A black hole's shadow isn't just a black circle. Usually, we see a dark void surrounded by a glowing ring of hot gas. But if a black hole wasn't born from a dying star but has existed forever, its shadow might hide a surprise.
The authors took such an eternal black hole and added a thin disk of gas — the same kind that swirls around ordinary black holes. But the key move: they replaced the singularity in the past (the point where physics breaks down) with a surface that radiates heat, like a hot stove. This surface sits a tiny distance from the center — where, according to Markov's hypothesis, the density of matter stops growing.
Light rays that fall into the black hole and travel backward in time hit this hot surface. For a distant observer, they create a bright spot right in the middle of the shadow. If the surface is very close to the singularity, the spot's center turns bluish — light shifts toward blue due to gravity. If it's farther out, the spot reddens and dims.
The scientists even figured out how to look for such a spot using radio interferometers. Usually, the brightness of the disk and photon ring falls off with distance as a power law. But for the spot, the falloff is a pure exponential. It's like a fingerprint: if you see that signal, you can immediately say — something from the past is shining there.
If our universe were an eternal black hole, at the center of its shadow we'd see light from the Big Bang, gravitationally shifted toward blue.
In Bob Shaw's novel "Light of Other Days," people use slow-glass to see the past — here, the past itself shines at us from the depths of a black hole.