In truth, axions belong to a wider class of “ultra-light” darkish matter particle candidates, which might have plenty all the way down to 10^-24 eV. That is a number of billions of instances lighter than the WIMPs—and certainly many of the particles of the Commonplace Mannequin.
Which means axions and their buddies act nothing like many of the particles of the Commonplace Mannequin.
First off, it might not even be acceptable to consult with them as particles. They’ve such little mass that their de Broglie wavelength—the scale of the quantum wave related to each particle—can stretch into macroscopic proportions. In some instances, this wavelength could be a few meters throughout. In others, it’s akin to a star or a photo voltaic system. In nonetheless others, a single axion “particle” can stretch throughout a complete galaxy.
On this view, the person axion particles could be subsumed into a bigger quantum wave, like an ocean of darkish matter so massive and huge that it doesn’t make sense to speak about its particular person parts.
And since axions are bosons, they will synchronize their quantum wave nature, turning into a definite state of matter: a Bose-Einstein condensate. In a Bose-Einstein condensate, many of the particles share the identical low-energy state. When this occurs, the de Broglie wavelength is bigger than the common separation between the particles, and the waves of the person particles all add up collectively, creating, in essence, a super-particle.
This fashion, we could get axion “stars”—clumps of axions appearing as a single particle. A few of these axion stars could also be just a few thousand kilometers throughout, wandering throughout interstellar house. Nonetheless others stands out as the measurement of galactic cores, which could clarify a difficulty with the normal WIMP image.
The very best description of darkish matter generally is that it’s “chilly,” that means that the person particles don’t transfer quick in comparison with the pace of sunshine. This enables them to gravitationally work together and kind the seeds of constructions like galaxies and clusters. However this course of is a bit too environment friendly. In response to simulations, chilly darkish matter tends to kind extra small, sub-galactic clumps than we observe, and it tends to make the cores of galaxies a lot, a lot denser than we see.
