astronomers have revealed the first image of the black hole at the center of our galaxy, the Milky Way. The image was taken by the Event Horizon Telescope (EHT) Collaboration, an international team of more than 300 scientists on five continents, including Africa.
Black holes were predicted by Albert Einstein’s General Theory of Relativity more than a century ago. They are areas of space so dense that nothing, including light, can escape. Their boundary is known as the event horizon, which marks the point of no return. That’s just one of the reasons why these objects are hidden from our eyes. The other is that they are extremely small when placed in their cosmic context. If our Milky Way galaxy were the size of a football field, the event horizon of the black hole would be a million times smaller than a pinprick in its midfield.
How can one photograph them? Our team did this by collecting light from the hot swirling gas in the immediate vicinity of the black hole. This light, with a wavelength of 1 millimeter, is registered by a worldwide network of antennas that together form one virtual telescope on Earth.
The light is more like a ring, a characteristic signature that is the direct result of two important processes. First, the black hole is so dense that it bends the path of light close to it. Second, it catches light that falls too close to the event horizon. The combined effect produces what’s called a black hole shadow – a brighter ring surrounding a distinct lack of light centered on the black hole. In the case of our black hole in the Milky Way, this ring is the apparent size of a donut on the moon, requiring extraordinary engineering effort to get it into view.
Revealing an image of our black hole, Sagittarius A*, is not just a huge moment for science. It could also be an important catalyst for diversifying African astrophysics research using existing strengths. We were the only two of more than 300 EHT team members on the African continent. The continent is not home to EHT telescopes – we were brought on board because of the expertise we developed in preparation for the world’s largest radio telescope, the Square kilometer array (SKA), co-hosted by South Africa and Australia.
Why the image is important
This isn’t the first time an image of a black hole has caught people’s attention. We were also part of the team that created the very first photo of a black hole in 2019 (it is in the center of another galaxy, Messier 87, which is 55 million light-years away). It’s estimated that more than 4.5 billion people saw that image. Sagittarius A* also dominated the headlines and captured people’s imaginations.
But there’s more to this result than just an incredible image. A plethora of rich scientific results have been described in ten publications by the squad. Here are three of our main highlights.
First, the image is a remarkable confirmation of Einstein’s general theory of relativity. The EHT has now imaged two black holes with masses that differ by a factor of more than 1000. Despite the dramatic difference in mass, the measured size and shape are consistent with theoretical predictions.
Second, we have now imaged black holes with very different environments. A wealth of previous research from the past two or three decades shows strong empirical evidence that galaxies and their black holes co-evolve over cosmic time, despite their completely disparate sizes. By zooming in on the event horizons of black holes in giant galaxies like M87, as well as more typical galaxies like our own Milky Way, we learn more about how this seemingly unlikely relationship between the black hole and its host galaxy plays out.
Third, the image gives us new insights about the central black hole in our own galactic home. It’s the closest animal to Earth, so it provides a unique laboratory for understanding this interplay – no different than examining a tree in your own yard to better understand the forests on the far horizon.
Southern Africa’s geographic advantage
We are proud to be part of the team that created the first black hole images. Going forward, we believe that South Africa and the wider African continent (including a joint Dutch-Namibian initiative), could play a crucial role in the making of the first black hole films.
As has been the case with the country’s key role in paleoanthropology, there are contributions to global astronomy that can only be made from South African soil. Sagittarius A* is in the southern sky, right above South Africa. That’s a big reason why this image of the center of the Milky Way, taken by the MeerKAT (a precursor to the SKA), is the best out there.
South Africa also has well-established infrastructure at its astronomical sites, which are protected by law. And it has world-class engineers at the forefront of their craft. This ensures low-cost, high-quality telescopes that are delivered on time and within budget.
New technology is on our side too: An advanced simultaneous multi-frequency receiver design developed by our Korean colleagues means that EHT sites no longer have to be the most pristine, high-altitude sites on Earth.
All the elements are in place for a dramatic increase in the number of young Africans participating in this new era of black hole imaging and precision testing of gravity. In the coming years, we hope to write about findings that could not have been made without technology on South African soil, as well as African scientists leading high-impact, highly visible EHT science in synergy with our multi-wavelength astronomy and programs. for high-energy astrophysics.
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