Dark Matter: Neutron Stars May Be Enveloped in Axion Clouds

An elusive form of matter

Dark matter is a form of matter that remains completely invisible to our detection instruments because it does not emit light or any other form of electromagnetic radiation. Despite its elusive nature, however, scientists have deduced its existence by studying gravitational effects which it exerts on visible matter, such as stars and galaxies.

To illustrate this phenomenon, let's take the example of spiral galaxies. These immense cosmic structures rotate at an impressive speed. According to the laws of physics, in particular Newton's law of gravitation, the rotation speed of a galaxy should decrease as we move away from its center, where most of the visible matter is concentrated. , like stars and gas. However, astronomers have observed that the rotation of galaxies does not follow this predicted pattern. In fact, stars located on the outskirts of these galaxies rotate at surprising speeds, almost as fast as those near the center.

This observation therefore suggests the presence of an invisible mass which contributes to the gravity of the galaxy. Current cosmological models suggest that this dark matter could represent around 27% of the total composition of the universe, while visible matter would only constitute around 5%.

What hypotheses?

Scientists are exploring various hypotheses to explain the nature of dark matter. Some theories suggest that it could be made up of exotic particles called WIMPs (Weakly Interacting Massive Particles). The latter, being very heavy, would only interact very weakly with ordinary matter, which would make them difficult to detect. Other proposals suggest that dark matter could be a modified form of gravity, but no experiments have yet confirmed this idea.

Another promising hypothesis concerns axions, particles introduced in the 1970s to solve several problems in theoretical physics, notably the problem of CP symmetry invariance in quantum chromodynamics (QCD), the theory that describes the interactions between quarks and gluons. These particles are postulated to be extremely light, almost massless. Their weak interaction with other particles means they could pass through matter without being absorbed, making them very different from the particles we commonly observe.

But then, how can we detect them? This is where neutron stars come into play.

Axion clouds

Neutron stars are the result of the gravitational collapse of massive stars at the end of their lives. These stars are among the densest objects in the universe. Imagine the mass of a Sun-like star is compressed into a sphere just 12 to 15 kilometers across. This compression then creates extreme conditions, with densities so high that a single cubic centimeter of material from a neutron star would weigh around a billion tons.

In addition to their incredible density, these stars generate magnetic fields of phenomenal intensity, which can be billions of times stronger than those found on Earth. These influence the behavior of particles in their neighborhood. However, a key mechanism proposed for the production of axions is based on the conversion of photons (particles of light) into axions in an intense magnetic field. This process is often described by the theoretical model of quantum electrodynamics.

In addition to their magnetic field, neutron stars exhibit extreme thermal conditions. These high temperatures can excite particles and increase the energy in the field, thereby promoting the interactions necessary for the potential production of axions.

Finally, we know that neutron stars are often surrounded by dense matter, such as star debris or plasma. This material could also play a role in providing an environment where axions can form and interact.

Once produced, the axions could then form a cloud around the neutron star, where they would be held by the star's gravity.

dark matter star neutrons axions

A cloud of axions around a neutron star. If some axions escape the gravitational attraction of the star, many remain bound to the star and over a long period form a cloud that surrounds it. Credit: University of Amsterdam

Potentially observable signals

Unlike other regions of the universe where axions could be present in low concentrations, axion clouds around neutron stars, formed by complex interactions in extreme environments, could provide a favorable setting for their detection.

The first way axions could manifest is through continuous emission of light or electromagnetic radiation. In this dense cloud, axions interact with the powerful magnetic field of the neutron star, emitting photons that propagate outward. These signals could be picked up by ground-based or space-based telescopes, allowing astronomers to study the behavior and density of axions over an extended period of time.

Another observation opportunity would arise at the end of the neutron star's life. When it stops emitting energy, it can enter an unstable phase, triggering a cataclysmic event. This phenomenon could release an immense amount of energy, creating a unique flash of light. Scientists believe this flash could result from the sudden decay of accumulated axions, producing a bright explosion detectable from Earth.

Of course, this work will require the development of more advanced telescopes and sophisticated analysis methods to isolate axion signals from other astronomical phenomena, but it would be worth the effort. The detection of these light signals would indeed be of crucial importance for modern physics. Not only would this provide direct proof of the existence of axions, but it would also allow scientists to learn more about their properties, such as their mass and their interaction with matter. This could offer clues to the nature of dark matter itself, helping to solve one of the greatest mysteries of the universe.

Source: Physical Review

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