Mass Inflation and the C² -inextendibility of Spherically Symmetric Charged Scalar Field Dynamical Black Holes

It has long been suggested that the Cauchy horizon of dynamical black holes is subject to a weak null singularity, under the mass inflation scenario. We study in spherical symmetry the Einstein–Maxwell–Klein–Gordon equations and while we do not directly show mass inflation, we obtain a “mass inflation/ridigity” dichotomy. More precisely, we prove assuming (sufficiently slow) decay of the charged scalar field on the event horizon, that the Cauchy horizon emanating from time-like infinity CHi+ can be partitioned as CHi+=D∪S for two (possibly empty) disjoint connected sets D and S such thatD (the dynamical set) is a future set on which the Hawking mass blows up (mass inflation scenario).S (the static set) is a past set isometric to a Reissner–Nordström Cauchy horizon i.e. the radiation is zero on S. As a consequence of this result, we prove that the entire Cauchy horizon CHi+ is globallyC2-inextendible̲, extending a previous local result established by the author. To this end, we establish a novel classification of Cauchy horizons into three types: dynamical (S= ∅), static (D= ∅) or mixed. As a side benefit, we prove that there exists a trapped neighborhood of the Cauchy horizon, thus the apparent horizon cannot cross the Cauchy horizon, which is a result of independent interest. Our main motivation is to prove the C² Strong Cosmic Censorship Conjecture for a realistic model of spherical collapse in which charged matter emulates the repulsive role of angular momentum. In our case, this model is the Einstein–Maxwell–Klein–Gordon system on space-times with one asymptotically flat end. As a consequence of the C²-inextendibility of the Cauchy horizon, we prove the following statements, in spherical symmetry: 1.Two-ended asymptotically flat space-times are C²-future-inextendible i.e. C² Strong Cosmic Censorship is true for Einstein–Maxwell–Klein–Gordon, assuming the decay of the scalar field on the event horizon at the expected rate.2.In the one-ended case, under the same assumptions, the Cauchy horizon emanating from time-like infinity is C²-inextendible. This result suppresses the main obstruction to C² Strong Cosmic Censorship in spherical collapse. The remaining obstruction in the one-ended case is associated to “locally naked” singularities emanating from the center of symmetry, a phenomenon which is also related to the Weak Cosmic Censorship Conjecture.