Researchers have discovered a way of sizing up the ‘shadows’ of two
supermassive black holes in the process of colliding, giving astronomers a
potentially new tool to measure black holes in distant galaxies and test
alternative theories of gravity
Three years ago, the first ever image of a black hole stunned the world. A
black pit of nothingness enclosed by a fiery ring of light. That iconic
image of the black hole at the center of galaxy Messier 87 came into focus
thanks to the Event Horizon Telescope, 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 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. From this sideways vantage
point, as one black hole passes in front of the other, you should be able to
see a bright flash of light as the glowing ring of the black hole farther
away 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 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, and how
closely entwined their orbits. If you measure how long the dip lasts, the
researchers say, you can estimate the size and shape of the shadow cast by
the black hole’s event horizon, the point of no exit, where nothing escapes,
not even light.
“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. 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.
The researchers became interested in flaring supermassive black holes after
spotting a suspected pair of supermassive black holes at the center of a
far-off galaxy in the early universe. 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
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.
Reference:
Jordy Davelaar et al, Self-Lensing Flares from Black Hole Binaries:
Observing Black Hole Shadows via Light Curve Tomography, Physical Review
Letters (2022).
DOI: 10.1103/PhysRevLett.128.191101
Jordy Davelaar et al, Self-lensing flares from black hole binaries:
General-relativistic ray tracing of black hole binaries, Physical Review D
(2022).
DOI: 10.1103/PhysRevD.105.103010
Tags:
Space & Astrophysics