PART 1
First Part
Chapter 1
A short scrapbook on classical black holes
There is not a general definition of a black hole. The intuitive idea is that a black hole appears when there is a trapping region from which nothing can escape, bounded by an horizon, the event horizon. In this chapter we will recall the definition of black holes in a particular class of spacetimes, which are asymptotically flat and with a far future predictable from Cauchy surfaces, as introduced by Hawking and Ellis [Hawking and Ellis (2011)]. Next, we will illustrate the classical “physical” examples of Schwarzschild and Kerr black holes. One could expect that, given the present knowledge of cosmological data, the de Sitter versions of such solutions would be more physical. These can also be considered, but, then, one should include black holes in a eventually closed expanding universe. To our knowledge, no rigorous definition of such a black hole, generalizing the one of Hawking and Ellis, exists in the last class of spacetimes. Thus, in more general situations the intuitive definition of a black hole must be sufficient. In all the explicit known examples of black hole, the event horizon is also a Killing horizon. Since some properties of Hawking radiation (and more general phenomena) are related to this fact, we will end this chapter by defining Killing horizons and illustrating some of their properties.
1.1Mathematical black holes
Following [Hawking and Ellis (2011)], we now will introduce a rigorous mathematical definition of a black hole. The main purpose is to collect some of the mathematical tools which will allow to get some exact results as illustrated in chapter 7. This general definition may be easily extended to the cases in presence of electromagnetic fields end/or a cosmological constant, but not to all possible physically interesting spacetimes, so the reader not interested in mathematically exact results can skip this section.
A D-dimensional spacetime is a D-dimensional smooth manifold M, endowed with a Lorentzian metric g. We will indicate it with a pair (M, g). It is said to be time orientable if it admits a continuous vector field everywhere non-spacelike (i.e. never spacelike) and future directed. In an orientable spacetime, a non-spacelike curve is future (past) directed if its tangent vector is everywhere future (past) directed. Given two subsets A, B ⊂ M, one defines
•the chronological future of A relative to B as the set I+(A, B)of all points of B that can be reached from A along future directed timelike curves;
•the chronological past of A relative to B as the set I−(A, B) of all points of B that can be reached from A along past directed timelike curves;
•the causal future of A relative to B as the set J+(A, B) obtained from the union of A ⋂ B to all points of B that can be reached from A along future directed non-spacelike curves;
•the causal past of A relative to B as the set J−(A, B) obtained from the union of A ⋂ B to all points of B that can be reached from A along past directed non-spacelike curves.
In particular, I±(A):= I± (A, M) and J±(A):= J±(A, M).
A set
A is said to be a
future set if it properly contains its chronological future,
I+(
A) ⊂
A. A is said to be
achronal if
A ⋂
I+(
A) =
. It happens that the boundary of a future set is an achronal (
D − 1)-dimensional
C1 manifold. A spacetime is said to satisfy the
chronological condition if it does not contain closed time-like curves. It satisfies the
causality condition...