Mass inflation and the $C^2$-inextendibility of spherically symmetric charged scalar field dynamical black holes

Abstract: 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 \textit{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 is $\mathcal{CH}{i^+}= \mathcal{D} \cup \mathcal{S}$ for two (possibly empty) disjoint connected sets $\mathcal{D}$ and $\mathcal{S}$ such that: _$\mathcal{D}$ (the dynamical set) is a past set on which the Hawking mass blows up (mass inflation scenario). _$\mathcal{S}$ (the static set) is a future set isometric to a Reissner--Nordstr\"{o}m Cauchy horizon i.e.\ the radiation is zero on $\mathcal{S}$. As a consequence, we establish a novel classification of Cauchy horizons into three types: dynamical ($\mathcal{S}=\emptyset$), static ($\mathcal{D}=\emptyset$) or mixed, and prove that $\mathcal{CH}{i^+}$ is globally $C^2$-inextendible. Our main motivation is the $C^2$ 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 the Einstein-Maxwell-Klein-Gordon system on one-ended space-times. As a result, we prove in spherical symmetry that: - two-ended asymptotically flat space-times are $C^2$-future-inextendible i.e. $C^2$ 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. - In the one-ended case, the Cauchy horizon emanating from time-like infinity is $C^2$-inextendible. This result suppresses the main obstruction to $C^2$ Strong Cosmic Censorship in spherical collapse.