Since the 1960s, astronomers have theorized that the universe could be filled with a mysterious mass that only interacts with “normal matter” through gravity. This mass, nicknamed dark matter (DM), is crucial for solving problems between astronomical observations and general relativity. In recent years, scientists have suggested that DM may consist of axions, a class of hypothetical elementary particles with low mass within a certain range. These particles were first proposed in the 1970s to solve problems in the Standard Model of particle physics and have emerged as leading candidates for DM.
Adding to the growing evidence that this may be the case, researchers at CERN are developing a new telescope that could help the scientific community search for axions – the CERN Axion Solar Telescope (CAST). According to new research from an international team of physicists, these hypothetical particles could exist in large clouds around neutron stars. These axions could be the long-awaited explanation for dark matter that cosmologists have been searching for for decades. Furthermore, their research suggests that these axions may not be very difficult to observe from Earth.
The team was led by Dion Noordhuis, a Ph.D. Student at the GRavitational AstroParticle Physics Amsterdam (GRAPPA) Institute, the Institute for Theoretical Physics (ITP) and the Delta Institute for Theoretical Physics at the University of Amsterdam (UvA). He was joined by researchers from the Center for Theoretical Science (PCTS) at Princeton University, the University of Barcelona and the Rudolf Peierls Center for Theoretical Physics at the University of Oxford. The paper describing their results was published in the journal Physical Review X on October 17, 2024.
As with DM, the existence of axions was postulated to fill gaps in our understanding of the behavior of another elementary particle – the neutron. However, like DM, these hypothetical particles have not yet been discovered after decades of research. This is understandable because if such particles existed, they would be extremely light, making them very difficult to detect through experiments or astronomical observations. For this reason, axions are considered promising candidates to explain DM, which theoretically accounts for 85% of the matter in our universe.
While DM theoretically interacts with visible matter via gravity, this does not necessarily mean that there are no other interactions that would be detectable. For example, axions are expected to convert into photons when exposed to electric and magnetic fields, which we can observe. However, the corresponding interaction strength and the amount of light generated should be very small. Therefore, they would likely go unnoticed unless there was an environment with massive axion clouds in a very strong electromagnetic field.
This led Noordhuis and his team to consider neutron stars because they are the densest class of stars in the universe and produce very strong electromagnetic fields. In fact, neutron stars produce magnetic fields billions of times stronger than Earth's magnetosphere. In addition, astronomers have been using supernovae and cooling neutron stars to constrain the properties of axons, including their mass and interactions with other particles. Recent research also supports the idea that neutron stars can produce large amounts of axions due to their strong magnetic fields near their surfaces.
In a previous study, Noordhuis and his colleagues examined how axions can escape from a neutron star. This included calculating the number of axions produced, the trajectories they would follow, and how converting them into light might result in an observable signal. In their latest work, the researchers focused on the axions, which are theoretically captured by the gravity of a neutron star. Due to the very weak nature of their interactions, these particles are likely to remain bound to their stars for millions of years.
Artist's impression of an axion cloud around a neutron star. Photo credit: UvA
As they argue in their paper, they would gradually form a hazy cloud around the neutron star that could be visible to telescopes. The team also studied the formation, properties and evolution of these axion clouds and found that they would likely form around most or even all neutron stars (accounting for a wide range of axion properties). They also calculated that these clouds would be up to twenty orders of magnitude larger than the local DM density, resulting in strong observational signatures.
These could take the form of a continuous signal emitted throughout much of a neutron star's lifetime, or a one-off burst of light at the end of its life. These signatures would be detectable with current radio telescopes and could be used to study the interaction between axions and photons. Although axion clouds have not yet been observed, the team's study gives astronomers parameters for what to look for. In addition to the search for axion clouds, this research offers additional opportunities for further theoretical research.
This includes follow-up work by one of the study's co-authors on how the axion clouds can change the dynamics of neutron stars themselves. There is also the possibility of exploring numerical modeling of axion clouds to further constrain what and where astronomers should look. Finally, the present work deals with single neutron stars, but there are also possibilities for binary stars, which consist of two neutron stars and a neutron star with a black hole as a companion. By using next-generation instruments in addition to current ones, these observations could be a step toward finding the elusive DM particle.
These studies could also have applications in other research areas, such as particle physics, astrophysics, plasma physics and radio astronomy. In short, this latest study offers opportunities for interdisciplinary research that could solve some of the biggest mysteries in astronomy and cosmology today.
Further reading: University of Physics
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