When a giant star runs out of fuel, it collapses into a black hole. Such a hole has an outer boundary — the event horizon, beyond which even light cannot escape. But many black holes also have an inner boundary — a second horizon that separates the outer layers from the singularity core. For a long time, it was believed that any such inner horizon is doomed: the slightest disturbance triggers an avalanche — energy piles up like a snowball, and spacetime tears apart. This is known as the classical instability of mass inflation.

However, a team of physicists led by Roberto Peccei discovered a subtlety. They considered not an eternal black hole but a realistic scenario: a black hole forms from collapsing matter and eventually evaporates due to quantum effects — Hawking radiation. In this picture, the inner horizon ceases to be a "Cauchy horizon" — a boundary beyond which the predictions of physics break down. Without this status, quantum fields behave much more calmly: their energy does not blow up to infinity but remains finite everywhere.

But the most interesting part is the difference between ordinary and "extreme" inner horizons. For ordinary ones, energy still grows — exponentially, like a chain reaction, but only while the hole exists. For extreme horizons, where surface gravity is zero, the growth becomes power-law — much more gradual. Such a hole can be compared to a boiler where pressure builds up not explosively but slowly, like in a cooling teapot.

This means that the interior of real black holes may be far more stable than old models suggested. The inner horizon does not necessarily tear spacetime apart — it can persist for quite a while, giving hope that an infalling observer will not meet an instant singularity but will see something more intricate.