Much of the Universe behaves as if there's more matter there than we can see. Dark matter explains this by positing that there's matter present that we can't see, and evidence has piled up in favor of this idea. By contrast, evidence for the identity of dark matter has behaved in an opposite manner: everything we've done to look for it has come up empty.

The latest bit of emptiness was published this week, and it seemingly puts an end to one of the possible remaining explanations for dark matter: black holes that formed shortly after the Big Bang and have been structuring the Universe ever since. While earlier studies have seemingly ruled out larger versions of these primordial black holes, the new study closes the window on anything more massive than a large asteroid. And it was all accomplished with just a single night of telescope time.

From the dawn of time

Black holes would seemingly make fantastic candidates for dark matter, given that they're black and thus difficult to detect. But there's a number of reasons they don't work especially well. For one, while light may not escape a black hole, it's often produced in prodigious amounts by the material just outside a black hole. So it's not clear whether large numbers of black holes could somehow go undetected.

Then there's the timing. The only black holes we know exist were formed by the death of stars. Yet, the Cosmic Microwave Background and the structure of the Universe itself both indicate that dark matter was there right from the start.

The latter issue was handled by what are termed "primordial black holes." These were hypothesized to form early in the Universe's history, when the density of matter was much higher, and persist to this day. Modeling their formation suggested that, under the right conditions, enough could form to account for dark matter.

Evidence, however, has not been kind to the idea. Some of that evidence has come from LIGO's detection of the gravitational waves produced by black hole collisions. If the primordial black holes are heavy enough, the rate of detections would be much higher than it is. Other limits have been placed by studies of what's called "microlensing." We've identified a number of cases where massive objects twist the fabric of space in a way that forms a lens, magnifying galaxies in the distance beyond the lens-forming object. Smaller objects can do this on a smaller scale, causing a temporary brightening of individual stars.

If primordial black holes were common, they'd create plenty of these smaller microlensing events, and we should be able to spot them. But dedicated searches for them came up with very few of the events, as did the Kepler planet-hunting telescope, which stared at a large field of stars on and off for several years. Combined, these suggested that any primordial black holes would have to be extremely odd, weighing less than our Sun.

But these small, primordial black holes couldn't be ruled out on theoretical grounds. This led a group of Japanese researchers to try to rule them out on observational grounds.

What’s Suprime?

Searching for microlensing events is relatively simple: simply stare at a bunch of stars and wait for one of them to brighten and fade as a dense object moves across your line of sight. The challenge comes in making sure you look at enough stars to make detection likely. A number of past surveys have used one of the dwarf galaxies that orbit the Milky Way to provide the stars. The duration of the microlensing event will then depend on the mass of the object doing the lensing (smaller means a shorter event), as well as the speed at which it crosses our line of sight, which is in turn related to the distance between us and the object.

Enabled by some new telescope hardware, the Japanese researchers decided to go bigger. The hardware is something called the Hyper Suprime-Cam, an 870 megapixel monster attached to an 8 meter telescope. Configured properly, it could capture the entire Andromeda galaxy in a single frame, and it can do so about every 90 seconds. That's fast enough that even a light black hole can be captured multiple times during the microlensing. To make sure they could capture as many events as possible, the researchers were given an entire night with the telescope all to themselves, with seven hours of total observation time.

That, they calculate, would let them cover much smaller masses than we've looked at before. At the low end, this would get down to black holes of about 10-14 solar masses—roughly in the neighborhood of the mass of the asteroid Eros. While black holes of this mass range can't form during the deaths of stars, it is possible that they could have formed early in the Universe.

So many pixels

Although the Hyper Suprime-Cam has a lot of pixels, Andromeda has even more stars. To detect individual lensing events, the team used software that compared consecutive images and highlighted any pixels that showed changes between them. After a night of observations, there were more than 15,500 events that had to be sorted through. But these included things like variable stars, stellar flares, and eclipses in binary star systems. Focusing on short-term changes cut the number to about 12,000, while searching for symmetric "bumps" in which the light intensity goes up and drops down again left the researchers with a total of 66 possible microlensing events.

At this point, they simply examined each event manually. Most of them were simply imaging artifacts caused by bright stars shifting near pixel boundaries; at least one was an asteroid. By the time all of them had been examined, there was only one candidate that could possibly be a microlensing event. It wasn't a great match, but there were no other obvious explanations for it.

These observations should have caught microlensing caused by primordial black holes in either the Milky Way or Andromeda. As such, if there were a significant population of these black holes, the telescope should have picked up more of them. So, the conclusion is that, if primordial black holes did form early in the Universe's history, they weren't formed in sufficient numbers to account for dark matter. This may not be the end of the search for primordial black holes—the authors suggest they can put even more stringent limits on the probability with another 10 days of observation time. But the study does slim down the already thin chances that they are the source of dark matter's effects.

Nature Astronomy, 2019. DOI: 10.1038/s41550-019-0723-1 (About DOIs).