Despite evidence of antihelium, researchers say antistars would be exceedingly rare

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Antihelium? Antistars? It all sounds like science fiction, but one of those is real. (NASA)

Cautioning that the existence of antistars would shatter known models of the universe, researchers put the prospect of finding such an object at one in every 400,000 regular stars within the Milky Way galaxy — 20 times lower than what previous studies suggest and the strictest reported limit to date.

Their study, published April 20 in Physical Review D, also identifies 14 potential candidates for the still-hypothetical stellar objects, but with a caveat: It's probably the case that none of them will pass the test.

"If they exist, it would change so [many] things," study author Simon Dupourqué told The Academic Times. "We must be very careful and do it the right way."

In 2017, NASA's space-borne Alpha Magnetic Spectrometer opened the door to the possibility of antistars with its detection of antihelium particles. But, Dupourqué and his team are urging that the features of antistars be properly laid out to prevent premature conclusions.

"We were able to constrain the existence of the subject; we shrink it as much as we could — we did it in a way to disprove the existence, actually," said Dupourqué, a Ph.D. student at the Research Institute in Astrophysics and Planetology in France.

Taking a process-of-elimination approach, Dupourqué believes that the day one fails at invalidating an antistar candidate will be the day a revolutionary discovery is made.

"The existence of antistars would give credit to alternative scenarios for the formation of the universe — to be short," he remarked.

Famously studied at the European Council for Nuclear Research, antimatter, which forms the composition of potential antistars, is exactly the same as normal matter except its particles have the opposite charge from what one would expect. Electrons are positive, called positrons, and protons are negative. 

"Nowadays, we think that all the antimatter is gone; that after the Big Bang something happened that we can't explain, that has led us to a universe made exclusively of matter and no antimatter," Dupourqué said.

But extremely light particles of antimatter, namely positrons or antiprotons, are sometimes discovered in phenomena called cosmic rays, particularly with NASA's groundbreaking ray detector.

Catapulting at almost exactly the speed of light, cosmic rays are among the most energetic things ever found in nature — and no one knows what they are or where they come from. These rays, which are really particles, periodically rain down on Earth and are sometimes blamed for electronic device malfunctions. 

"We observe antimatter in cosmic rays, but in the form of positrons and very light particles, such as antineutrons and antiprotons," Dupourqué said.

While very peculiar, those antiparticles are believed to result from the ultrahigh energy of the rays that can rip apart elements to make antimatter-like materials. But when NASA's spectrometer picked up signals of antihelium, a much heavier form of antimatter made up of two antiprotons and one positron, the plot thickened.

"You cannot expect antihelium from already known processes," Dupourqué said. "It can sound very strange, but it should be easier to create antihelium using antistars from what we know today."

He continued, "An antistar would fuse antiprotons into antihelium and then could eject it in the interstellar medium through its stellar wind."

Parallel to antimatter, an antistar would have the same properties as a normal star — at least, in the way humans would view it. Beneath superficial similarities, however, the team recognized that the variation between a normal star and an antistar would lie in chemical composition, indicated by spectroscopic information. So, the researchers sifted through a decade's worth of data from the Fermi Large Area Telescope that orbits Earth. 

That telescope collects spectroscopic information about gamma rays, which are thought to originate from star explosions or black holes. From this, the team drew its conservative constraints on where these antistars may appear and collected the 14 candidates discussed in the paper.

But in line with the healthy dose of skepticism these researchers are taking, Dupourqué says that the spectral shapes highlighted as markers can be attributed to other occurrences in the universe, too — particularly, normal stars.

The study, "Constraints on the antistar fraction in the solar system neighborhood from the 10-years Fermi Large Area Telescope gamma-ray source catalog," published April 20 in Physical Review D, was authored by Simon Dupourqué, Luigi Tibaldo, and Peter von Ballmoos, Research Institute in Astrophysics and Planetology.

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