One of Stephen Hawking's most famous theories about black holes just suffered a huge shock

One of Stephen Hawking's most famous theories on dark matter – that this mysterious and invisible substance is made up of primordial black holes – has recently been shocked. This conclusion comes from a massive telescope that captured at one time the image of a whole galaxy.

The results do not completely exclude the famous notion of Stephen Hawking. But they suggest that primordial black holes should be really tiny to explain dark matter.

Mystery of dark matter

Dark matter is the name given by physicists to explain a particularly mysterious phenomenon: everything in the universe moves, revolves around it and turns as if there was more mass than we can detect. Explanations about dark matter range from ghostly particles called neutrinos to unknown particles, to the new laws of physics. In the 1970s, Stephen Hawking and his colleagues hypothesized that the Big Bang could have created a large number of relatively small black holes, each the size of a proton. It would be difficult to see these tiny and old black holes, while exerting a considerable force of attraction on the other objects – the two known properties of dark matter. [The 11 Biggest Unanswered Questions About Dark Matter]

Until now, this theory could only be tested for primordial black holes heavier than the moon. But with the improved technology, scientists were able to take more and more accurate pictures of the space. The Hyper Suprime-Cam (HSC) digital camera installed on the Subaru telescope in Hawaii is a state-of-the-art imaging technology that allows the entire Andromeda galaxy (closest to ours) to be photographed at one time. Masahiro Takada and his team at the Kavli Institute for Physics and Mathematics of the Universe in Japan used this camera to look for primordial black holes; their results were published earlier this month in the journal Nature Astronomy.

Black holes do not emit light, however, supermassive black holes, like the one in the heart of the Messier 87 galaxy, are lined with shiny hot-material disks. The primordial black holes, however, are billions of times smaller and are surrounded by no visible, glowing matter. Instead, looking for small black holes means looking for places where their powerful gravitational fields bend light – a phenomenon called microlens.

Telescopes detect microlensed black holes by taking many different pictures of stars over time. A black hole passing in front of this star will distort its light by making it blink; the smaller the black hole, the faster the flash. "If a microlens object has, say, a solar mass," Takada told Live Science, referring to the mass of the sun, "the time scale [of the microlensing ‘flash’] it's like a few months or a year. "But the primordial black holes that they were looking for had only a small fraction of that mass, roughly the mass of the moon, which means that their lightning would be much shorter." Takada said that It allowed them to take pictures of all the stars of the Andromeda galaxy at the same time, at breathtaking exposure intervals (for astronomers) – each interval only being 2 minutes.

Takada and his team took about 200 photos of the Andromeda galaxy in 7 hours a clear night. They found only one potential microlens event. According to Takada, if primordial black holes were a significant fraction of the dark matter, they should have seen about 1,000 microlens signals.

"The microlens are the gold standard for detecting black holes or eliminating them," said Simeon Bird, black hole physicist at the University of California at Riverside, who did not participate to work. "This work eliminates primordial black holes as dark matter in a mass range where the previous constraints were not as strong or as robust as this new one." It's a very nice result. "

Was it the last nail in the coffin? Is Hawking's theory really dead? This is not the case, according to Bird and Takada, who claim that the primordial black holes of a number of masses have still not been totally eliminated as candidates. [Stephen Hawking’s Most Far-Out Ideas About Black Holes]

"There are still masses where the stresses are low, around 20 to 30 solar masses," Bird told Live Science. "These could still represent between 1% and 10% dark matter … and there is always a window for the 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 not exclude these tiny black holes because the flashes of these black holes would be much too short. "So we have to think of another method to do it."

There was however a "flash" detected in their investigation. Although it is a single preliminary result, it could prove extremely important: the very first detection of a primordial black hole, which would constitute a revolutionary validation of some Hawking works.

"One observation is not convincing," said Takada. "We need more observations 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 search for more microlenses.

Originally published on Science live.