What's that ring that only now appears in black hole illustrations?
If you'll notice, space time starts to deform near the black hole, but then at considerable distance from the black hole' surface, images in the background become ultra-distorted, but then as you look even closer to the black hole, the distortion starts to look normal again almost like nothing is happening except for a minor fish-eye lens effect.
If space-time distorts in a monotonic way as you approach the surface of a black hole, how does this ring of ultra-distortion exist? Why doesn't the image look even more distorted at the surface than at that ring?
This is due to photons actually going in circles around the black hole multiple times.
Far away from the BH, the image is only slightly distorted. These are photons that deviated only a little bit.
As you get closer, you're seeing photons that deviated more and more. At some point, you'll see photons that came from behind you, went around the BH, and hit your eye. Eventually, close enough to the BH, you'll see photons that did a full 360 degree turn. The BH acts like a fisheye camera.
Looking closer and closer to the disk, you get photons that did 1.5 turns, 2 turns, 3 turns and so on.
What you see is concentric ring images, each one of them containing the full picture of everything around the BH but compressed in a ring. There's essentially an infinity of concentric images, but you only see clearly the first few of them.
It should be noted that the black disk in the middle is not the event horizon. It's simply a region in space where no light comes from that could reach your eye. The actual event horizon is about 2x smaller. You could think of it as the image of the event horizon "magnified" by the spacetime distortion, but the analogy is not perfect.
A black hole is, in many ways, like a very strange "lens".
Why is the black region 2 × the one delimited by the horizon? Why is it bkack if light can escape or move around?
@Alchimista Any photon that could hit your eye coming in from that direction would have been swallowed by the black hole to begin with. Those are trajectories that are simply too close to the BH.
Or is light that doesn't sink in the hole but is still bent enough no to arrive at my eyes?
@Alchimista Try reading this sourc. The answer basically comes down to the fact that outside the Event Horizon is something known as a photon sphere where photons physically can, but are very unlikely to escape the black hole, making that region seem dark as well.