(1+1)d collapsing null-shell? I am trying to understand the following Penrose diagram (from https://arxiv.org/abs/1507.03489)

According to the authors, it is depicting the formation of a (1+1)d black hole from a collapsing null shell. But to me it looks like it is simply a null ray, that moves from past minus infinity to future plus infinity. Why does a black hole form there?
 A: A collapsing null shell in $\text{(1+1)D}$ is two oncoming photons meeting each other in the center of the diagram. They move up in time on the diagram, one (the dotted line) from left to right; the other (the red line) from right to left. Once they meet, a black hole is formed initially at the intersection point (event) in the center of the diagram. Then the event horizon expands (as a continuation of the same two lines up on the diagram) at the speed of light. As the authors state, "the horizon is the future light cone of the [central] point". Therefore this black hole expands at the speed of light to consume more and more space until the entire space becomes an infinite line of the spacelike singularity (the wavy line) in the infinite future of external observers, but in a finite proper time of those falling through the horizon.
A black hole is formed, because, when the photons meet, they create a concentration of energy in a small space, similarly to a shell in $\text{(3+1)D}$ collapsing to its Schwarzschild radius. However note that simply writing the Einstein field equations in $\text{(1+1)D}$ does not work, as both the spacetime curvature and stress-energy tensor vanish. I am not sure what approach these authors employ, but often an
alternative theory of gravity is used in $\text{(1+1)D}$, for example, the direct $\text{(1+1)D}$ analog of a theory of gravity in $\text{(3+1)D}$ proposed by Nordstrøm in $\text{1913}$:
https://en.wikipedia.org/wiki/Nordstr%C3%B6m%27s_theory_of_gravitation
