Dark matter is a thorn in astronomers’ collective side. This stuff, detectable only by its gravitational effect, appears to make up more than 80% of the universe’s matter. But what is it?
One contender making a comeback is primordial black holes. These objects might have been born in the earliest age of the universe, back when the cosmos was nothing but a hot plasmatic soup — mostly radiation, in fact. This radiation-rich plasma wasn’t uniform; its density fluctuated from patch to patch. If a patch were excessively dense compared to its surroundings, then it would naturally collapse and create a black hole, a primordial relic from long before the first star shone. If enough of these black holes were forged, the thinking goes, they could provide the invisible mass that forms the substrate of galaxies, galaxy clusters, and the cosmic web.
“I personally find it really cool that dark matter could be (even in part) made out of light that collapsed into black holes,” says Yacine Ali-Haïmoud (New York University). “I find it to be sufficient motivation to study how large of an abundance is allowed by observations.”
Astronomers began looking in earnest for primordial black holes, or PBHs, after a 1986 paper by Bohdan Paczyński suggested a way to find them. Searches didn’t pan out, and interest waned.
But PBHs reentered the scientific mainstream a few years ago, after LIGO turned up its first black holes. At tens of solar masses, the merging black holes surprised astronomers as unexpectedly beefy for supernovae-made objects. Scientists began reconsidering whether LIGO’s sources might be PBHs instead of the cores of dead stars. Whether that possibility is still viable today depends on whom you ask, but PBHs continue to enjoy their second wind.
Astronomers look for PBHs using microlensing, the boost in starlight created when a black hole passes in front of a more distant star and its gravity bends some of the star’s light toward us. This lensing effect creates multiple images of the star too tiny to resolve individually, but combined they create a bright blip. Previous microlensing surveys have found a handful of candidate PBHs, but not a single definitive discovery, says Nathan Golovich (Lawrence Livermore National Laboratory). Such searches are whittling down the fraction of dark matter that could be these marauding black holes, but the remaining fraction depends on the swath of possible PBH masses you consider.
Several types of observations have constrained how large a fraction of dark matter could comprise primordial black holes, depending on the primordial black hole mass considered. Shaded regions show excluded regions where existence of such primordial black holes are not consistent with various observation data. The red region is the current study's result.
H. Niikura et al.
As part of this ongoing effort, Hiroko Niikura (Kavli Institute for the Physics and Mathematics of the Universe, University of Tokyo) and colleagues turned the 8.2-meter Subaru Telescope’s Hyper Suprime-Cam on our cosmic neighbor, the Andromeda Galaxy (M31). They stared at the galaxy for 7 hours, measuring the light from an estimated 100 million stars. The stars crowd together in the resulting images, each pixel containing the light from several suns. This blending is a common problem for microlensing hunters, and it means that, to find a microlensing event, the astronomers couldn’t look at an individual star’s behavior. Instead, they searched for pixels that flashed (presumably because one of the stars it contained had briefly brightened) and marked the event as a candidate.
This approach turned up 15,571 candidates. Through a series of elimination rounds, the astronomers reduced the candidates to those that only flashed once (and so probably weren’t variable stars), brightened and faded in the right way, and weren’t red herrings created by their image processing. If dark matter in the Milky Way and Andromeda galaxies is primarily made of PBHs with masses in the range of Earth’s mass down to that of Saturn’s moon Mimas, the team hypothesized, then the search should have turned up roughly a thousand events.
They found one.
Unfortunately, the astronomers can’t determine with current data whether this single candidate is the flash from a primordial black hole passing in front of a star, they write April 1st in Nature Astronomy. The case of the long-sought PBHs remains open.
“This is an impressive measurement,” Golovich says. The team has essentially crossed out a large chunk of the contribution PBHs in this mass range might make to dark matter: The remaining fraction is less than a hundredth.
Microlensing searches are not perfect, however. Among the complexities they’re subject to, the black holes’ distances determine how fast the black hole would move across the field of view and, thus, the kind of events observations can catch. Niikura’s team estimates that they could “recover” only 20% to 30% of events for the majority of stars in their images. (The number is much higher — 60% to 70% — for the subset of brighter stars.) This fraction is on par with that of other studies, which range from 10% to 50%, says coauthor Masahiro Takada (Kavli IPMU, University of Tokyo).
The team continues to explore their M31 data. Once they’ve built up enough candidates, they may rope in citizen scientists to help them investigate, Takada says.