Researchers say they have seen a brief microlensing event—nicknamed “Phoebe”—that could match a primordial black hole about three times the mass of the Moon. The claim, presented in two arXiv papers posted May 19 by a team led by Renee Key, comes with clear we
For an hour in 2019, a distant star brightened dramatically and then faded almost as quickly as it had appeared. Renee Key’s team interpreted that fleeting change as the telltale signature of a primordial black hole—an object so hard to detect that. if it’s real. it could reshape how scientists think about dark matter.
Key and colleagues. in two papers posted to the preprint server arXiv.org on May 19. describe what they believe they saw: a potential primordial black hole nicknamed “Phoebe. ” glimpsed as it drifted through the Milky Way’s halo—an extended region of the galaxy thought to host much of its dark matter. The putative black hole would be about three times the mass of Earth’s moon. moving through the halo at about 300 kilometers a second and located roughly 60. 000 light-years from Earth.
The story begins with the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile. Across five nights of observations in 2019. Key and her team captured images of about 10 million stars in the Large Magellanic Cloud. a dwarf galaxy about 163. 000 light-years away. Every minute. they scanned for stars that momentarily brightened as a compact object passed in front and magnified their light—a gravitational effect called microlensing.
The candidate event lasted about an hour. Key’s team estimates that the star involved was about twice the size of the Sun. and that it brightened before returning to baseline just as quickly. Crucially. microlensing is a one-off geometric alignment: once the object has moved away from the line of sight. the same lens cannot be seen again. That makes confirmation difficult, and the team knows it.
Key acknowledges “weaknesses with our data. ” even as she frames the possibility as potentially epochal—one that could solve a central mystery in modern astrophysics and rewrite parts of the universe’s early history. The core idea dates back to the 1960s. when primordial black holes were first proposed. and was later explored in detail by physicist Bernard Carr and the late physicist Stephen Hawking in the 1970s. Their scenario suggested that in the first quadrillionths of a second after the Big Bang. extremely dense regions of the expanding universe could have collapsed under their own gravity. forming vast numbers of black holes across a wide range of masses.
If some of those primordial black holes survive today. they could be a form of dark matter—an invisible. lightless substance that acts like gravitational glue. binding galaxies and galaxy clusters. But the problem has always been evidence. Over decades, astronomers developed constraints that narrow the plausible mass range where primordial black holes could account for dark matter. Djuna Croon. a theoretical particle physicist at Durham University in England who was not involved with Key’s studies. points to the scale of those limits. saying. “There’s a really big wealth of constraints on PBHs. ” and that finding one would be an “extraordinary” discovery.
In the years since, scientists have also expanded the debate beyond black holes. The prevailing view for dark matter particles spans a huge mass range—anywhere between trillionths of an electron’s mass and about 1. 000 times the mass of a proton. Hunting them is described as looking for a needle in a cosmic haystack when the needle size is unknown. Primordial black holes. by contrast. face tighter constraints; if they made up most or all of the dark matter. a typical black hole would have roughly the mass of an asteroid. meaning the search might be more focused on a smaller “needle.” “Phoebe. ” however. would be heavier-than-the-moon and therefore an outlier.
That outlier nature is precisely where doubt enters. Key’s team considers alternative explanations for the hour-long brightening. The flare could be caused by stellar variability—“a burp from the star” rather than light being magnified by a passing black hole. Another possibility is that a free-floating planet (FFP) somewhere in the galaxy created a microlensing-like effect of its own. The object’s nickname reflects that connection: “Phoebe” comes from the acronyms FFP and PBH.
After exhaustive modeling of these scenarios. the researchers say the best fit remains a primordial black hole about three times the mass of the Moon. roughly 60. 000 light-years away. Key adds that the black hole itself would still be tiny despite its mass—spanning “less than the diameter of a human hair.”.
Because the lens can’t be observed again, the most promising test is indirect. The researchers plan to monitor the distant star for signs of repeat behavior. If the star brightens again in a similar way. Ken Freeman. an astronomer at the Australian National University and a co-author on the papers. says. “then you would be very suspicious indeed that this has nothing to do with microlensing.”.
This claim lands at a moment when dark matter searches have been coming up empty, pushing scientists toward unconventional ideas. Even so, “Phoebe” is not the end of the story—it’s the start of a fight over interpretation.
The same basic mechanics that would make primordial black holes a dark matter candidate also raise a different question: how they could connect to the universe’s earliest and fastest-growing monsters. If primordial black holes formed at the dawn of time. they might help explain the murky origins of supermassive black holes—the million- to billion-solar-mass bodies found at the centers of most large galaxies. Observations by the James Webb Space Telescope have found these large black holes earlier and earlier in cosmic time. including a recent discovery of a 50-million-solar-mass black hole seen just 700 million years after the Big Bang. Scientists have struggled to explain how such titans grew so quickly, and PBHs are one proposed mechanism. “maybe these supermassive black holes had a head start,” says David Kaiser, a physicist at the Massachusetts Institute of Technology.
Still, skepticism is hard to shake. Przemek Mróz. an astronomer at the University of Warsaw. argues that if there really were a lunar-mass black hole. similar objects should have shown up in other microlensing surveys. Mróz points to the Optical Gravitational Lensing Experiment (OGLE). which includes a microlensing survey of the Large Magellanic Cloud and the Small Magellanic Cloud and where he is a team member. “We should see hundreds of such microlensing events in our data. ” he says. adding that this expectation makes other explanations more likely—“This is consistent with just a mundane variable star.”.
Key offers a different possibility: she says it could be that the team was simply “entirely lucky” in seeing this particular event. If most primordial black holes are smaller—around the mass of an asteroid—then larger objects like Phoebe would be rarer. and the first detection might be a matter of timing and chance.
Recent observations from the Subaru Telescope in Hawaii provide some support for the idea that lunar-mass-scale candidates might exist. In a preprint paper posted in February. a team led by Sunao Sugiyama of the Kavli Institute for the Physics and Mathematics of the Universe in Japan reported 12 microlensing events in observations of the Andromeda galaxy. They described some of those events as potentially caused by primordial black holes in the Milky Way’s halo. “Our candidates are also in the lunar-mass scale,” Sugiyama says. Mróz counters that none of those cases are actual microlensing events. arguing instead that they reflect the fluctuations of ordinary variable stars.
The difficulty of these studies is more than philosophical. Microlensing searches require images taken at a high cadence—at least every few minutes—to catch the brightness tweak from a relatively small primordial black hole. The data deluge is staggering. Key’s five nights of observations produced a terabyte of data. New projects designed to handle such volumes, and equipped with panoramic optics, may be better positioned. The Vera C. Rubin Observatory in Chile and NASA’s Nancy Grace Roman Space Telescope—scheduled to launch later this year—are cited as future platforms suited to such work.
Even if microlensing is the best-established route, it isn’t the only one on the table. Kaiser and his Ph.D. student Alexandra Klipfel previously suggested that a powerful neutrino detected in a partially complete detector called KM3NeT off the coast of Sicily might have been caused by an exploding primordial black hole. The mechanism they invoked is Hawking radiation: black holes shed particles and effectively evaporate over time. with lower-mass black holes evaporating faster. In their picture. lower-asteroid-mass primordial black holes would be the ones going off now. releasing high-energy radiation at different epochs of the universe and producing “bang”-like endings. Their idea remains contested. “I am doubtful that makes sense,” says Ignacio Taboada, a neutrino astrophysicist at the Georgia Institute of Technology. “If this neutrino had really been from a primordial black hole, we should have seen it in gamma rays somehow.”.
Kaiser is also working with a team of astronomers in France to look for changes in the position of Mars that might be caused by an occasional passage of a primordial black hole through the solar system. It is a long shot, Kaiser admits, but one he still wants to explore. “I’m still enamored to the idea,” he says.
Then there is the gravitational-wave trail. A merger of two objects spotted via gravitational waves by the LIGO-Virgo-KAGRA collaboration last year has drawn attention because both objects might be less than a solar mass. If they were black holes. Freeman says. “the only known way they could arise would be through primordial production. ” adding. “There’s nothing much else a black hole of that mass can be other than primordial.”.
For Key, though, the immediate path forward remains the same: keep scanning for brief brightenings. She is already sifting through additional data from the Dark Energy Camera. targeting 100 million stars to look for more microlensing events. The hope is straightforward and fragile—maybe the next time the universe lines up the geometry just right. the passage of another candidate primordial black hole like Phoebe will be caught in the act. and the debate will move from “could be” to something closer to certainty.
primordial black holes dark matter microlensing Dark Energy Camera Cerro Tololo Inter-American Observatory Milky Way halo arXiv Renee Key Phoebe Hawking radiation KM3NeT LIGO-Virgo-KAGRA