Even though physicists’ best models predict antimatter should have arisen in equal proportion to the regular matter at the big bang, antimatter appears to be the stuff of science fiction. Researchers do, however, consistently make antimatter particles in their tests, and they have a theory for why it isn’t seen in nature: when antimatter and normal matter collide, they mutually annihilate in an explosion of energy. Antimatter would have been entirely wiped off the celestial map at the beginning of time, but for its periodic formation in cosmic-ray impacts, human-made particle accelerators, and maybe certain postulated interactions between particles of dark matter.
That’s why physicists were perplexed in 2018 when the leader of the Alpha Magnetic Spectrometer (AMS) experiment, which is mounted on the outside of the International Space Station, announced that the instrument had detected two antihelium nuclei in addition to the six that had previously been detected. Known natural processes would struggle to make enough antihelium for any of it to end up in our space-based detectors, regardless of how you slice it. But, of all those difficult processes, cooking up antihelium within antistars—which, of course, do not appear to exist—would be the simplest. Even though the AMS results have yet to be validated, let alone publicly published, experts have taken them seriously, with some scrambling to come up with answers.
Based on a count of presently unexplained gamma-ray sources discovered by the Fermi Large Area Telescope and inspired by the preliminary AMS discoveries, a group of academics recently released a paper calculating the maximum number of antimatter stars that might be hidden in our universe (LAT). The estimate was made after looking for antistar candidates in a decade’s worth of data from the LAT by 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).
Antistars would glow in the same way as regular stars do, emitting light of the same wavelengths. They would, nevertheless, exist in a cosmos dominated by matter. As particles and gases consisting of conventional matter were drawn into the gravitational pull of such a star and collided with its antimatter, a burst of high-energy light would follow from the annihilation. This light appears as a distinct hue of gamma rays. The researchers analyzed data from the previous ten years, which totaled over 6,000 light-emitting items. They whittled the list down to sources that emitted the correct gamma frequency and could not be attributed to any already cataloged celestial objects. “This left us with 14 choices, none of whom, in my judgment and that of my co-authors, are antistars,” Dupourqué adds. However, if all of those sources were antistars, the study calculated that there would be around one antistar for every 400,000 conventional stars in our stellar neighborhood.
Instead of hypothetical antistars, these gamma flashes might be emanating from pulsars or supermassive black holes at galaxies’ cores, according to Dupourqué. Alternatively, they might simply be detector noise. The next stage would be to direct telescopes at the 14 potential sources to see if they seem like a star or a common gamma-emitting item.
Calculating the hypothetical “upper limit” to the number of antistars is a long way from actually detecting such astronomical objects, given some fascinating but dubious gamma sources, therefore most researchers aren’t leaning toward that conclusion. “There should be no antistars in our galaxy, according to both theory and measurements of extragalactic gamma rays… According to Floyd Stecker, an astronomer at NASA’s Goddard Space Flight Center who was not involved in the study, “one would only anticipate top limits commensurate with zero.” “However, more observational data showing this is always beneficial.”
Why are antistars worth debating if scientists, including the authors, are doubtful of their existence? The puzzle is on those bothersome probable antihelium detections made by the AMS, which remain unsolved. Antiparticles can be produced by two known natural sources: cosmic rays and dark matter, although the chances that either is to blame appear to be vanishingly small.
According to Vivian Poulin, a CNRS cosmologist located in Montpelier, France, as the size of an atom grows larger, producing an antiparticle gets more difficult. This “means that it is becoming increasingly unusual that it occurs, yet it is permitted by physics.” Antiprotons are reasonably easy to create, but bigger elements like antideuterium (an antiproton plus one antineutron) and antihelium (two antiprotons plus generally one or two antineutrons) become more difficult to generate as they become larger. Poulin utilized the AMS’s probable antihelium detections to derive an approximate estimate of the prevalence of antistars in a report released in 2019, which prompted Dupourqué’s latest investigation.
According to Pierre Salati, a particle astrophysicist at the Annecy-le-Vieux Particle Physics Laboratory, high-energy cosmic rays from exploding stars can ram into interstellar gas particles in a process known as spallation, who contributed to Poulin’s 2019 research. According to Salati, the team in charge of the AMS antiparticle detections believes it may have found six antihelium-3 nuclei, which would be extremely rare spallation products, and two antihelium-4 nuclei, which would be practically statistically impossible to produce from cosmic rays. (The addition of one antineutron is the difference between the two isotopes.)
Certain theories suggest that dark matter particles can destroy one another, perhaps creating antiparticles in the process.
However, this mechanism may not be capable of producing antihelium-4 in sufficient numbers for humans to have a reasonable possibility of perceiving it (if such speculative models reflect reality at all). As a result, the antistar idea remains viable. Antihelium detections that have been verified would be a good signal of the presence of antistars, but Salati points out that the AMS is the only experiment that has provided such evidence—which has yet to be published in a peer-reviewed journal.
“It’s a difficult analysis because there are 100 million conventional helium events for every one antihelium event,” says Ilias Cholis, an astronomer from Oakland University who also collaborated on Poulin’s paper. It’s possible, he and others argue, that the detections are a result of a difficult study gone wrong.
The AMS team is led by Nobel Laureate physicist Samuel Ting of the Massachusetts Institute of Technology, who first publicly announced the two most recent probable antihelium detections—the antihelium-4 candidates—in 2018. He states, “We are not yet ready to disclose any heavy antimatter data.” “Before making any [additional] statement, we are gathering additional data.”
It’s feasible that a different experiment would provide results sooner. This year, the General AntiParticle Spectrometer (GAPS) experiment will use a balloon to search for antiparticles above Antarctica. Finding additional antiparticles with the GAPS detector—in particular, antiideuterons or antihelium, according to Cholis—would make the AMS results considerably more credible.
If antistars were shown to be the culprit, it would need a radical rethinking of the universe’s evolution: we couldn’t put antistars and other hypothetical antimatter-based astrophysical phenomena on the periphery of acceptable conjecture anymore. Even if antistars exist, Salati believes they aren’t developing right now since their putative natal clouds of antihydrogen would have a difficult time surviving for the previous 13 billion years or more. As a result, any antistars discovered would most certainly be extremely ancient leftovers of the early cosmos. If it were the case, one deep enigma would be replaced by another: How did such old artifacts make it to the present day? A new finding, as is often the case, raises many more questions than it answers.