The axion may help clean up the messy business of dark matter

In fact, axes belong to a much wider class of candidates of “ultra -light” dark matter particles, who can have masses at 10 ^ -24 EV. These are several billion times lighter than the WIMP – and in fact most of the particles of the standard model.

This means that axes and their friends do not act like most of the particles of the standard model.

First of all, it may not even be appropriate to qualify them as particles. They have so little mass that their broglie wavelength – the size of the quantum wave associated with each particle – can stretch in macroscopic proportions. In some cases, this wavelength can be a few meters in diameter. In others, it is comparable to a star or a solar system. In still others, a single “particle” with axion can stretch through an entire galaxy.

From this point of view, individual axion particles would be subsumed in a larger quantum wave, like an ocean of black matter so large and vast that it has no sense to speak of its individual components.

And because axes are bosons, they can synchronize the nature of their quantum wave, becoming a distinct state of matter: a condensate of Bose-Einstein. In a Bose-Einstein condensate, most particles share the same low energy state. When this happens, Broglie’s wavelength is greater than the average separation between particles, and the waves of individual particles add up all together, creating, in substance, a superparticles.

In this way, we can get “stars” of axion – axle balls acting as a single particle. Some of these axion stars can have a few thousand kilometers in diameter, wandering in the interstellar space. Still others can be the size of galactic nuclei, which could explain a problem with the traditional image of Wimp.

The best description of dark matter in general is that it is “cold”, which means that individual particles do not move quickly compared to the speed of light. This allows them to interact gravitally and to form structure seeds such as galaxies and clusters. But this process is a little too effective. According to simulations, cold dark matter tends to form smaller under-galactic tufts that we observe, and it tends to make the nuclei of galaxies much denser than we see it.