A black hole is like a perfectly smooth sphere. It has no features — only mass, charge, and spin. This rule is called "no-hair." But nature loves to complicate things: sometimes a cloud of a scalar field (a special field described by a single number at each point) spontaneously appears around a black hole. This process, spontaneous scalarization, adds "hair" to the black hole.
For a long time, it was thought that such a cloud destroys the inner horizon — the Cauchy horizon. It acts like a one-way membrane: once you cross it, you can never return. But the authors of the new work have shown that destruction is not inevitable. It all comes down to how the electromagnetic field acts on the scalar field. Usually, this action is unidirectional, like the flow of a river. However, in some theories of nonlinear electrodynamics, it can change sign between the horizons, as if the river briefly flows backward.
Such a "reversal" allows the scalar field to coexist with the Cauchy horizon. The authors derived a general condition for this and pointed to candidate theories. Now we can study scalarization in the region between horizons and its connection with mass inflation — the avalanche-like growth of energy that usually destroys the Cauchy horizon.
The Cauchy horizon is named after the French mathematician Augustin-Louis Cauchy, who contributed to the theory of differential equations describing the evolution of physical systems over time.
In Michael Crichton's novel "Sphere," scientists encounter a mysterious object that might be linked to an alternate reality — something akin to what lies beyond the Cauchy horizon.