The Event Horizon Telescope (EHT) has added more observatories to its global network of radio telescopes and the first image of our galaxy's black hole could be less than a year away.
"As soon as next spring, the Event Horizon Telescope is going to produce images of the black hole at the center of the Milky Way," said Avery E. Broderick, assistant professor of the Department of Physics & Astronomy at the University of Waterloo, during a presentation at the Perimeter Institute's Convergence conference on June 23.
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Broderick, who is also a faculty member at Perimeter, outlined some of the mind-blowing possibilities for research into the extremes of spacetime surrounding the supermassive black hole at the center of the Milky Way, called Sagittarius A* (or Sgr. A*), that the EHT will focus on to study strong gravity.
"There's really only two places where you can study (strong gravity): over very, very large scales (cosmology) ... and around compact objects," he said.
Likening the study of general relativity - which Albert Einstein formulated 100 years ago - with the early maps created by mariners, Broderick said that there are many "monsters" in the uncharted depths of space, one of them being the gravitational conditions surrounding a black hole. But this is about to change for the first time in human history.
Event Horizon Angular Resolution The EHT is composed of many radio observatories around the world. Through the technique of Very Long Baseline Interferometry (VLBI), many independent radio antennae separated by hundreds or thousands of miles can be used in concert to create a "virtual" telescope with a diameter of the entire planet.
The most powerful optical telescopes on the planet are severely limited when observing even the most massive objects known in the universe. Massive they may be, but black holes are extremely compact, appearing as a tiny speck in the sky. When active, the supermassive black holes known to exist in the cores of most galaxies can outshine their host galaxies and generate vast streamers of relativistic gases blasting into intergalactic space. They can also have powerful impacts on the evolution of galaxies, so observing these gravitational behemoths is one of the most important aims of modern astrophysics.
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But to observe Sgr. A*'s event horizon, you need extremely small angular resolution. The event horizon of Sgr. A* is estimated to be 50 micro arcseconds (μas) in diameter (arcseconds is a fine measurement of angular resolution used by astronomers). One of the most powerful optical/infrared observatories on the planet, the twin Keck telescopes atop Mauna Kea in Hawaii, can resolve down to 20,000 μas, way short of resolving a 50 μas object. Likewise, the planned Thirty-Meter Telescope (TMT) will only be able to resolve 7,000 μas. Although these are terrific resolutions for optical astronomy, it's a "complete bust" for event horizon astronomy, Broderick added.
When complete, the EHT will be operating at resolutions of 10 μas, something we are very close to achieving. And this has some intriguing possibilities for not only producing the first image of a black hole, it will also test Einstein's general relativity at the extremes and could also uncover physics beyond general relativity.
"This is not a future experiment, it's happening now," Broderick said, pointing out that the "prototype" EHT already has 9 years of data. But the observations of Sgr. A* up to now have come from only 3 participating EHT radio antennae, so the data has been too sparse to create an image of the black hole's predicted event horizon "ring." These observations, however, have been critical in constraining certain characteristics of the black hole, helping theoretical physicists envisage what Sgr. A* may look like, but many unknowns remain.
"Even for the black hole at the center of the Milky Way, there are grave uncertainties regarding the geometry and dynamics of the region responsible for producing the (sub-resolution) image that we see," he said.
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The "classical" idea of a black hole includes a point of no return - the region where the warping of gravity becomes so extreme that even light cannot escape a black hole's gravitational grasp, called an event horizon. There's also an accretion disk, where material falls onto the region surrounding the event horizon, generating high-energy phenomena. But we don't really know if the accretion disk is big, small, thin or thick. We also don't know if the disk is misaligned to the direction of black hole spin (astrophysicists are pretty sure that Sgr. A* is a spinning black hole). Does our black hole even have a disk? For now, the morphology of the structures surrounding a black hole remain a mystery.
This mystery presents something of an "existential crisis" for the EHT, cautions Broderick.
"For something named the 'Event Horizon Telescope,' you really want to make sure that event horizons are out there, otherwise we're going to have to rename it," he said. "Luckily, the EHT has found the best evidence to date that this is true (and event horizons do exist)."
"Twinkle, Twinkle Sagittarius A*"
For now, barring an unlikely twist of physics, the event horizon is there and physicists have managed to narrow down what it will look like to a very small set of variables and current models appear to be agreeing with the data starting to come from the EHT.
"I'm not sure if I'm excited by this or I'm depressed by this, I kinda hoped we'd find some unexpected things by now, but success is OK," joked Broderick.
Although the EHT is already producing results and hopes are high for the first images of Sgr. A*'s event horizon in a matter of months, there is still a long way to go and challenges to overcome.
One challenge pointed out by Broderick is a similar problem experienced by ground-based observatories. When peering through the atmosphere, we see stars twinkle. This twinkling is caused by a myriad of atmospheric effects, including turbulence in the upper atmosphere and moisture. When observing the supermassive black hole at the center of our galaxy, there's an analogous effect that causes Sgr. A* to also "twinkle."
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As stars and interstellar plasma pass between us and Sgr. A*, scintillation in the received signal can occur, creating small-scale anomalies that need to be corrected for. In a few years, when the EHT is carrying out full-scale operations, zooming in on small-scale phenomena at the event horizon will be crucial to understanding what drives accretion flow in and around the black hole. But if scintillation effects aren't accounted for, phenomena such as magnetohydrodynamic (MHD) turbulence (a mechanism that drives the magnetic environment around a black hole) may become too blurry to study.
Like the ground-based observatories that fire lasers into the upper atmosphere to measure the amount of turbulence and correct for it via adaptive optics, the EHT may adopt an adaptive optics scheme (but without the lasers) in the analysis to remove this effect.
Spacetime Tomography Perhaps the most stunning application of the EHT will come when studying the flares Sgr. A* is known to periodically generate.
Approximately once a day, multiple observatories have observed a flaring event that not only causes a brightening of Sgr. A*'s emission, but it also changes the structure of the emission region. This flare has been interpreted as a "hotspot" in the accretion flow close to the event horizon. When the EHT is fully operational, it will be able to track these hotspots, tracing their origin and watch them diminish. Not only that, astronomers hope to use hotspots as a tracer to map out the structure of spacetime in this strong gravity environment.
"This opens the door to doing spacetime tomography - these spots move around, they light up different regions of spacetime," said Broderick. "And as it happens once a day, you can catch many of them."
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As we are probing the deep unknown in this unique gravitational domain, although we think we have a pretty good handle on black hole dynamics in theory, it's fascinating to think that the EHT might find something new and, possibly, exotic about the extremely warped spacetime surrounding our nearest supermassive black hole.
"I'd like to think that as soon as we have an image like this, this will be the beginning of the 21st century's version of the pale blue dot - maybe with a more sinister connotation ... there are monsters lurking in the dark," Broderick concluded.