One Starry Night ... :)

Creativity and innovation go hand in hand, especially when it involves science and the solving of a wicket problem: to whit, do primordial black holes give rise to dark matter? If so, how do you find said entities? The answer, microlensing.

Gravitational microlensing relies on chance events where from our viewpoint, one star (or a black hole - ed) passes in front of another star. The farther star is usually a bright star, and the near one is normally one we couldn't ordinarily see from Earth. When it passes in front of the farther star, however, its gravity causes the light from the farther star to bend and the star is magnified from our point of view. 

The answer, thus far, no.

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.

Wicked problem No. 2 ... Could smaller primordial black holes give rise to dark matter?

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.

To find out, innovation and persistence comes into play.

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.

The Hpyper Suprime-Cam

End result ...

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 over 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.

Innovation & persistence indeed. :)