What is the inner horizon inside a Kerr black hole called?

• A. Event horizon.
• B. Cauchy horizon.
• C. Hawking horizon.
• D. Rindler horizon.

Answer: (B)

In a rotating (Kerr) black hole, spacetime contains two horizons: the outer event horizon and an inner boundary called the Cauchy horizon. The inner horizon sits inside the black hole and marks where the predictable evolution from initial data ceases to be guaranteed. Beyond it, the spacetime becomes non-globally hyperbolic, so information from outside cannot determine what happens next. In practice, infalling radiation gets enormously blue-shifted at this surface, causing instabilities known as mass inflation, which shows why classical predictions break down there.

This inner boundary is not the event horizon, and it isn’t the product of quantum effects like Hawking radiation, nor is it related to the Rindler horizon, which pertains to accelerating observers in flat spacetime.