A bosonic star may be nearby: what is it and how is it connected to dark matter?

If you look at the night sky, you can see millions of glowing stars, each with varying brightness. At the same time, scientists speculate that the cosmos may be filled with invisible bosonic stars made up of exotic forms of matter that do not emit light, as reported by Space.

Dark Matter Particles

According to current understanding of the universe, dark matter constitutes the majority of its mass. There is substantial indirect evidence supporting the existence of this form of matter. Scientists believe that dark matter is a yet undiscovered particle, though direct evidence for it remains elusive.

For the last approximately 20 years, scientists have theorized that dark matter is a hypothetical particle known as WIMP (Weakly Interacting Massive Particle). It was suggested that this new particle would have a mass similar to the heaviest particle in the universe, namely the top quark. However, WIMPs largely remain invisible particles, as they interact extremely rarely with ordinary matter. Despite extensive searches for WIMPs, no conclusive results have been found.

As a result, researchers proposed that dark matter might be a hypothetical, yet undiscovered particle called the axion. This particle was introduced to address a problem related to strong interactions, one of the four fundamental forces of nature. Observations indicate that strong interactions obey two important symmetries in the universe: charge and parity. This means that if you take a strong interaction, reverse the charges of all particles, and observe the reaction in a mirror, you will get the same result.

However, nothing in the theory suggests that strong interactions must adhere to these symmetries. Physicists attempted to correct this by adding a new parameter to the equations and setting it to zero, but this did not resolve the issue. Subsequently, scientists speculated that this parameter might represent a new quantum field, and that interactions with this field would naturally create the required symmetry. This is the axion, which addresses the symmetry problem.

If axions exist, they would be well-suited to serve as dark matter, as they should be extremely abundant and interact very rarely, if at all, with ordinary matter. However, axions have some additional characteristics.

Bosonic Stars

These particles are believed by physicists to be very light. Their mass should be trillions of times smaller than that of the lightest particle in the universe—the neutrino. Therefore, the quantum-wave nature of axions should manifest on macroscopic scales. Although each particle is associated with a wave, this is typically not considered unless discussing subatomic quantum systems. However, axions can spread their wavelength throughout the galaxy.

Additionally, physicists believe that axions are bosons. This type of particle can exist in the same quantum state. In other words, you can fit as many as you want into a compact volume. Thus, bosons resemble photons but differ from other particles, like electrons, which can only occupy a compact volume in limited quantities.

These two properties of axions imply that they can be compressed to incredibly high densities, drawn together by their own gravity. Scientists think they could form a kind of star. However, this star would be completely invisible, as it does not emit light and does not interact with anything.

Such stars are referred to as bosonic stars, axion stars, and dark stars. It is believed that they can be the size of a typical star, but they can also be so massive that they encompass the entire core of a galaxy.

On one hand, detecting a bosonic star is extremely challenging, unless it is located within the Solar System or passes through Earth, allowing axions to be detected by terrestrial detectors. On the other hand, bosonic stars might do everything possible to be discovered. For instance, they could interfere with nuclear fusion in the cores of ordinary stars or explode like supernovae.