If scientists, including the authors, are skeptical of antistars’ very existence, why are they worth discussing? The mystery lies in those pesky possible detections of antihelium made by the AMS, which remain unexplained. “However, it is always good to have further observational data confirming this.” One would only expect upper limits consistent with zero,” says Floyd Stecker, an astrophysicist at NASA’s Goddard Space Flight Center, who was not involved in the research. “According to both theory and observations of extragalactic gamma rays, there should be no antistars in our galaxy. Given some interesting but questionable gamma sources, calculating the conceivable “upper limit” to the number of antistars is a long shot from actually discovering such astrophysical objects, So most researchers are not leaning toward that conclusion. The next step would be to point telescopes at the locations of the 14 candidate sources to find out if they resemble a star or a prosaic gamma-emitting object. Or they might simply be some kind of detector noise. In place of any putative antistars, Dupourqué says, these gamma flashes could instead be coming from pulsars or the supermassive black holes at the centers of galaxies. If all of those sources were such stars, however, the group estimated that about one antistar would exist for every 400,000 ordinary ones in our stellar neck of the woods. “So this left us with 14 candidates, which, in my opinion and my co-authors’ opinion, too, are not antistars,” Dupourqué says. They pared the list down to sources that shone with the right gamma frequency and that were not ascribed to previously cataloged astronomical objects. The team took 10 years of data, which amounted to roughly 6,000 light-emitting objects. We can see this light as a specific color of gamma rays. As particles and gases made of regular matter fell into such a star’s gravitational pull and made contact with its antimatter, the resulting annihilation would produce a flash of high-energy light. But they would exist in a matter-dominated universe. Simon Dupourqué, the study’s lead author and an astrophysics graduate student at the Research Institute in Astrophysics and Planetology at the University of Toulouse III–Paul Sabatier in France and the French National Center for Scientific Research (CNRS), made the estimate after looking for antistar candidates in a decade’s worth of the LAT’s data.Īntistars would shine much as normal ones do-producing light of the same wavelengths. Inspired by the tentative AMS findings, a group of researchers recently published a study calculating the maximum number of antimatter stars that could be lurking in our universe, based on a count of currently unexplained gamma-ray sources found by the Fermi Large Area Telescope (LAT). Despite the fact that the entirely unexpected AMS results have yet to be confirmed, let alone formally published, scientists have taken them seriously, and some have scrambled to find explanations. But the easiest of all those hard methods would be to cook up the antihelium inside antistars-which, of course, do not seem to exist. Any way you slice it, known natural processes would struggle to produce enough antihelium for any of it to end up in our space-based detectors. That is why physicists were so greatly puzzled back in 2018, when the head of the Alpha Magnetic Spectrometer (AMS) experiment mounted on the exterior of the International Space Station announced that the instrument might have detected two antihelium nuclei-in addition to six that were possibly detected earlier. The slimmest overabundance of normal matter at the beginning of time would have therefore effectively wiped antimatter off the celestial map, save for its occasional production in cosmic-ray strikes, human-made particle accelerators and perhaps certain theorized interactions between particles of dark matter. But researchers do regularly produce particles of antimatter in their experiments, and they have the inklings of an explanation for its cosmic absence: Whenever antimatter and normal matter meet, they mutually annihilate in a burst of energy. Antimatter may seem like the stuff of science fiction-especially because scarcely any of it can be seen in our universe, despite physicists’ best theories suggesting antimatter should have arisen in equal proportion to normal matter during the big bang.
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