A hypothetical particle that could make up the universe’s dark matter may be produced by and hang around neutron stars, some of the densest objects in the universe, according to a team of physicists.
The particles are axions, one of several proposed candidates for so-called dark matter, the enigmatic stuff that makes up over a quarter of the universe’s matter. A team of researchers from the universities of Amsterdam, Princeton, and Oxford now posit that axions could form clouds around neutron stars, which are the incredibly dense, collapsed remnants of dead stars. The finding offers a new arena where researchers can focus astrophysical searches for dark matter, while highlighting the potential utility of a radio telescope in space.
Possible dark matter factories
The team suggests that some axions produced inside neutron stars could convert into photons and escape into space. But many of these particles would remain trapped by the star’s gravity, forming an axionic cloud around the neutron star. The group’s research describing the idea was recently published in Physical Review X and follows up on an earlier work by the team that explored axions that could escape the gravitational fields of the neutron stars that produce them.
“When we see something, what is happening is that electromagnetic waves (light) bounce off an object and hit our eyes. The way we ‘see’ axions is a little different,” said Anirudh Prabhu, a research scientist at the Princeton Center for Theoretical Science and co-author of the paper, in an email to Gizmodo. “While light can ‘bounce’ off of axions, this process is extremely rare. The more common way to detect axions is through the Primakoff effect, which allows axions to convert into light (and vice versa) in the presence of a strong magnetic field.”
Some neutron stars can be among the most magnetic objects in the universe, and therefore are given a special label: magnetars. This extremely magnetized environment is fertile breeding grounds for axions’ conversion into light, Prabhu said, which then could be detectable by space-based telescopes.
Dark matter and axion waves in the universe
Dark matter is the catch-all name for the 27% of stuff in the universe that scientists cannot directly observe because it does not emit light and only appears to interact with ordinary matter through gravitational interactions. Other candidates include Weakly Interacting Massive Particles (or WIMPs), dark photons, and primordial black holes, to name a few. Axions were originally proposed as a solution to a problem in particle physics: Basically, some of the predicted characteristics of the neutron aren’t observed in nature. Hence their name—axions—which comes from a cleaning product brand. After all, the axion was proposed as a way to clean up some of the nasty conundrums that arose around the Standard Model of particle physics. Last year, a different team of researchers studied Einstein rings—areas of space where light has been bent strongly by gravity, forming a visible “ring” in space—and found evidence boosting axions as a candidate for dark matter.
The electromagnetic waves (i.e., light) produced by converting axions could have wavelengths a fraction of an inch up to more than half a mile (one kilometer) long, Prabhu noted. But Earth’s ionosphere blocks very long wavelengths from Earth-based telescopes, so space-based observatories might be our best bet for spotting evidence of axions.
Neutron stars and axions have a history
“It is well established in the field of axion physics that if you have large, time-varying electric fields parallel to magnetic fields you end up with ideal conditions for producing axions,” said Benjamin Safdi, a particle physicist at UC Berkeley who was not affiliated with the recent paper, in an email to Gizmodo. “In retrospect, it is obvious and clear that if this process happens in pulsars a sizable fraction of the axions produced could be gravitationally bound due to the strong gravity of the neutron star. The authors deserve a lot of credit for pointing this out.”
In 2021, Safdi co-authored a paper positing that axions may be produced in the Magnificent Seven, a group of neutron stars in our own galaxy. The Magnificent Seven produce high-frequency X-rays, and the team proposed that axions converting into photons could produce X-rays like those observed by some telescopes. But many of the axions produced at the cores of those neutron stars stay closer to the source, the recent team said, and build up a large population over hundreds of millions—if not billions—of years.
“These axions accumulate over astrophysical timescales, thereby forming a dense ‘axion cloud’ around the star,” the team wrote in the paper. “While a deeper understanding of the systematic uncertainties in these systems is required, our current estimates suggest that existing radio telescopes could improve sensitivity to the axion-photon coupling by more than an order of magnitude.”
“There are a lot of uncertainties, however, in the calculations presented in this work — this is no fault of the authors; it is simply a hard, dynamical problem,” Safdi added. “I would also like to see more thorough work on the detection prospects for this signal, including a better job modeling the neutron star population and estimating the sensitivity with existing and upcoming instruments.”
So how can we detect and identify dark matter?
But the state-of-the-art telescopes in space are not radio telescopes. The Webb Space Telescope, launched in 2021, observes some of the oldest light we can see at infrared and near-infrared wavelengths. ESA’s Euclid Space Telescope, launched last year with the specific goal of improving our understanding of the universe’s dark matter, also sees the cosmos in the infrared. In fact, one of the most compelling options for a radio-based observatory is the Lunar Crater Radio Telescope (LCRT), which is exactly what it sounds like: a huge radio telescope that would make a dish out of a lunar crater on the dark side of the Moon.
“Axions are one of our best bets for new physics,” Safdi said, though they are “notoriously difficult to probe given their feeble interactions with ordinary matter.”
“These feeble interactions can be magnified in extreme astrophysical environments such as those found in neutron star magnetospheres,” he added. “Work like this could thus easily open the pathway towards discovery.”
There are plenty of radio telescopes doing fantastic work on Earth—MeerKAT, the Very Large Telescope, and ALMA, to name a few—but it seems we may need a new space-based mission if we want to have a chance of seeing axionic waves. No pressure, NASA coffers!
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