Integrable singularities obstruct Cauchy horizons and mass inflation

Abstract

Black-hole interiors with an inner (Cauchy) horizon are unstable through the Poisson-Israel-Ori mass-inflation mechanism, governed by the Dray-'t Hooft-Redmount relation and widely regarded as a dynamical realization of strong cosmic censorship. We show that static, spherically symmetric anisotropic interiors supported by a marginally divergent but integrable density $ρ\sim A/r^2$, with $A>1/(8π)$ and mild screening at finite radius, are incompatible with this mechanism. The compactness $q=m/r$ decreases monotonically from a super-critical central value to $1/2$ at the unique event horizon, producing a strictly positive compactness gap throughout the trapped region and excluding any second zero of the lapse. This gap enters the dynamics quantitatively. (i) Under the null energy condition and a dynamical screening condition, $2m/r$ is non-decreasing along both future null directions of a trapped domain, so the gap propagates under nonlinear spherically symmetric evolution. (ii) For ingoing radiation (generalized Vaidya), the gap bounds the blueshift: the advanced time spent in the gapped region is finite, whereas it diverges at a Cauchy horizon. (iii) For counter-streaming null dust, the radiative part $μ$ of the Misner-Sharp mass obeys exactly $\partial_u\partial_vμ=2\,\partial_uμ\,\partial_vμ/(r|f|)$, of which the DTR relation is the thin-shell form; the in-out coupling $2/(r|f|)$ is uniformly bounded on the interior precisely when the gap is positive. This yields an a priori mass bound and, in the Ori thin-shell model, an e-folding rate bounded by the influx luminosity instead of tending to the inner-horizon surface gravity. Integrable-singularity interiors thus realize a class of sources for which the Poisson-Israel-Ori amplification channel is structurally inoperative in the models in which mass inflation was originally established.