Figure 5: is a dynamical
horizon, foliated by marginally trapped surfaces . is the unit time-like normal to and the unit space-like normal
within to the
foliations. Although is
space-like, motions along can be regarded as ‘time evolution with respect to
observers at infinity’. In this
respect, one can think of as a
hyperboloid in Minkowski space and as the intersection of the hyperboloid with space-like
planes. In the figure, joins
on to a weakly isolated horizon
with null normal at a cross-section .
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