One of Stephen Hawking’s most famous hypotheses on dark matter — that this mysterious and invisible element is made up of primordial black holes — was recently destroyed. A massive telescope took an image of a whole galaxy in one shot, leading to this conclusion.
The findings do not completely invalidate Stephen Hawking’s famous hypothesis. But they suggest that primordial black holes would have to be truly tiny to explain dark matter.
The Hyper-Suprime-Cam captured this image of the Andromeda Galaxy. A recent snapshot of Andromeda discovered only one signal that could have originated from a medium-sized primordial black hole or one produced shortly after the Big Bang. (Image credit: Copyright HSC-SSP and NAOJ)
Dark matter mystery
Dark matter is the name given by physicists to an especially mysterious phenomenon: Everything in the universe moves, orbits, and rotates as though it had greater mass than we can see. Explanations for dark matter range from ghostly particles known as neutrinos to unknown particles and new physical laws. Stephen Hawking and his colleagues proposed in the 1970s that the Big Bang created a large number of relatively small black holes, each approximately the size of a proton. These tiny, ancient black holes would be difficult to see, yet they would exert a large gravitational pull on other objects — the two known properties of dark matter.
Although black holes do not emit light, supermassive black holes, such as the one at the center of the galaxy Messier 87, are surrounded by bright disks of hot matter. Primordial black holes, however, are billions of times smaller and have no visible, glowing matter surrounding them. Instead, searching for small black holes means looking for places where their powerful gravitational fields bend light — a phenomenon called microlensing.
Microlensing black holes are discovered by taking many different photographs of a star over time. A black hole passing in front of that star will distort its light, making it flash; the smaller the black hole, the quicker the flash. Takada told Live Science that “if a microlensing object has, say, one solar mass,” referring to the mass of the sun, “the timescale [of the microlensing ‘flash’] is like a few months or a year.” However, the primordial black holes they were looking for had a fraction of that mass, roughly the mass of the moon. As a consequence, their flashes would be much shorter.
Takada described the HSC as “unique” in that it allowed them to picture all of the stars in the Andromeda galaxy at once, at breathtakingly fast (to astronomers) exposure intervals – each interval being only 2 minutes long.
Takada and his colleagues shot the Andromeda galaxy for about 7 hours on a clear night. They discovered just one possible microlensing event. Takada believes that if primordial black holes were a significant fraction of dark matter, they should have noticed 1,000 microlensing signals.
“Microlensing is the gold standard for detecting black holes or ruling them out,” said Simeon Bird, a black- hole physicist at the University of California — Riverside, who was not involved in the work. “This work rules out primordial black holes as dark matter in a range of masses where the previous constraints were not as strong nor as robust as this new one. It’s a very nice result.”
Was this the final nail in the coffin? Is Hawking’s theory really dead? According to Bird and Takada, primordial black holes of a specific mass range have not been completely dismissed as candidates.
“There are still some masses where the constraints are weak, around 20-30 solar masses,” Bird told Live Science. “Those could still be 1% to 10% of dark matter … and there’s still a window at lower masses, like the mass of a very small asteroid.”
“Our physicists are very excited because there is still a window,” Takada said. The data can’t rule out those teensy tiny black holes because the flashes from those black holes would be much too short, “so we need to think of another method to do it.”
However, one “flash” was discovered in their survey. While it was a single, preliminary result, it could end up being incredibly important: the very first detection of a primordial black hole, which would be a groundbreaking validation of some of Hawking’s work.
“Only one observation is not that convincing,” Takada said. “More observations are required to confirm. If it was really [a primordial black hole], we should continue to find the same thing” as they continue to use the HSC to look for more microlensing.