Scaled “shadows” of two colliding supermassive black holes

The process of merging supermassive black holes has a new way to measure the vacuum

Scientists have found a way to quantify the “shadows” of two colliding supermassive black holes, giving astronomers a potential new tool to measure black holes in distant galaxies and test alternative gravitational theories.

Three years ago, the world was shocked by the first image of a black hole. A black hole from nowhere, surrounded by a ring of fiery light. This iconic image[{” 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.

Simulation of gravitational lensing in a pair of supermassive compact black holes. Credit: Jordy Devalar

This technique has only two requirements. First, you need a pair of supermassive black holes in the middle of a merger. Secondly, you should be looking at the pair from a roughly lateral angle. From this side view, as one black hole passes in front of another, you should see a flash of bright light as the black hole’s bright ring is magnified by the black hole closest to you, a phenomenon known as gravitational lensing.

The lensing effect is well known, but what the researchers found here was a subtle sign: a characteristic dip in brightness corresponding to the “shadow” of a black hole in the background. This subtle dimming can last from hours to days, depending on the size of the black holes and the entanglement of their orbits. If you measure how long the fall lasts, the researchers say, you can estimate the size and shape of the shadow cast by a black hole’s event horizon, a cul-de-sac from which nothing, not even light, escapes.

In this simulation of a pair of merged supermassive black holes, the black hole closest to the observer approaches and therefore appears blue (Box 1), inflating the redshifted black hole from behind through a gravitational lens. As the nearby black hole amplifies the light from the distant black hole (Box 2), the observer sees a burst of bright light. But when a nearby black hole passes in front of the abyss or shadow of a more distant black hole, the observer sees a slight decrease in brightness (Table 3). This decrease in brightness (3) is clearly visible in the light curve data below the images. Credit: Jordy Devalar

“It took years and a huge effort on the part of dozens of scientists to make this high-resolution image of M87’s black holes,” said first study author Jordy Davilar, a postdoctoral fellow at Columbia and the Flatiron Center for Computational Astrophysics. “This approach only works with the closest and largest black holes – a couple at the core of M87 and possibly our own Milky Way.”

He added: “With our method, you measure the brightness of black holes over time, and you don’t have to spatially resolve each object. It should be possible to find this signal in many galaxies.”

The shadow of a black hole is its most mysterious and instructive feature. “This dark spot tells us about the size of the black hole, the shape of the space-time around it, and how matter gets into the black hole near its horizon,” said study co-author Zoltan Hyman, professor of physics at Columbia University.

When a supermassive black hole merger is observed from the side, the black hole closest to the observer enlarges the distant black hole through a gravitational lens effect. The researchers found a small drop in brightness corresponding to the “shadow” of a distant black hole, allowing the observer to measure its size. Credit: Nicoletta Barolvini

The shadows of a black hole may hide the secret of 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. So physicists turned to them to test alternative theories of gravity, trying to reconcile two competing ideas about how nature works: Einstein’s general theory of relativity, which explains large-scale phenomena such as the rotation of the planets and the expanding universe, and quantum physics, which explains how nature works. small particles such as electrons and photons occupy several states at the same time.

Researchers interested in lighting up supermassive black holes master A suspicious pair of supermassive black holes at the center of a distant galaxy at the beginning of the 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.

References:

“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