On the first anniversary of the first black hole image comes research that could revolutionise the imaging of such objects and reveal secrets locked beneath their surface.

April 2020 marks the first anniversary of the image of the environment directly surrounding the black hole at the centre of the M87 galaxy —captured by the Event Horizon Telescope (EHT). In the twelve months since this spectacular achievement, researchers have not rested on their laurels. Several teams have been hard at work imagining and theorising ways to build a better black hole image.

Building a better black hole image ((Center for Astrophysics | Harvard & Smithsonian/ adjustments made by Robert Lea)

Just this week, as that anniversary approaches, researchers have published a study that promises to both improve the imaging of black holes but also increase the amount of information that can be derived from such images.

The team, led by scientists at the Center for Astrophysics, Harvard and Smithsonian (CfA), have calculated that an intricate substructure within black hole images could be revealed by extreme gravitational lensing — the phenomena by which objects with great mass cause the curvature of space and thus, bend the path of light travelling through it.

M87’s Black Hole: Lord of the Rings

The team conducted their study, published in the journal Science Advances, by examining the image of M87, in particular, the golden ring that dominates it. They note that this ring should contain a sub-structure of smaller rings, as predicted by the theory of general relativity, which the EHT was unable to resolve. And within this sub-structure of rings is information about the black hole in question.

When looking at the image of M87 it is impossible not to notice that it is dominated by a bright gold ring. Einstein’s theory of general relativity, which is used to predict the properties of black holes, says that within this bright ring there should be a ‘photon ring’ which is composed of a sequence similar sub-rings.

“The image of a black hole actually contains a nested series of rings,” explains Michael Johnson of the CfA. “Each successive ring has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer.

“With the current EHT image, we’ve caught just a glimpse of the full complexity that should emerge in the image of any black hole.”

The image of a black hole has a bright ring of emission surrounding a “shadow” cast by the black hole. This ring is composed of a stack of increasingly sharp subrings that correspond to the number of orbits that photons took around the black hole before reaching the observer (George Wong (UIUC) and Michael Johnson (CfA))

The reason black holes have been so difficult for astronomers to spot relates to the quality through which they earned their moniker in the first place. At the edge of a black hole exists a boundary known as the ‘event horizon’ this is the point at which its gravitational influence on space is so extreme that not even photons can escape its pull.

This trapping of photons means the black hole casts a shadow on the bright emission of the gas and dust that surrounds it, gradually falling onto its surface. Around this shadow is a ring of photons produced by the strong gravitational effect outside the event horizon in the vicinity of the black hole. Trapped, but still circling the object.

Black holes cast a shadow on the image of bright surrounding material because their strong gravitational field can bend and trap light. The shadow is bounded by a bright ring of light, corresponding to photons that pass near the black hole before escaping. The ring is actually a stack of increasingly sharp subrings, and the n-th subring corresponds to photons that orbited the black hole n/2 times before reaching the observer. This animation shows how a black hole image is formed from these subrings and the trajectories of photons that create the image. (Center for Astrophysics | Harvard & Smithsonian)

This photon ring contains characteristic information about the black hole — its size, shape, its angular momentum or spin — and thus can be used as a tool to study the black hole itself. Perhaps, an even more stunning revelation about these rings is that general relativity tells us that each ring is composed of trapped photons that together represent a picture of the Universe as seen from the face of the black hole.