On the night of November 23, 2014, a powerful telescope at Mauna Kea, Hawaii, attempted to detect the mysterious movements of a black hole traveling through space. In the seven hours that the telescope peeped into the cosmos, it may have captured one, as an Earth-sized structure obscured a star in our nearest galactic neighbor, Andromeda, about 2.5 million light-years away. Amid the 188 relatively dull images taken of the galaxy that night, the candidate event in the black hole was a moment of literal enlightenment.
“If it goes exactly along the line of sight, the light bends [the black hole]. Not only the rays of light that indicate that you are with you, but also the rays that would have passed you bend towards you, ”said Alexander Kusenko, astrophysicist at UCLA and the Kavli Institute for the Physics and Mathematics of the Universe , in a video call. “It will make the star appear brighter for a second. It’s a bit counterintuitive. “
Kusenko is lead author of a recent article on the event, published in the journal Physical Review Letters in October. The research suggests that some and all of the dark matter in the universe could be explained by primal black holes – hypothetical small and very old versions of the classic cosmic character that only pictured directly for the first time in 2019. All black holes, regardless of size, are celestial objects that exert so much gravity that nothing, not even light, can escape them.
One idea is that, at the beginning of the universe, small fluctuations in density in the incredibly dense inflating universe would have been enough to spawn black holes from the pre-stellar plasma, especially if heavy particles interacted with them through an unknown force. (By default, known black holes are usually formed by collapsing stars.)
“If you take a spoonful of primordial plasma, it’s almost like a black hole,” Kusenko said, referring to the universe’s initial density. “Squeeze it a little bit and the light wouldn’t escape.”
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Some of these theoretical black holes, in accordance with Einstein’s theory of gravity, would have been of a certain critical size to be seen as constantly expanding to an observer in the black hole – while remaining static size to the external observer. This idea may spawn ideas in our own universe, but keep in mind that primal black holes themselves are only theoretical at this point.
And that’s the immediate concern of Kusenko’s team: proving their existence. Primordial black holes would have to be numerous if they explained some of the universe’s dark matter – mysterious things that seem to make up about 27% of the universe – but too small to be detectable, as their confirmed counterparts have been.
Kusenko and his colleagues (the October paper involved researchers from UCLA and Kavli IPMU) cast a wide net for black hole candidates using the Hyper Suprime-Cam, a lens barrel nearly six feet long that has been attached the Subaru telescope on Mauna Kea at the mirror of almost 9 meters. The camera can image the entire Andromeda galaxy every few minutes. Since a candidate for a primordial black hole was selected in 2014 during the seven-hour round trip of the cosmos, Kusenko hopes future sightings can gather more events to unravel.
The 2014 observation was not easy to find across all data. That team narrowed down a catalog of more than 15,000 candidate stars to check for light distortion, and found nearly 50 “deceptive” events, including those caused by bright stars. One trickster was even triggered by a passing asteroid. But after much star sorting, one candidate seemed bona fide.
If more candidate events are identified, there will be more runway for the team’s theory that many miniature black holes are responsible for the excessive gravity measured in many galaxies (it’s that extra gravity that tipped scientists to the existence of dark matter in the 1970s). To put things in perspective, the smallest known black holes are in the realm of 5 solar masses (that is, five times the size of the sun). The recent candidate black hole was only the size of our planet.
If a black hole the size of Earth sounds hard to believe, it’s not even the smallest black hole proposed. Last year, physicists proposed one black hole the size of a bowling ball to explain a hypothetical object in our solar system known as Planet Nine.
Kusenko’s team made another round of sightings at Mauna Kea in late 2020, and they now have to do the painstaking work of searching the data. We could find out later this year if they found possible black holes– and our fingers are tightly crossed for good news.