Columbia University scientists have found a way to measure the ‘shadows’ of two colliding supermassive black holes, giving astronomers a potentially new tool to measure black holes in distant galaxies and test alternative theories of gravity.
In 2019, the world was surprised by the very first image of a black hole. A black abyss of nothingness surrounded by a ring of fiery light. This iconic image of a black hole at the center of the Messier 87 galaxy was unearthed thanks to the Event Horizon Telescope – a global network of radio-synchronized antennas that act like a giant telescope.
Now, two UC scientists have developed a potentially easier way to peer into the abyss. Their imaging technique, described in the companion publications Physical Review Letters and Physical Review D, could allow astronomers to study black holes smaller than M87, a monster with a mass of 6.5 billion suns in galaxies more distant than M87, which is 55 million light years old and still relatively close to our Milky Way.
This technique has only two requirements. First, you need a pair of merging supermassive black holes. Second, you need to watch this pair closely. From this side view, as one black hole passes in front of the other, we should be able to see a bright flash of light because the bright ring of the farthest black hole is amplified by the nearer black hole, a phenomenon known as gravitational lensing.
The lensing effect is well known, but scientists have found a hidden signal here: a characteristic gradation corresponding to the “shadow” of the black hole behind it. This subtle eclipse can last from several hours to several days, depending on the mass of the black holes and the proximity of their orbits. Scientists say that if the duration of this eclipse is measured, the size and shape of the shadow cast by a black hole’s event horizon can be estimated.
It took years and enormous effort by dozens of scientists to produce a high-resolution image of M87’s black holes Says the book’s first author, Jordy Davelaar, a doctoral student at Columbia University and the Center for Computational Astrophysics at the Flatiron Institute. This approach only works for the largest and closest black holes – the pair at the core of M87 and potentially in our Milky Way.
He added that our technique measures the brightness of black holes over time, there is no need to spatially separate each object. It should be possible to find this signal in many galaxies.
The black hole’s shadow is both its most mysterious and most instructive feature. The 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 Says co-author of the book, Zoltan Haiman, professor of physics at Columbia University.
Black hole shadows may also hold a secret about 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. Therefore, physicists seek them out to test alternative theories of gravity, trying to reconcile two competing concepts of how nature works: Einstein’s theory of general relativity, which explains large-scale phenomena such as planets in circle and the expanding universe, and quantum physics, which explains how tiny particles such as electrons and photons can be in multiple states at once.
Scientists became interested in bright supermassive black holes after spotting a suspicious pair of supermassive black holes at the center of a distant galaxy in the early universe. The Kepler space telescope, in search of planets, looked for small dips in brightness corresponding to the planet passing in front of its host star. Instead, Kepler detected flare-ups of what Haiman and his colleagues say are a pair of merged black holes.
They named the distant galaxy “Spikey” because of the spikes in brightness caused by its alleged black holes that grow larger with each full revolution due to lensing. To learn more about flares, Haiman built a model with his PhD student Davelaar.
However, they were confused when the pair of black holes they simulated caused an unexpected but periodic dip in brightness each time one rotated past the other. At first they thought it was a coding error. However, further verification led them to trust the signal.
Looking for a physical mechanism to explain this, they realized that each decrease in brightness corresponds to the time it takes for the black hole closest to the viewer to pass the shadow of the black hole behind it.
Scientists are now looking for data from other telescopes to try to confirm the decline they saw in the Kepler data and to verify that Spikey is indeed hiding a pair of merged black holes. If everything is confirmed, the technique could be applied to several other suspected pairs of connected black holes out of about 150 that have been spotted so far and are awaiting confirmation.
Source: Columbia University
In the illustration: In the simulation of a supermassive black hole merger, the blueshifted black hole closest to the viewer enhances the redshifted black hole behind through gravitational lensing. Source: Jordy Davelaar.