Axions represent a category of dark matter particle candidates described as “ultra-light,” with potential masses as low as 10^-24 eV. This makes them significantly lighter than Weakly Interacting Massive Particles (WIMPs) and virtually all particles included in the Standard Model of particle physics.
This remarkable lightness means that axions and similar particles behave differently from most known particles in the Standard Model.
Notably, it may not be entirely accurate to classify them strictly as particles. Their minimal mass allows for their de Broglie wavelength—the quantum wave linked to each particle—to extend into macroscale dimensions. These wavelengths can stretch several meters long in some instances, and in other cases, they can be akin to the size of a star, a solar system, or even an entire galaxy.
In this framework, individual axion particles merge into a more extensive quantum wave, creating a vast “ocean” of dark matter so expansive that it becomes impractical to identify its individual components.
Moreover, since axions are classified as bosons, they have the ability to synchronize their quantum wave properties, resulting in a state of matter known as a Bose-Einstein condensate. In such a state, the majority of particles occupy the same low-energy configuration. This phenomenon occurs when the de Broglie wavelength exceeds the average distance separating the particles, causing the individual waves to combine, essentially creating what can be viewed as a super-particle.
This concept gives rise to the idea of axion “stars”—aggregations of axions acting cohesively as a single entity. Some of these axion stars could span thousands of kilometers, drifting through interstellar space, while others might be as massive as galactic cores, potentially offering solutions to discrepancies in the conventional WIMP framework.
Generally, dark matter is characterized as “cold,” indicating that its particles do not travel at speeds close to that of light. This property facilitates gravitational interactions, allowing for the formation of structures such as galaxies and clusters. However, this mechanism seems overly effective. Simulations suggest that cold dark matter tends to create more small, sub-galactic clumps than what is observed, and it also tends to produce much denser galactic cores than currently measured.
