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A year ago, on 10 April at 4.08 pm Finnish time, we saw for the first time what a black hole looks like. The historical image interested people around the world and decorated the front pages of all the world's most prestigious magazines. The magazines Science and Physics World declared it the scientific breakthrough of the year. The photo was also selected to the permanent collection of the Museum of Modern Art in New York (MoMa). For the Event Horizon Telescope research team, one of the highlights of the year was being awarded the Breakthrough Prize, known as the “Oscar of Science”.

Research continues

The international Event Horizon Telescope (EHT) research team continues to investigate black holes. Currently, the team is working on two large projects.

In the first of the projects, the researchers are trying to obtain an image of the black hole in the centre of the Milky Way. It is not an easy thing to do, however, as the black hole in the centre of our galaxy is smaller than the black hole in the galaxy M87, the image of which we admired a year ago. Due to the “small” size of the black hole in the Milky Way, it takes less than 20 minutes for matter to orbit around it. In the galaxy M87, it takes days or even weeks for plasma to orbit around the black hole. The exposure time of the image is approximately eight hours, in which time the matter has orbited the black hole in the Milky Way several times. Moving objects are harder to photograph.

Tuomas Savolainen says that obtaining an image is difficult, but not impossible. To work around the problem, researchers are experimenting with imaging algorithms that produce a film instead of a single image. A moving image would show how the matter circulates around the black hole.

A crescent or a peanut?

Based on theory, researchers conclude that the black hole in the centre of the Milky Way is similar to the black hole in M87, with a dark region in the middle, surrounded by a bright ring.

‘But will the image be an equally pretty ring as the one we saw a year ago – that remains to be seen. The pattern it makes may look more like a crescent or a peanut.’

In the second project, the researchers are analysing the polarisation data that gives them more detailed information about the magnetic field surrounding the black hole in the galaxy M87. Polarisation is related to how light travels as a transverse wave. This spring, the EHT research community had planned to make more observations, but the coronavirus has closed the observatories almost completely, so observation-making has been postponed to next year.

Aren't you getting tired of the black holes?

‘I've been studying black holes for a long time, but they do not bore me. Particularly now, when I've just been involved in making a major breakthrough. I believe that we will have an opportunity to get more interesting research results in the near future.’

Docent Tuomas Savolainen works at Aalto University Metsähovi Radio Observatory, which is the only astronomical radio observatory and continuously operational astronomical observation station in Finland. Savolainen is the only Finnish member in the EHT, and he was engaged in analysing the huge amount of data used to produce the image of the black hole.