A hypothetical particle that might make up the universe’s darkish matter could also be produced by and dangle round neutron stars, among the densest objects within the universe, based on a staff of physicists.
The particles are axions, one in all a number of proposed candidates for so-called dark matter, the enigmatic stuff that makes up over 1 / 4 of the universe’s matter. A staff of researchers from the colleges of Amsterdam, Princeton, and Oxford now posit that axions may type clouds round neutron stars, that are the incredibly dense, collapsed remnants of dead stars. The discovering provides a brand new enviornment the place researchers can focus astrophysical searches for darkish matter, whereas highlighting the potential utility of a radio telescope in house.
Potential darkish matter factories
The staff means that some axions produced inside neutron stars may convert into photons and escape into house. However many of those particles would stay trapped by the star’s gravity, forming an axionic cloud across the neutron star. The group’s analysis describing the thought was not too long ago published in Bodily Evaluate X and follows up on an earlier work by the staff that explored axions that might escape the gravitational fields of the neutron stars that produce them.
“Once we see one thing, what is occurring is that electromagnetic waves (mild) bounce off an object and hit our eyes. The best way we ‘see’ axions is just a little completely different,” stated Anirudh Prabhu, a analysis scientist on the Princeton Middle for Theoretical Science and co-author of the paper, in an electronic mail to Gizmodo. “Whereas mild can ‘bounce’ off of axions, this course of is extraordinarily uncommon. The extra frequent technique to detect axions is thru the Primakoff impact, which permits axions to transform into mild (and vice versa) within the presence of a robust magnetic subject.”
Some neutron stars could be among the many most magnetic objects within the universe, and subsequently are given a particular label: magnetars. This extraordinarily magnetized surroundings is fertile breeding grounds for axions’ conversion into mild, Prabhu stated, which then may very well be detectable by space-based telescopes.
Darkish matter and axion waves within the universe
Darkish matter is the catch-all identify for the 27% of stuff within the universe that scientists can’t instantly observe as a result of it doesn’t emit mild and solely seems to work together with strange matter by gravitational interactions. Different candidates embody Weakly Interacting Large Particles (or WIMPs), dark photons, and primordial black holes, to call a number of. Axions have been initially proposed as an answer to an issue in particle physics: Mainly, among the predicted traits of the neutron aren’t noticed in nature. Therefore their identify—axions—which comes from a cleansing product model. In spite of everything, the axion was proposed as a technique to clear up among the nasty conundrums that arose across the Normal Mannequin of particle physics. Final yr, a special staff of researchers studied Einstein rings—areas of house the place mild has been bent strongly by gravity, forming a visual “ring” in house—and located evidence boosting axions as a candidate for darkish matter.
The electromagnetic waves (i.e., mild) produced by changing axions may have wavelengths a fraction of an inch as much as greater than half a mile (one kilometer) lengthy, Prabhu famous. However Earth’s ionosphere blocks very lengthy wavelengths from Earth-based telescopes, so space-based observatories could be our greatest guess for recognizing proof of axions.
Neutron stars and axions have a historical past
“It’s effectively established within the subject of axion physics that if in case you have giant, time-varying electrical fields parallel to magnetic fields you find yourself with superb situations for producing axions,” stated Benjamin Safdi, a particle physicist at UC Berkeley who was not affiliated with the latest paper, in an electronic mail to Gizmodo. “Looking back, it’s apparent and clear that if this course of occurs in pulsars a large fraction of the axions produced may very well be gravitationally sure because of the sturdy gravity of the neutron star. The authors deserve lots of credit score for pointing this out.”
In 2021, Safdi co-authored a paper positing that axions could also be produced within the Magnificent Seven, a gaggle of neutron stars in our personal galaxy. The Magnificent Seven produce high-frequency X-rays, and the staff proposed that axions changing into photons may produce X-rays like these noticed by some telescopes. However lots of the axions produced on the cores of these neutron stars keep nearer to the supply, the latest staff stated, and construct up a big inhabitants over lots of of tens of millions—if not billions—of years.
“These axions accumulate over astrophysical timescales, thereby forming a dense ‘axion cloud’ across the star,” the staff wrote within the paper. “Whereas a deeper understanding of the systematic uncertainties in these methods is required, our present estimates counsel that current radio telescopes may enhance sensitivity to the axion-photon coupling by greater than an order of magnitude.”
“There are lots of uncertainties, nevertheless, within the calculations introduced on this work — that is no fault of the authors; it’s merely a tough, dynamical downside,” Safdi added. “I’d additionally prefer to see extra thorough work on the detection prospects for this sign, together with a greater job modeling the neutron star inhabitants and estimating the sensitivity with current and upcoming devices.”
So how can we detect and establish darkish matter?
However the state-of-the-art telescopes in house are not radio telescopes. The Webb Space Telescope, launched in 2021, observes among the oldest mild we will see at infrared and near-infrared wavelengths. ESA’s Euclid Space Telescope, launched final yr with the particular objective of enhancing our understanding of the universe’s darkish matter, additionally sees the cosmos within the infrared. The truth is, one of the crucial compelling choices for a radio-based observatory is the Lunar Crater Radio Telescope (LCRT), which is strictly what it appears like: an enormous radio telescope that may make a dish out of a lunar crater on the darkish aspect of the Moon.
“Axions are one in all our greatest bets for brand spanking new physics,” Safdi stated, although they’re “notoriously tough to probe given their feeble interactions with strange matter.”
“These feeble interactions could be magnified in excessive astrophysical environments akin to these present in neutron star magnetospheres,” he added. “Work like this might thus simply open the pathway in the direction of discovery.”
There are many radio telescopes doing fantastic work on Earth—MeerKAT, the Very Giant Telescope, and ALMA, to call a number of—however it appears we might have a brand new space-based mission if we wish to have an opportunity of seeing axionic waves. No strain, NASA coffers!
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