Notes on the Area Theorem

Abstract

Hawking's area theorem can be understood from a quasi-stationary process in which a black hole accretes $positive$ energy matter, ``independent of the details of the gravity action''. I use this process to study the dynamics of the ``inner'' as well as the outer horizons for various black holes which include the recently discovered exotic black holes and three-dimensional black holes in higher derivative gravities as well as the usual BTZ black hole and the Kerr black hole in four dimensions. I find that the area for the inner horizon ``can decrease'', rather than increase, with the quasi-stationary process. However, I find that the area for the outer horizon ``never decrease'' such as the usual area theorem still works in our examples, though this is quite non-trivial in general. There exist the instability of the inner horizons and the connected effects of ``mass inflation'' but, according to more detailed analysis, it seems that the instability is not important in my analysis. I also find a ``generalized'' area theorem by combining those of the outer and inner horizons.