Three years ago, the very first image of a black hole stunned the world. A black pit of nothingness surrounded by a fiery ring of light. This iconic image of the black hole at the center of the Messier 87 galaxy was brought into focus using the Event Horizon Telescope, a global network of radio-synchronized antennas that act like a giant telescope.
Now, two Columbia researchers have come up with a potentially simpler way to look into the abyss. Described in additional studies in Physical examination letters and Physical examination D, their imaging technique could allow astronomers to study black holes smaller than that of M87, a monster with a mass of 6.5 billion suns, hosted in galaxies more distant than M87, which at 55 million suns light years, is still relatively close to our own Milky Way.
The technique has only two requirements. First, you need a pair of merging supermassive black holes. Second, you need to look at the pair from an almost sideways angle. From this sideways perspective, as one black hole passes in front of the other, you should be able to see a flash of light as the ring of light from the farthest black hole is amplified by the black hole closest to you, a phenomenon known as gravitational lensing.
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 to the rear. This subtle dimming can last from a few hours to a few days, depending on the mass of the black holes and the proximity of their orbits. If you measure the duration of the trough, the researchers say, you can estimate the size and shape of the shadow cast by the black hole’s event horizon, the no-exit point, where nothing escapes, not even the light.
“It took years and considerable effort by dozens of scientists to create this high-resolution image of M87’s black holes,” said study first author Jordy Davelaar, a postdoctoral fellow at Columbia and the Center for Science. Computational Astrophysics from the Flatiron Institute. “This approach only works for the largest and closest black holes – the pair at the core of M87 and potentially our own Milky Way. »
He added: “With our technique, you measure the brightness of black holes over time, you don’t have to resolve every object in space. It should be possible to find this signal in many galaxies. »
A black hole’s shadow is both its most mysterious and most informative feature. “This dark spot tells us about the size of the black hole, the shape of the spacetime around it, and how matter falls into the black hole near its horizon,” said co-author Zoltan Haiman, Professor of physics at Columbia.
Black hole shadows may also hold the secret to the true nature of gravity, one of the fundamental forces in our universe. Einstein’s theory of gravity, known as general relativity, predicts the size of black holes. So physicists sought them out to test alternative theories of gravity in an effort to reconcile two competing ideas about 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.
Researchers became interested in the splaying of supermassive black holes after spotting a suspected pair of supermassive black holes at the center of a distant galaxy in the early universe. NASA’s planet-hunting Kepler space telescope scanned the tiny dips in brightness corresponding to a planet passing in front of its host star. Instead, Kepler ended up detecting flares of what Haiman and his colleagues claim to be a pair of merged black holes.
They named the distant galaxy “Spikey” for the spikes in brightness triggered by its putative black holes magnifying each other with each full rotation via lensing. To learn more about the eruption, Haiman built a model with his postdoc, Davelaar.
They were confused, however, when their pair of simulated black holes produced an unexpected but periodic dip in brightness each time one rotated past the other. At first they thought it was a coding error. But further verification led them to trust the signal.
As they searched for a physical mechanism to explain it, they realized that each dip in brightness corresponded closely to the time it took for the black hole closest to the viewer to pass in front of the black hole’s shadow at the ‘back.
The researchers are now looking for more telescope data to try to confirm the dip they saw in the Kepler data to verify that Spikey is in fact home to a pair of merged black holes. If all holds true, the technique could be applied to a handful of other suspected pairs of merging supermassive black holes among the roughly 150 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, due to open this year, is targeting more than 100 million supermassive black holes. Further black hole reconnaissance 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 have the right conditions to measure our proposed effect, we could find many of these black hole troughs,” Davelaar said.
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In a pair of merged supermassive black holes, a new method to measure the vacuum – News Astronomy and astrophysics
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