{"id":17237,"date":"2020-01-10T06:31:54","date_gmt":"2020-01-10T06:31:54","guid":{"rendered":"https:\/\/www.revoscience.com\/en\/?p=17237"},"modified":"2020-06-09T12:12:38","modified_gmt":"2020-06-09T12:12:38","slug":"on-the-hunt-for-primordial-black-holes","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/on-the-hunt-for-primordial-black-holes\/","title":{"rendered":"On the hunt for primordial black holes"},"content":{"rendered":"\n

By AsiaResearch News <\/p>\n\n\n\n

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\u00a0The theory that dark matter could be made of primordial black holes a fraction of a millimetre in size has been ruled out by a team of researchers led by the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU).<\/p>\n\n\n\n

In 1974, physicist Stephen Hawking described how primordial black holes could have formed in the fraction of a second after the Big Bang. Primordial black holes could have masses ranging from a tiny speck to 100,000 times our sun. In contrast, supermassive black holes detected by astronomical observations started forming at least hundreds of thousands of years later, and are millions or billions times larger than our sun. Since primordial black holes of any size have not been detected, they have\u00a0been an intriguing candidate for elusive dark matter.<\/p>\n\n\n\n

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As far as we currently know, matter that makes up humans, plants, the Earth, other planets, stars and galaxies only makes up 5% of all matter in the universe. The rest is either dark matter (27%) or dark energy (68%), both of which have not yet been physically detected. But researchers are confident that dark matter exists because we can see its effect on our universe. Without the gravitational force from dark matter, the stars in our Milky Way Galaxy would be flying apart.<\/p>\n\n\n\n

To test the theory that primordial black holes, specifically those about the mass of the moon or less, could be dark matter, Kavli IPMU researchers Masahiro\u00a0Takada, Naoki Yasuda, Hiroko Niikura and collaborators from Japan, India and the USA searched for these tiny black holes between Earth and the Andromeda Galaxy, Earth\u2019s closest neighbour galaxy 2.5 million light years away.<\/p>\n\n\n\n

\u201cWhat made me interested in this project was the tremendous impact it would have on uncovering the nature of dark matter,\u201d says Niikura. \u201cDiscovering primordial black holes would be a historical achievement. Even a negative result would be valuable information for researchers piecing together the scenario of how the universe began.\u201d<\/p>\n\n\n\n

To look for black holes, the team used the \u2018gravitational lensing effect\u2019. Gravitational lenses were first explained by Albert\u00a0Einstein, who said it was possible for an image of a distant object, such as a star, to become distorted due to the gravitational effect of a massive object between the star and Earth. The massive object\u2019s gravity could act like a magnifying glass lens, bending the star\u2019s light and making it appear brighter or distorted to human observers on Earth.<\/p>\n\n\n\n

Because a star, a black hole and the Earth are constantly moving in interstellar space, a star would gradually grow brighter, then dimmer to observers on Earth, as it moves across the path of a gravitational lens. So the researchers captured 190 consecutive images of the entire Andromeda Galaxy, thanks to the Hyper Suprime-Cam digital camera on the Subaru Telescope in\u00a0Hawaii. If dark matter is made of primordial black holes and, in this case, ones lighter than the moon, the researchers expected to find 1,000 gravitational microlenses. They calculated this estimate by assuming dark matter in the entire galaxy\u2019s halo is made up of primordial black holes, and taking into consideration the number of stars in the Andromeda Galaxy that could be affected by a primordial black hole, and finally the chances of their equipment capturing a gravitational microlens event.<\/p>\n\n\n\n

The telescope photographed 90 million stars. It took two years for the team to filter out all of the noise and non-gravitational lens events from the data. In the end, they could only identify one star that brightened then dimmed \u2013 suggesting a possibleprimordial black hole \u2013 meaning it is unlikely that they make up all of dark matter.<\/p>\n\n\n\n

Even so, Niikura explains that there is still a lot to learn about primordial black holes.\u00a0The researchers had only debunked the theory for a specific mass: black holes with a mass similar to or less than the moon. Previous studies have ruled out other masses, or to what extent they could account for dark matter. But there is still a chance that primordial black holes of varying sizes might be out there. The analytical approach developed by the Kavli team could be used in future primordial black hole studies, including trying to determine if black holes discovered by the Laser Interferometer Gravitational Wave-Observatory (LIGO) in the USA could in fact be primordial.\u00a0
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