A new solution to Einstein's equations has been proposed, uniting the rotating Kerr black hole with Friedmann–Lemaître–Robertson–Walker cosmology. This metric reduces to the Kerr–de Sitter solution in the limit and describes the expansion of the universe both outside and inside the black hole. The model predicts that with cosmic expansion, the ergosphere (the region where rotation drags space) and the event horizon shrink relative to a stationary cosmic background. Interestingly, the ergosphere gradually disappears—as if a giant centrifuge is slowing down due to the stretching of the very fabric of reality.
Imagine an air mattress being stretched, with a whirlpool of water spinning on it. As you stretch, the whirlpool tightens—its size relative to the whole surface shrinks. That's exactly how a rotating black hole behaves in an expanding universe: its event horizon (the boundary of no return) and ergosphere (the twisted region of space) gradually shrink, even though the mass stays the same.
For the first time, physicists have mathematically combined the description of a rotating hole with the expanding cosmos model created by Friedmann and Lemaître. Previously, this was only possible for static holes. A surprising result: dark energy, discovered by Adam Riess, is indifferent to the furious spin—it continues to stretch spacetime without noticing the vortex. It's as if stretching the mattress doesn't care whether water is spinning on it or not.
🎯 The shrinking event horizon works like a patch on an air mattress: when the fabric stretches, the patch tightens but the thread stays intact.
🎬 Sci-fi writers imagine black holes as portals, but in this work, they are merely obedient objects, shrinking in the swelling cosmos.