On the night of November 23, 2014, a powerful telescope on Mauna Kea in Hawaii was trying to pick out the enigmatic movements of a black hole travelling through space. In the seven hours the telescope peered at the cosmos, it may have caught one, as a structure about the size of Earth eclipsed a star in our nearest galactic neighbour, Andromeda, about 2.5 million light-years away. Amid the 188 relatively bland images taken of the galaxy that night, the candidate black hole event was a moment of literal illumination.
“When right along line of sight, the light bends around [the black hole]. Not just the light rays pointing at you to being with, but also those that would’ve gone past you bend towards you,” Alexander Kusenko, astrophysicist at UCLA and the Kavli Institute for the Physics and Mathematics of the Universe, said in a video call. “It’ll make the star appear brighter for a second. It’s a little counterintuitive.”
Kusenko is lead author of a recent paper discussing the event, published in the journal Physical Review Letters in October. The research suggests that between some and all of the universe’s dark matter could be explained by primordial black holes — hypothesised petite and very ancient versions of the classic cosmic character that was only imaged directly for the first time in 2019. All black holes, regardless of their size, are celestial objects that exert so much gravitational force that nothing, not even light, can escape from them.
One idea is that, at the universe’s outset, slight density fluctuations in the incredibly dense inflating universe would have been enough to spawn black holes out of the pre-stellar plasma, especially if interacted with by heavy particles via some unknown force. (Run-of-the-mill, known black holes are typically formed from collapsing stars.)
“If you take a spoonful of primordial plasma, it’s almost a black hole,” Kusenko said, referring to the universe’s initial density. “Squeeze it a little bit, and light wouldn’t escape.”
Some of these theoretical black holes would have been of some critical size, in accordance with Einstein’s theory of gravity, to be perceived as continuously expanding to an observer within the black hole — while remaining a static size to the outside observer. This idea may spawn notions of “baby universes” within our own, but keep in mind that primordial black holes are themselves merely theoretical for the time being.
And that’s the immediate concern of Kusenko’s team: proving their existence. Primordial black holes would have to be numerous if they were to account for some amount of the universe’s dark matter — mysterious stuff that seems 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) are casting a wide net for black hole candidates using the Hyper Suprime-Cam, a nearly 1.83 m-long lens barrel attached to the nearly 9.14 m mirror of the Subaru telescope on Mauna Kea. The camera is capable of imaging the entire Andromeda galaxy every few minutes. Since one candidate for a primordial black hole was picked out in the seven-hour tour of the cosmos back in 2014, Kusenko hopes future observations will be able to collect more events to unpack.
The 2014 observation wasn’t easy to find in all the data. That team narrowed down a catalogue of over 15,000 candidate stars to check for light warping, and in the process found nearly 50 “impostor” events, caused by bright stars, among other things. One impostor was even caused by a passing asteroid. But after plenty of star sorting, one candidate seemed bonafide.
If more candidate events are identified, there will be more runway for the team’s theory of lots of miniature black holes accounting for the excess gravity measured in many galaxies (it’s that extra gravity that tipped scientists off to the existence of dark matter back 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 merely the size of our planet.
If an Earth-size black hole sounds hard to believe, well, that’s not even the smallest proposed black hole. Last year, physicists suggested a bowling ball-size black hole to explain a hypothetical object in our solar system known as Planet Nine.
Kusenko’s team made another round of observations at Mauna Kea at the end of 2020, and they now must do the painstaking work of sifting through the data. We could know later this year if they found any potential black holes — and our fingers are firmly crossed for good news.