Figure 5

Figure 5: H is a dynamical horizon, foliated by marginally trapped surfaces S. a t is the unit time-like normal to H and ra the unit space-like normal within H to the foliations. Although H is space-like, motions along ra can be regarded as ‘time evolution with respect to observers at infinity’. In this respect, one can think of H as a hyperboloid in Minkowski space and S as the intersection of the hyperboloid with space-like planes. In the figure, H joins on to a weakly isolated horizon D with null normal a l at a cross-section S0.