Physicists have discovered that very light particles called axions might exist in substantial groups surrounding neutron stars. These axions could help explain the mysterious dark matter that cosmologists have been searching for and may even be relatively easy to detect.
A collaborative team of physicists from Amsterdam, Princeton, and Oxford has revealed that exceptionally light particles known as axions may form extensive clouds around neutron stars. These axions could potentially account for the enigmatic dark matter pursued by cosmologists, and interestingly, they might not be overly challenging to detect.
This week, their latest findings were published in the journal Physical Review X. This research builds on prior studies where the same authors examined axions and neutron stars but from a different angle. While their previous study focused on axions that escape from the neutron star, this time, the researchers are investigating those that remain, specifically the axions that are captured by the star’s strong gravitational pull. Over time, these particles are expected to gradually cluster into a misty cloud around the neutron star, which might indeed be detectable with our telescopes. But what is it about these distant star clouds that captivates astronomers and physicists?
Axions: the connection from soap to dark matter
Many are familiar with protons, neutrons, electrons, and photons. However, axions are less known, and for a good reason: as of now, they are only a theoretical-type particle that has not yet been observed. Their name is derived from a brand of soap, reflecting the role they were proposed to play in resolving a problem pertaining to one well-studied particle: the neutron. Despite their theoretically appealing nature, if axions do exist, they would be extremely light, making them very difficult to detect through experiments or observations.
Currently, axions are recognized as a leading candidate for explaining dark matter, one of the most significant puzzles in modern physics. Numerous observations imply that about 85% of the universe’s matter is ‘dark’, meaning it is not comprised of any known or currently observable type of matter. Instead, dark matter’s existence is inferred through its gravitational effects on visible matter. Luckily, this does not inherently imply that dark matter has no measurable interactions with visible matter; if such interactions exist, they are likely very weak. Consequently, any potential dark matter candidate is extremely challenging to observe directly.
Given this context, physicists have surmised that axions might be the solution to the dark matter mystery. A particle that remains undetected, possesses a tiny mass, and has minimal interaction with other particles… could axions potentially be part of the dark matter puzzle?
Neutron stars as observatories
The notion of axions serving as dark matter candidates is appealing, yet in physics, an idea is only truly worthy if it leads to observable outcomes. Is there a way to finally detect axions fifty years after they were initially proposed?
When subjected to electric and magnetic fields, axions are predicted to convert into photons — the particles of light — and vice versa. While we know how to observe light, as previously mentioned, the interaction strength is expected to be minimal, resulting in only a small amount of light emitted by axions. That is, unless the environment comprises a vast number of axions in strong electromagnetic fields.
This consideration led researchers to focus on neutron stars, the most dense stars in our universe. These stars possess masses comparable to our Sun but are condensed into a size of merely 12 to 15 kilometers. Such extreme density engenders equally extreme environments, characterized by immense magnetic fields that are billions of times stronger than any found on Earth. Recent findings suggest that if axions indeed exist, neutron stars’ magnetic fields could facilitate the mass production of these particles near their surface.
The axions that remain
In previous research, the focus was on axions that managed to escape the star — analyzing how many would be produced, their trajectories, and how their transformation into light might yield a faint but potentially observable signal. This time, however, they are examining axions that don’t escape — the ones that fall victim to the considerable gravity of the neutron star despite their minuscule mass.
Given the axion’s incredibly weak interactions, these particles linger, gradually accumulating around the neutron star over millions of years. This process can lead to the formation of dense axion clouds surrounding neutron stars, creating new exciting possibilities for axion investigation. The researchers have delved into the formation, properties, and evolution of these axion clouds, asserting that they should and often must exist. In fact, the authors contend that if axions are real, these clouds should be common (forming around most, if not all, neutron stars across a variety of axion properties), notably very dense (potentially 20 orders of magnitude denser than local dark matter densities), and thus capable of producing strong observational signals. These signals could take multiple forms; the authors discuss two: a continuous signal emitted over much of a neutron star’s life cycle, and a brief burst of light at the end of a neutron star’s life when it ceases its electromagnetic emissions. Both potential signatures could be detected and utilized to investigate the interaction between axions and photons beyond existing limits, even with current radio telescopes.
What’s next?
While no axion clouds have been detected yet, the new findings provide clarity on what precisely to look for, making the search for axions more attainable. Therefore, the primary item on the agenda is the ‘search for axion clouds’, but this study also paves the way for various new theoretical inquiries.
One of the authors is already working on subsequent research examining how axion clouds might influence neutron star dynamics. Another critical direction for future research is the numerical modeling of axion clouds: although the present paper indicates significant discovery potential, additional modeling is required for a more precise identification of what and where to look. Lastly, current results relate solely to individual neutron stars, yet many are found in binary systems — either with another neutron star or a black hole. Grasping the physics of axion clouds within such systems and understanding their observational signals would be incredibly beneficial.
In summary, this research marks a significant advancement in an exciting new area of study. Achieving a comprehensive understanding of axion clouds will necessitate collaborative efforts across multiple scientific fields, including particle (astro)physics, plasma physics, and observational radio astronomy. This work inaugurates a new cross-disciplinary field brimming with opportunities for future exploration.
