What if dark matter was not an exotic particle that stubbornly eludes discovery, but rather a swarms of tiny “primordial” black holes born in the first second of the universe?
Once considered fringe science, this bizarre idea is seeing a real comeback as research into dark matter continues. arrive empty. Yet concrete evidence for the existence of primordial black holes remains rarepotentially making it another cosmic case of wishful thinking – unless scientists have finally spotted one.
In two papers published on the preprint server arXiv.org on May 19, researchers led by Renee Key of Swinburne University of Technology in Australia say they have done just that. Their potential primordial black hole (PBH) would be an object three times more massive than Earth’s Moon, briefly seen as it drifted through the halo of the Milky Way– the sparsely starry periphery of our galaxy, believed to host most of its dark matter.
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The result is controversial and Key acknowledges that there are “weaknesses in our data”. But the possibility of a historic discovery that would radically change our understanding of the history of the universe and solve one of the greatest mysteries of modern astrophysics is too enticing to ignore. And while this statement fades upon closer examination, it nonetheless highlights the extent to which scientists must get off the beaten track as the hunt for dark matter continues to flounder.
First proposed in the 1960s, PBHs were explored in detail by physicist Bernard Carr and the late physicist Stephen Hawking in the 1970s. Carr and Hawking suggested that, in the first quadrillion seconds after the big bang, particularly matter-dense regions of the expanding universe could have collapsed under their own gravity, leading to the formation of countless black holes with a wide range of masses, from particles lighter than subatomic particles to much heavier than stars.
Such black holes would be extremely difficult to spot and could therefore explain some or all of the dark matter in the universe – invisible, lightless matter. something it seems to act like gravitational glue, connect galaxies and galaxy clusters. But in the decades since PBHs were first proposed, astronomers have found clever ways to narrow their plausible range of masses, thereby ruling out many scenarios in which these black holes could explain dark matter. “There is a very rich wealth of constraints on PBHs,” says Djuna Croon, a theoretical particle physicist at the University of Durham in England, who was not involved in Key and his colleagues’ studies. Finding one, she adds, would be an “extraordinary” discovery.
Today, experts say dark matter particles could lie in an extremely wide mass range, from a trillionth the mass of an electron to about 1,000 times the mass of a proton. Looking for them is like looking for a needle in a cosmic haystack: if the “haystack” is made up of different sized needles and you don’t know how big your target needle should be. The much tighter constraints for PBHs suggest that a typical PBH should have roughly the mass of an asteroid if it constitutes most or all of the dark matter – a very small “needle”, to be sure, but still a more achievable task.
The presumed heavier-than-Key’s moon black hole, named “Phoebe,” would be an outlier. The team found it during five nights of 2019 observations with the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile. Every minute during their stay at the telescope, Key and his colleagues took images of some 10 million stars in the Large Magellanic Cloud, a dwarf galaxy about 163,000 light years away, looking for those that momentarily brightened with a rogue black hole briefly passing them and amplifying their light with its space-warping gravitational field.
This rare and ephemeral event, known as a microlensing event, is one of the main strategies for searching for PBHs in and around the Milky Way. And that’s what Key and his team think they saw when, for an hour, a star estimated to be twice the size of our sun suddenly became much brighter before returning to its baseline just as quickly.
The effect could likely come from stellar variability – a burp of the star rather than a light-magnifying PBH. Or maybe the brightening was due to a free-floating planet (FFP) somewhere in our galaxy, a world ejected from an alien planetary system this could create its own microlensing event mimicking the PBH. (Phoebe gets her name from the acronyms FFP and PBH.)
However, after exhaustive modeling of these scenarios, the best fit for what she saw was a black hole three times the mass of our moon, about 60,000 light years from Earth, moving through the Milky Way’s halo at about 300 kilometers per second. If correct, the black hole itself would be tiny despite its mass, spanning “less than the diameter of a human hair,” Key says.
Because microlensing depends on point geometric alignments, the distant object that caused the event by passing so perfectly into our field of vision can never be seen again. Of the few methods available to test Key’s claim, the most promising involves monitoring the distant star for any signs of stellar variability. If the star lights up the same way again, “then you would be really very suspicious that it has nothing to do with microlensing,” says Ken Freeman, an astronomer at the Australian National University and co-author of the papers.
If PBHs exist, they could explain much more than dark matter. Born at the dawn of time, they could also represent the obscure origins of supermassive black holesthe million to billion solar mass giants observed at the centers of most large galaxies. Observations with the James Webb Space Telescope have discovered such large black holes in galaxies from earlier and earlier in the universe, including the recent discovery of a Black hole of 50 million solar masses seen only 700 million years after the big bang. Until now, scientists have struggled to explain how these titans grew so quickly, but PBHs could be an answer. By growing from large PBHs, “these supermassive black holes may have had a head start,” says David Kaiser, a physicist at the Massachusetts Institute of Technology.
Unsurprisingly, not everyone is convinced that Phoebe is a real PBH. Przemek Mróz, an astronomer at the University of Warsaw, says that if it really is a lunar-mass black hole, we should have seen similar objects in other searches, such as a microlensing study of the Large Magellanic Cloud and Small Magellanic Cloud called the Optical Gravitational Lensing Experiment (OGLE), of which he is a team member. “We should see hundreds of such microlensing events in our data,” he says, making other explanations more likely. “This corresponds to a simple ordinary variable star.”
It’s possible, Key says, that his team was “completely lucky” to attend this event; it could be that most PBHs are smaller, asteroid mass, and some are larger like Phoebe and they spotted one by chance. Recent observations from the Subaru telescope in Hawaii support this idea to some extent. In a preliminary paper published in February, a team led by Sunao Sugiyama of the Kavli Institute for the Physics and Mathematics of the Universe in Japan observed the Andromeda Galaxy and reported 12 microlens events comparable to Phoebe, some of which may have been caused by PBHs in the Milky Way halo. “Our candidates are also on the lunar mass scale,” says Sugiyama. Mroz counters, however, that none of these cases constitute true microlensing events and are instead simple fluctuations in ordinary variable stars.
Obviously, doing this research is difficult. Images must be taken at a high rate, at least every few minutes or so, to spot telltale changes in a star’s brightness as it is microlensed by a relatively small PBH. Sifting through all these images poses additional challenges: Key’s five nights of observations, for example, produced a terabyte of data. New projects designed to cope with such data deluges and equipped with panoramic optics, such as the Vera C. Rubin Observatory in Chile and launch of NASA’s Nancy Grace Roman Space Telescope later this year– might be good for research.
PBHs can also disclose their presence by means other than microlensing. Last year, Kaiser and his Ph.D. student Alexandra Klipfel suggested that a powerful neutrino spotted in a partially complete detector called KM3NeT off the coast of Sicily may have been caused by an explosive PBH. A process called Hawking radiation causes black holes to shed particles and effectively evaporate over time. And the lower the mass of a black hole, the faster it evaporates, resulting in an exponentially accelerated release of high-energy radiation. This means that black holes are dying out in a big way, with lower mass PBHs exploding at different times in the universe. The smallest possible PBHs should have expired this way a long time ago, and today it would be the lowest asteroid mass PBHs that would explode; Kaiser and Klipfel suggested that one of these could have been the origin of the KM3NeT neutrino. This idea remains very controversial. “I doubt it makes sense,” says Ignacio Taboada, a neutrino astrophysicist at the Georgia Institute of Technology. “If this neutrino really came from a primordial black hole, we should have seen it somehow in gamma rays.”
Kaiser is also working with a team of astronomers in France to research any changes in the position of Mars this could be caused by the occasional passing of a PBH thanks to our solar system. It’s a long road, Kaiser admits, but it’s still intriguing to explore. “I’m still attracted to the idea,” he says.
Meanwhile, a merger of two objects spotted via gravitational waves by the LIGO collaboration -Virgo-KAGRA last year intrigued scientists– because the two objects could be less than a solar mass. If these objects were black holes, the only known way they could appear would be through primordial production. “There’s nothing a black hole of this mass could be other than primordial,” Freeman says.
For astronomers like Key, scanning the sky for brief increases in starlight remains the best hope for finding PBHs. She’s already looking at more data from the Dark Energy Camera, this time targeting 100 million stars, to look for more microlensing events. Maybe, just maybe, we’ll soon see other candidate primordial black holes like Phoebe pass by as they move around the perimeter of the Milky Way.
