At the center of our galaxy lurks a dark, beating heart: Sagittarius A*, a black hole some four million times more massive than our sun. We cannot see this fearsome object directly—only its shadow, a lightless bubble nestled within an “accretion disk” of infalling incandescent plasma. Spotting this light-ringed dark orb from our place in the Milky Way’s hinterlands is almost unfathomably difficult, like spotting a single virus particle that’s half a mile away.
But thanks to a global network of radio observatories called the Event Horizon Telescope (EHT), scientists can not only glimpse this glowing doughnut but also map its monstrous magnetic fields. Yet as remarkable as these observations may be, they pale in comparison to what some astronomers hope to see a decade from now with an upgraded EHT.
Black holes are among the most enigmatic and important characters in our great cosmic drama. Whether they form from a single star or weigh in at billions of stellar masses, they are thought to power many of the most energetic phenomena in the known universe, from quasars to gamma-ray bursts. And supermassive black holes play a huge role in shaping the structure and flow of matter within their home galaxies. As a supermassive black hole accretes matter, the energy released in the form of high-energy radiation can heat up and redistribute the surrounding gas—affecting, among other things, galactic rates of star formation. “They are literally the origin of life, in a way,” says Sara Issaoun, a postdoctoral fellow at the Center for Astrophysics | Harvard & Smithsonian (CfA).
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But so far, only two black holes have made it into the EHT’s high-resolution photo album: Sagittarius A* and the even larger M87*, a 6.5-billion-solar-mass gargantuan at the center of Messier 87, or M87, a giant elliptical galaxy some 55 million light-years from Earth. The first snapshot came in 2019 when the EHT released an image of the plasmatic torus encircling M87*. In 2022 the EHT followed suit with Sagittarius A*. These breakthrough observations sparked deeper looks: in 2021 researchers announced that they had measured how light is polarized across M87*’s inner accretion disk, which let them chart the magnetic fields around the black hole. Now the EHT team has done the same for Sagittarius A* and detailed its results in twostudies recently published in the Astrophysical Journal Letters.
“Imagine you’re at the beach, and you look out and you want to go swimming. If you don’t see the undertow, you can be in a lot of trouble,” says CfA astrophysicist Shep Doeleman, EHT’s founding director. “The polarization lets us see the structure below the glow that we’ve been seeing.”
The thousandfold mass difference between Sagittarius A* and M87* explains why the latter’s magnetic underpinnings were seen first. Because our own galaxy’s central black hole is roughly a thousand times less massive than M87*, the churning maelstrom around Sagittarius A* can change about a thousand times faster, too: features that may persist for days around M87* can come and go in mere minutes around Sagittarius A*. That transience made getting a sharp long-exposure image of the black hole over several hours especially tricky. “Sagittarius A* is such a problem-child black hole,” says Issaoun, who led the observational side of the analysis. “We were very concerned about being able to get an image at all. But it was actually pretty surprising; the polarization data ended up being a lot calmer than we had predicted.”
Despite the differences in mass and M87*’s prominent, iconic jet, the polarized light detected by the EHT around each supermassive black hole shows a striking similarity with profound implications: both boast accretion disks shaped by strong, ordered magnetic fields. “I’m quite excited,” says Alex Chen, a theoretical astrophysicist at Washington University in St. Louis, who is not a member of the EHT Collaboration. “This is a very, very interesting result that humanity has not been able to get—like, ever.”
And as the EHT pursues an ambitious upgrade plan, this surprising result may prove to be only the opening chapter of a grander tale that will reveal the quirks of the giant gravitational beasts lurking within the Milky Way, M87 and a host of other nearby large galaxies.
SANE or MAD?
Collectively, the EHT’s studies to date point toward an answer to a longstanding debate in astrophysics: Do the accretion flows around black holes tend to be SANE, or are they MAD?
Under the SANE (standard and normal evolution) model of accretion flows, magnetic fields around a black hole are relatively weak and constantly jostled by swirling, turbulent plasma within the accretion disk. In MAD (magnetically arrested disk) models, however, magnetic fields can be stronger and play a dominant role in shaping a black hole’s accretion disk, corralling the plasma into more orderly arrangements.
“It’s almost a little bit like watching a waterfall,” Doeleman says. “Sometimes when the conditions are just right, you’ll get this nice smooth flow of water over the waterfall. It’ll almost seem glassy. And then you’ll also see, with different conditions, turbulent flow with a lot of bubbles in it.”
A side-by-side comparison of two supermassive black holes—one in the elliptical galaxy M87 (left), and the other in the Milky Way (right)—captured in polarized light by the EHT. Despite M87’s black hole being some three orders of magnitude more massive than our own galaxy’s central giant, both share similarly structured magnetic fields.
These two different models each imply a different, distinctive polarization signature. Most of the glow that the EHT is picking up from the edges of Sagittarius A* and M87* is generated by what’s called synchrotron emission. As charged particles rapidly gyrate around the magnetic field lines around the black hole, they emit linearly polarized photons—and the specific pattern of those photons’ polarization angles depends on the structure of the underlying magnetic field lines. In the case of MAD, the polarization angles should reveal a majestic spiraling symmetry to the magnetic field. If SANE ruled the day, the polarization angles would be more scrambled. For both M87* and Sagittarius A*, the EHT’s polarization data suggest a highly ordered magnetic structure, favoring the MAD model.
That swing in MAD’s favor also hints at something interesting about Sagittarius A* and thus about the Milky Way: its likelihood of sporting black hole–powered jets like its larger cousin.
In computer simulations, black holes with MAD-like magnetic fields are particularly efficient at generating huge jets of charged particles, like the cosmic blowtorch that stretches out for thousands of light-years from M87*. Thanks to what’s known as the Blandford–Znajek mechanism, a spinning black hole wreathed by a strongly magnetized accretion disk can drag along the magnetic field lines threaded into it. This process creates two spiraling cones of magnetic field lines over the black hole’s poles like the twisted ends of a candy wrapper. Charged particles can then accelerate along these field lines up and away from the black hole, forming jets in the process.
But if Sagittarius A* has or once had such jets, astronomers have yet to conclusively see them. Instead they’ve surmised their possible existence from multiple lines of circumstantial evidence. Two vast plasma structures aglow with high-energy gamma rays, known as the Fermi bubbles, extend above and below the plane of the Milky Way in mute testament to past tumult in the galactic center. X-ray and radio images of our galaxy’s core reveal large-scale filaments of magnetized plasma. One study published last year in the Astrophysical Journal Letters analyzed the orientations of these filaments and found subtle hints of a jetlike outflow emanating from Sagittarius A* through the Milky Way’s starry disk.
At present, the EHT can’t see the jet of M87*, let alone any similar feature from Sagittarius A*. That’s because such jets are undetectably dim at the radio wavelengths used by the EHT—for now, anyway. The EHT team is lobbying for its ambitious plan to upgrade its facilities during the coming decade and radically transform our up-close views of black holes. “We’re really excited to see if this jet is really there,” Issaoun says. “I think we’d be able to see it.”
The Next Generation
The next-generation Event Horizon Telescope, or ngEHT for short, would add a dedicated network of approximately 10 new antennas to the collaboration’s existing international array.
The EHT team assembles its unique images by combining the data from a global network of millimeter-wave telescopes all looking for days on end at the same supermassive black hole. Through clever and intensive data processing that compares slight variations in the signals from each telescope in the network, the EHT effectively becomes a radio antenna the diameter of Earth. Although this virtual Earth-size telescope has astounding angular resolution, the vastness of intergalactic space serves as a spoiler—in its present form the EHT can only probe the black holes of our galaxy and of M87; all others in galaxies with favorable “disk-on” (rather than “edge-on”) alignments are too far away to be resolved. Additionally, the EHT’s images are effectively monochrome—limited to a small swath of radio wavelengths—and the project can’t easily monitor its targets for extended periods.
The new antennas would roughly double the number of participating telescopes, while also adding upgrades that let the EHT observe at two or possibly three different radio frequencies at once. A more broadband radio view of black holes in multicolor would be a scientific boon because it would let astronomers tease out certain effects of magnetic fields (which vary by frequency) and Einstein’s general relativity (which don’t) in exquisite detail. Adding multiple frequencies alone would improve ngEHT’s resolution by up to 50 percent over today’s EHT, according to a paper published last October in the journal Galaxies. In all, the ngEHT is aiming for a resolution of 15 microarcseconds: good enough to spot a single virus particle from more than three quarters of a mile away.
These upgrades would also greatly expand the number of black holes the EHT could meaningfully resolve from only two up to a few dozen or so. It could even spot two black holes at once. Years-long tracking of pulsars across the night sky strongly suggest that spacetime hums with a coffee-shop murmur of gravitational waves given off by pairs of supermassive black holes spiraling into each other in the center of distant galaxies. In principle, ngEHT could spatially resolve such a pair—and see two dark dots instead of one.
In addition, the team envisions using ngEHT to make “movies” of M87* and Sagittarius A*, which would still be, by far, the most favorable targets. “We want these black holes to be, for black hole astrophysics, as useful as the sun is for stellar astrophysics,” says CfA postdoctoral fellow Angelo Ricarte, who led the theory side of the new results mapping Sagittarius A*’s magnetic fields. “We want to understand these so well that we can use them as a baseline for other systems.”
Since 2019 the ngEHT effort has received more than $14.6 million from the U.S. National Science Foundation (NSF) to fund the design of new hardware and upgrades for the current crop of participating telescopes. Last September the Smithsonian Astrophysical Observatory selected mtex antenna technology, a German firm, to design, develop and build the necessary next-generation antennas. According to Doeleman, completing ngEHT will cost $88 million over several years, a sum that he and his colleagues requested from the NSF in December.
The NSF has not yet revealed whether it will fund a fully scoped ngEHT. That announcement isn’t expected any sooner than October, according to the agency’s FY2025 budget request.
The EHT Collaboration has even bigger, longer-term ideas for how to snap the best possible black hole picture. Even the ngEHT can only get so good; its virtual telescope’s effective light-gathering power is still limited by our planet’s size. To get an even better resolution, the EHT would have to increase its diameter beyond that of Earth’s—by putting at least one of its telescopes in space.
An EHT team led by CfA astrophysicist Michael Johnson is exploring a space mission concept known as Black Hole Explorer (BHEX). The idea is to place a satellite in orbit around Earth to act as another node of the EHT array. BHEX would boost the array’s overall resolution by a factor of three to five—enough, in principle, to allow scientists to see what’s known as a photon ring: a sharp, glowing circle formed by photons that repeatedly circle a black hole before escaping outward to sail across the cosmos.
Optimistic performance projections suggest this space-based expansion to ngEHT could even tease out the photon ring’s razor-thin substructures, which arise from variations in the number of orbits that different groups of photons complete around a black hole before escaping.
Such observations would provide extraordinarily strict tests of general relativity while also resolving black holes’ behavior as never before. How exactly do accretion disks around black holes behave? How do black holes launch their jets? These questions and more could be answered with ngEHT—revealing one of the universe’s most mysterious objects in eye-popping detail.
“When we first launched the EHT, we didn’t even know if black holes could be imaged,” Doeleman says. “Now that we have spectacularly retired that scientific risk, it’s time to take a big step.”
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