various detectors have been built to detect WIMPs’ weak interactions, but they have been unsuccessful. Moreover, if WIMPs were Standard Model particles, their presence could have been inferred through missing mass in particle colliders – but no such evidence has emerged. Consequently, the idea that WIMPs may not be the optimal solution to dark matter is gaining traction.
While WIMPs fit well with large-scale data, peculiarities emerge on the scale of individual galaxies. These peculiarities suggest a more intricate structure is needed for the dark matter halo surrounding a galaxy. Similar issues arise when mapping dark matter of single galaxies based on its ability to form a gravitational lens that manipulates space, magnifying and distorting background objects. The recent study suggests that these peculiarities may be linked to differences between WIMPs and axions. WIMPs, as their name suggests, should behave as separate particles, mainly interacting through gravity. Axions, on the other hand, should interact via quantum interference, resulting in wave-like patterns throughout a galaxy. Consequently, while the frequency of WIMPs should decrease gradually with distance from the galactic core, axions should form a standing wave (or soliton) near the core. In outer regions, complex interference patterns should produce areas devoid of axions, as well as areas with twice the average density. As axions continue to gain traction as a potential dark matter candidate, the search for the elusive substance constituting our Universe continues.