In the depths of the universe, far beyond our grasp, lie some of nature’s most baffling secrets. One of these mysteries — dark matter — may have found an unlikely accomplice: the dense, collapsed remnants of dead stars.
Known as neutron stars, these spinning, magnetic powerhouses might be churning out hypothetical particles called axions. If true, these particles could make up the universe’s mysterious dark matter.
A team of physicists from the universities of Amsterdam, Princeton, and Oxford has proposed that axions could be forming dense clouds around neutron stars. Their findings suggest that these “dark matter factories” might, under the right conditions, send axions cascading into space. And their signals may be faintly detectable as flashes of light. This means neutron stars could be our best bet yet for finding dark matter’s elusive particles.
Neutron Stars and the Dance of Axions
Axions were initially theorized to address unexplained behaviors in the Standard Model of particle physics, particularly with neutrons. Since the 1970s, they’ve been regarded as prime dark matter candidates, alongside other theoretical particles like WIMPs and dark photons.
According to Anirudh Prabhu, researcher at Princeton and co-author of the study, axions might convert into photons near neutron stars. This phenomenon is called the Primakoff effect, where strong magnetic fields facilitate this transformation.
“When we see something, what is happening is that electromagnetic waves (light) bounce off an object and hit our eyes,” Prabhu told Gizmodo. “The way we ‘see’ axions is a little different.” Axions don’t “bounce” light in the usual sense, but the Primakoff effect allows them to become visible as light under the right conditions.
Some neutron stars, known as magnetars, have some of the strongest magnetic fields in the universe. This makes them especially favorable for these axion-to-light transformations. But as compelling as this scenario sounds, detecting axions remains exceptionally difficult. Unlike visible light, these axion-born electromagnetic waves can range in wavelength from a fraction of an inch to more than half a mile. Earth’s atmosphere blocks many of these long wavelengths, meaning that a space-based radio telescope might be required for detection.
Catching a Flicker of Dark Matter
Searching for axions around neutron stars isn’t straightforward. Nobody is really sure they exist in the first place. And, if they are real, the signals they produce are faint and would require sensitive, specialized equipment. Some scientists, like particle physicist Benjamin Safdi of UC Berkeley, see the potential for axions in extreme astrophysical environments — but he notes the challenges ahead.
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“There are a lot of uncertainties,” Safdi said, “this is no fault of the authors; it is simply a hard, dynamical problem.”
Alternatively, axion clouds might release a sudden burst of light when a neutron star reaches the end of its life. Such a process could take trillions of years, making it impossible to observe in our lifetime.
Yet, despite the hurdles, the study provides essential constraints on axion properties, refining the search for this elusive particle and providing a roadmap for future studies. Prabhu’s team suggests that existing radio telescopes, given enough refinement, might improve sensitivity to axion detection. However, a dedicated space-based radio observatory could dramatically enhance our chances of detecting axions.
Proposals like NASA’s Lunar Crater Radio Telescope (LCRT), a massive radio telescope envisioned on the Moon’s far side, might bridge this gap. Such an instrument could be insulated from Earth’s interference and detect the long-wavelength signals axions are expected to emit.
Our Best Bet
Though still hypothetical, axions may finally be on the cusp of moving from theoretical speculation to detectable reality. As Safdi put it, “Axions are one of our best bets for new physics . . . work like this could thus easily open the pathway towards discovery.”
And so, in the quiet darkness of space, neutron stars continue their dance, perhaps in the company of dark matter’s elusive particles. If we’re lucky, and if the universe cooperates, a glimmer of that dark mystery may soon reveal itself. And we may come one step closer to understanding the invisible scaffolding of the cosmos.
The findings were described in the journal Physical Review X.