Measuring the “shadows” of two colliding supermassive black holes

In this simulation of a supermassive black hole merger, the blue-shifted black hole closest to the viewer amplifies the red-shifted black hole behind by gravitational lensing. The researchers found a sharp dip in brightness as the nearest black hole passed in front of its counterpart’s shadow, an observation that could be used to gauge the size of the two black holes and test alternative theories of gravity. 1 credit

Inside a pair of merged supermassive black holes, a new way to measure vacuum

Scientists have discovered 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.

Three years ago, the world was stunned by the first-ever image of a black hole. A black pit of nothingness surrounded by a fiery ring of light. This iconic image of[{” attribute=””>black hole at the center of galaxy Messier 87 came into focus thanks to the Event Horizon Telescope (EHT), a global network of synchronized radio dishes acting as one giant telescope.

Now, a pair of Columbia researchers have devised a potentially easier way of gazing into the abyss. Outlined in complementary research 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.

A simulation of gravitational lensing in a pair of merged supermassive black holes. 1 credit

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 is what is called the gravitational lens.

The lensing effect is well known, but what the researchers discovered here was a hidden signal: a characteristic 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 tightness 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.

Supermassive black hole merger simulation

In this simulation of a pair of merged supermassive black holes, the black hole closest to the viewer approaches and therefore appears blue (image 1), magnifying the red-shifted black hole behind by gravitational lensing. As the nearest black hole amplifies the light from the farthest black hole (image 2), the viewer sees a flash of light. But when the nearest black hole passes in front of the chasm, or shadow, of the farthest black hole, the viewer sees a slight dip in brightness (image 3). This drop in brightness (3) is clearly visible in the light curve data below the images. 1 credit

“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.

Observing the merger of supermassive black holes

Observing a merger of supermassive black holes from the side, the black hole closest to the viewer magnifies the black hole further via gravitational lensing. The researchers discovered a brief dip in brightness corresponding to the “shadow” of the more distant black hole, allowing the viewer to gauge its size. 1 credit

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 attempt to reconcile two competing ideas about how nature works: Einstein’s general relativity, which explains large-scale phenomena like orbiting planets and 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.[{” attribute=””>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.

They were confused, however, when their simulated pair of black holes produced an unexpected, but periodic, dip in brightness each time one orbited in front of the other. At first, they thought it was a coding mistake. But further checking led them to trust 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 the 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.


“Self-Lensing Flares from Black Hole Binaries: Observing Black Hole Shadows via Light Curve Tomography” by Jordy Davelaar and Zoltán Haiman, 9 May 2022, Physical Review Letters.
DOI: 10.1103/PhysRevLett.128.191101

“Self-lensing flares from black hole binaries: General-relativistic ray tracing of black hole binaries” by Jordy Davelaar and Zoltán Haiman, 9 May 2022, Physical Review D.
DOI: 10.1103/PhysRevD.105.103010

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