Astronomers are reconsidering primordial black holes as an answer to the invisible matter mystery, but recent observations disfavor at least some sizes of black hole.

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?

primordial black hole illo
Artist's impression of a black hole passing in front of a star in the Andromeda Galaxy.
Kavli IPMU

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.

primordial black holes ruled out
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.


Reference: Hiroko Niikura et al. “Microlensing Constraints on Primordial Black Holes with Subaru/HSC Andromeda Observations.” Nature Astronomy. April 1, 2019.


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April 3, 2019 at 1:29 pm

Dark matter is a supersolid that fills 'empty' space, strongly interacts with ordinary matter and is displaced by ordinary matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the supersolid dark matter. The state of displacement of the supersolid dark matter is gravity.

The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.

Displaced supersolid dark matter is curved spacetime.

In the Bullet Cluster collision the dark matter has not separated from the ordinary matter. The collision is analogous to two boats that collide, the boats slow down and their bow waves continue to propagate. The water has not separated from the boats, the bow waves have. In the Bullet Cluster collision the galaxy's associated dark matter displacement waves have separated from the colliding galaxies, causing the light to lense as it passes through the waves.

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Mario Zepeda

April 6, 2019 at 7:52 pm

Is "Time" taken into the equation?
As a variable.

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April 9, 2019 at 8:10 am

The rate at which an atomic clock ticks is determined by the state of the supersolid dark matter in which it exists. The greater the gravitational pressure exerted toward and throughout an atomic clock the slower the clock ticks. The faster an atomic clock moves with respect to the state of the supersolid dark matter in which it exists the greater the displacement of the supersolid dark matter by the clock, the greater the pressure exerted toward and throughout the clock by the displaced supersolid dark matter, the slower the clock ticks.

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April 6, 2019 at 3:14 pm

I would guess that smaller black holes and associated micro lensing would be spread out across the sky fairly evenly, so looking at one patch of the sky would be a fair evaluation compared to any other patch of sky? Looking at a patch of sky for a long period, and capturing contiguous data of that field would seem a reasonable approach to capturing these lensing events? So would the archival Kepler data provide any useful insight? Would there be a significant difference in the transit shape and signal between a large Jupiter sized planet transiting and a smaller black hole passing between the star and Earth.

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Andre Duval

May 7, 2019 at 10:44 am

I may be missing something, but it sounds like they might be vastly underestimating the number of black holes if they only count candidates that flash just once. Wouldn't a black hole in orbit around another star flash periodically? Of course, this would only happen if we're observing them in the plane of their orbits, but there might be quite a lot of these in an entire galaxy. (I'm thinking of all the planets discovered in the Kepler mission as briswold mentions above.) In fact, I'd almost guess that most of the microlensing events would be caused by double-star black holes. Or would they be too close to their companion stars to bend their light properly? Could we tell?

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Monica Young

May 7, 2019 at 1:12 pm

Dear Andre - the black holes in question are lensing background stars. These chance alignments only happen once, and briefly.

For a star in orbit around a black hole, the effect of accretion of stellar matter onto the black hole would be far greater than gravitational lensing (though lensing would still be present!)

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Andre Duval

May 8, 2019 at 10:58 pm

Thanks for your explanation Monica.
Wouldn't the accretion depend on the distance of the black hole from the other star though? Obviously a hole that was close enough to actually suck in stellar matter would develop a fairly impressive accretion disk, but what if it were as far as, say, Earth or Venus is from the Sun? And if the star in question were a red dwarf it could orbit much more closely, and have a much shorter orbital period.
On the other hand, it would be absorbing some matter from the solar (excuse me, stellar) wind, I'm not sure how much of an accretion disk it would develop from that.
Also, the gravitation effect on the light from a star that was in the same planetary system as the hole would have to be different than that on light from a star parsecs away. Presumably astronomers on Earth would easily be able to tell the difference,
Or would they? Obviously, I don't have the figures here. 🙂

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Umesh Verma

August 18, 2019 at 6:12 am

Microlensing due to stars illuminations arond the BH may confuse in interpretating the effect In(actual practice)and acertaining the source of pull.Hoever Its true Pull due to BH may be catestrophic rather than DM pul.Eye on the spots after a certain period may conclude inference.

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June 12, 2019 at 2:58 pm

Duh, I've explained this in my book: GOD is Blank Space,

A black hole is heavy concentration of base matter. A recycle activity that converts matter into simpler form, which is blank space to us Humans. This simple form of matter that is blank space can be used to created any desirable element. This happens back and forth as time progress.

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