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. 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. 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.
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. 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. 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. 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.
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). 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.
Antistars would shine much as normal ones do—producing light of the same wavelengths. But they would exist in a matter-dominated universe. 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. We can see this light as a specific color of gamma rays. The team took 10 years of data, which amounted to roughly 6,000 light-emitting objects. They pared the list down to sources that shone with the right gamma frequency and that were not ascribed to previously cataloged astronomical objects. “So this left us with 14 candidates, which, in my opinion and my co-authors’ opinion, too, are not antistars,” Dupourqué says. 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.
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. Or they might simply be some kind of detector noise. 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.
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. “According to both theory and observations of extragalactic gamma rays, there should be no antistars in our galaxy.... 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. “However, it is always good to have further observational data confirming this.”
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. Antiparticles can be created from two known natural sources—cosmic rays and dark matter—but the odds that either of them are responsible appear to be vanishingly slim.
As we increase the size of an atom, it becomes harder and harder to produce as an antiparticle, says Vivian Poulin, a CNRS cosmologist based in Montpelier, France. This “means that it’s rarer and rarer that it occurs, but it’s allowed by physics.” An antiproton is relatively easy to form, yet anything heavier, such as antideuterium—an antiproton plus an antineutron—or antihelium—two antiprotons plus typically one or two antineutrons—gets progressively harder to make as it gets more massive. In a paper published in 2019, Poulin used the AMS’s potential antihelium detections to calculate a rough estimate of the prevalence of antistars, which inspired Dupourqué’s new study.
In a process called spallation, high-energy cosmic rays from exploding stars can ram into interstellar gas particles, says Pierre Salati, a particle astrophysicist at the Annecy-le-Vieux Particle Physics Laboratory, who worked on Poulin’s 2019 study. The team responsible for the AMS’s antiparticle detections claim it may have detected six antihelium-3 nuclei, which would be incredibly rare products of spallation, and two antihelium-4 nuclei, which would be almost statistically impossible to form from cosmic rays, Salati says. (The difference between the two isotopes is the addition of one antineutron.)
As for dark matter, certain models predict that dark matter particles can annihilate one another—a process that could also create antiparticles. But this process still might not be able to make antihelium-4 in high enough quantities for us to have a realistic chance of ever seeing it (if such speculative models reflect reality at all). That is why the antistar hypothesis is still on the table. Verified antihelium detections would be a good indicator for the existence of antistars, but so far the AMS is the lone experiment to offer any such evidence—which has yet to be granted peer-reviewed publication, Salati notes.
“It’s a very challenging analysis because, for every one antihelium event, there are 100 million regular helium events,” says Ilias Cholis, an astrophysicist at Oakland University, who also worked on Poulin’s study. It is possible, he and others say, that the detections turn out to be a fluke of a very complicated analysis.
Samuel Ting, a Nobel laureate physicist at the Massachusetts Institute of Technology, heads the AMS team and first publicly presented the two latest possible antihelium detections—the antihelium-4 candidates—in 2018. “We are not yet ready to publish any heavy antimatter results,” he says. “We are collecting more data before any [further] announcement is made.”
It is possible that a different experiment may give answers sooner. The General AntiParticle Spectrometer (GAPS) experiment is a balloon-borne detector that will hunt for antiparticles above Antarctica this year. Finding more antiparticles—antideuterons or even antihelium, in particular, according to Cholis—with the GAPS detector would make the AMS results far more convincing.
If antistars were found to be the culprit, that discovery would require a major reenvisioning of the universe’s evolution: no longer could we relegate antistars and other hypothetical astrophysical objects composed of antimatter to the fringes of reasonable speculation. Even if they do exist, however, antistars probably are not forming now, Salati says, because their presumptive natal clouds of antihydrogen would face steep odds of avoiding annihilation for the past 13 billion years or so. Thus, any antistars that might be found likely would be exceedingly old remnants of the early universe. If so, one deep mystery would be replaced with another: How, exactly, did such ancient relics manage to survive to today? As is often the case, a new discovery raises far more questions than it answers.