The hypothetical particles that make up the dark matter of the universe may be produced and hang around neutron stars, some of the densest matter in the universe, according to a group of physicists.
These particles are axions, one of several proposed components of so-called dark matter, the mysterious substance that makes up a quarter of the matter in the universe. A team of researchers from the universities of Amsterdam, Princeton, and Oxford now say that axions can form clouds around neutron stars, which are the incredibly dense, collapsing remnants of dead stars. The findings provide a new platform on which researchers can focus their astronomical search for dark matter, while highlighting the potential use of radio telescopes in space.
Dark potential industries
The team suggests that some of the axions produced inside neutron stars can be converted into photons and escape into space. But many of these particles will remain trapped by the star’s gravity, forming an axionic cloud around the neutron star. A group study explaining the idea was recently published in Physical Review X and follows up on previous work by the group that explored axions that can escape the gravitational fields of the neutron stars that produce them.
“When we see something, what happens is that electric waves (light) bounce off something 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. Although light can ‘bounce’ off axions, this process is very rare. The most common way to detect axions is the Primakoff effect, which allows axions to transform into light (and vice versa) in the presence of a strong magnetic field.”
Some neutron stars can be among the most highly magnetized objects in the universe, so they are given a special label: magnets. This highly magnetic field is fertile ground for the conversion of axions into light, Prabhu said, which could be detected by telescopes in space.
Dark matter and axion waves in the universe
Dark matter is the catch-all term for the 27% of matter in the universe that scientists can’t see directly because it doesn’t emit light and appears to interact with ordinary matter through gravity. 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 properties of neutrons are not observed in nature. Hence their name – axions – from the cleaning product. After all, the axion was proposed as a way to clean up some of the ugly controversies that arose around the Standard Model of particle physics. Last year, a separate team of researchers examined Einstein rings—areas of space where light is bent by gravity, creating a visible “ring” in space—and found evidence that boosts axions as a candidate for dark matter.
Electromagnetic waves (that is, light) produced by converting axions can have wavelengths from half an inch to more than half a mile (one kilometer) long, notes Prabhu. But Earth’s ionosphere blocks the longest wavelengths from Earth-based telescopes, so observatories in space may be our best bet for seeing evidence of the axion.
Neutron stars and axions have a history
“It’s well established in the field of axion physics that when you have large time-varying electric fields coupled with magnetic fields you end up with the right conditions to produce axions,” said Benjamin Safdi, a particle physicist at UC Berkeley who was absent. related to the latest paper, in an email to Gizmodo. “In retrospect, it is clear and obvious that if this process occurs in pulsars a large fraction of the axions produced may be gravitationally bound by the gravitational force of the neutron star. The writers deserve a lot of credit for bringing this up.”
In 2021, Safdi co-authored a paper showing that axions can be produced in the Magnificent Seven, a group of neutron stars in our galaxy. The Magnificent Seven produces X-rays from the surface, and the group proposed that axions converted into photons could produce X-rays similar to those observed by other telescopes. But most of the axions produced in the cores of those neutron stars stay close to the source, the latest team says, and form large populations over hundreds of millions—if not billions—of years.
“These axions accumulate over astronomical timescales, thus forming a dense ‘axion cloud’ around the star,” the group wrote in the paper. “While a deeper understanding of the systematic uncertainties in these systems is needed, our current measurements suggest that existing radio telescopes can improve the sensitivity in axion-photon coupling over an order of magnitude.”
“There are many uncertainties, however, in the figures presented in this work – this is not the fault of the authors; it’s a difficult, dynamic problem,” added Safdi. “I would also like to see more comprehensive work on the prospects for detecting this signal, including better work on modeling the population of neutron stars and measuring sensitivity with existing and future instruments.”
So how can we identify and identify dark matter?
But modern space telescopes are not radio telescopes. The Webb Space Telescope, launching in 2021, is seeing 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 aim of improving our understanding of the dark matter of the universe, 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 large radio telescope that can image a lunar crater in the dark. side of the Moon.
“Axions are one of our best bets for new physics,” Safdi said, although “they are very difficult to investigate given their weak interaction with normal matter.”
“This weak interaction can be amplified in extreme astrophysical environments such as those found in neutron star magnetospheres,” he added. “Work like this could easily pave the way to discovery.”
There are plenty of radio telescopes doing great 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’re to have a chance of seeing sound waves. . No pressure, NASA bag!
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