The world was astonished three years ago when the first black hole image was captured. a flaming ring of light enclosing a black pit of emptiness. The Event Horizon Telescope, a global network of synchronized radio dishes operating as one enormous telescope, was responsible for bringing into focus that famous image of the black hole at the center of galaxy Messier 87.
Now, two Columbia University researchers have developed a method that might make peering into the void simpler. Outlined in complementary studies in Physical Review Letters and Physical Review D, their imaging technique could allow astronomers to study black holes smaller than M87’s, a monster with a mass of 6.5 billion suns, harbored in galaxies more distant than M87, which at 55 million light-years away, is still relatively close to our own Milky Way.
The technique has just two requirements. First, you need a pair of supermassive black holes in the throes of merging. Second, you need to be looking at the pair at a nearly side-on angle. You should be able to see a dazzling flash of light as one black hole passes in front of the other from this sideways vantage point. This is due to gravitational lensing, where the closer black hole enlarges the blazing ring of the farther away black hole.
The lensing effect is well known, but what the researchers discovered here was a hidden signal: a distinctive dip in brightness corresponding to the “shadow” of the black hole in back. This subtle dimming can last from a few hours to a few days, depending on how massive the black holes, are and how closely entwined their orbits.
The length of the dip, according to the researchers, can be used to calculate the size and shape of the shadow produced by the event horizon of a black hole, the point beyond which nothing, not even light, can escape.
“It took years and a massive effort by dozens of scientists to make that high-resolution image of the M87 black holes,” said the study’s first author, Jordy Davelaar, a postdoc at Columbia and the Flatiron Institute’s Center for Computational Astrophysics. “That approach only works for the biggest and closest black holes the pair at the heart of M87 and potentially our own Milky Way.”
He added, “with our technique, you measure the brightness of the black holes over time, you don’t need to resolve each object spatially. It should be possible to find this signal in many galaxies.”
The shadow of a black hole is both its most mysterious and informative feature. “That dark spot tells us about the size of the black hole, the shape of the space-time around it, and how matter falls into the black hole near its horizon,” said co-author Zoltan Haiman, a physics professor at Columbia.
Black hole shadows may also hold the secret to the true nature of gravity, one of the fundamental forces of our universe. Einstein’s theory of gravity, known as general relativity, predicts the size of black holes.
It took years and a massive effort by dozens of scientists to make that high-resolution image of the M87 black holes. That approach only works for the biggest and closest black holes the pair at the heart of M87 and potentially our own Milky Way.
Jordy Davelaar
Physicists, therefore, have sought them out to test alternative theories of gravity in an effort to reconcile two competing ideas of how nature works: Einstein’s general relativity, which explains large-scale phenomena like orbiting planets and the expanding universe, and quantum physics, which explains how tiny particles like electrons and photons can occupy multiple states at once.
After detecting what appeared to be a pair of supermassive black holes at the heart of a distant galaxy in the early universe, astronomers were interested in supermassive black holes that flare.
NASA’s planet-hunting Kepler space telescope was scanning for the tiny dips in brightness corresponding to a planet passing in front of its host star. Instead, Kepler ended up detecting the flares of what Haiman and his colleagues claim are a pair of merging black holes.
They named the distant galaxy “Spikey” for the spikes in brightness triggered by its suspected black holes magnifying each other on each full rotation via the lensing effect. To learn more about the flare, Haiman built a model with his postdoc, Davelaar.
However, they were perplexed when their fictitious black hole pair unexpectedly but consistently generated a decrease in brightness whenever one orbited in front of the other They initially believed it to be a coding error. But more investigation made them decide to believe the signal.
As they looked for a physical mechanism to explain it, they realized that each dip in brightness closely matched the time it took for the black hole closest to the viewer to pass in front of the shadow of the black hole in back.
The researchers are currently looking for other telescope data to try and confirm the dip they saw in the Kepler data to verify that Spikey is, in fact, harboring a pair of merging black holes. If it all checks out, the technique could be applied to a handful of other suspected pairs of merging supermassive black holes among the 150 or so that have been spotted so far and are awaiting confirmation.
As more powerful telescopes come online in the coming years, other opportunities may arise. The Vera Rubin Observatory, set to open this year, has its sights on more than 100 million supermassive black holes. Further black hole scouting will be possible when NASA’s gravitational wave detector, LISA, is launched into space in 2030.
“Even if only a tiny fraction of these black hole binaries has the right conditions to measure our proposed effect, we could find many of these black hole dips,” Davelaar said.
