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    Topological regular black holes without a Cauchy horizon

    Marco Calzá1,2,*, Massimiliano Rinaldi1,2,†, and Sergio Zerbini1,‡

    • 1Department of Physics, University of Trento, Via Sommarive 14, 38123 Povo (Trento), Italy
    • 2TIFPA-INFN, Trento, Via Sommarive 14, 38123 Povo (Trento), Italy

    • *Contact author: marco.calza@unitn.it
    • †Contact author: massimiliano.rinaldi@unitn.it
    • ‡Contact author: sergio.zerbini@unitn.it

    Phys. Rev. D 112, 024024 – Published 8 July, 2025

    DOI: https://doi.org/10.1103/dyy8-qjnd

    Abstract

    Regular and spherically symmetric black holes that solve the singularity problems of the Schwarzschild solution are phenomenologically viable at large distances but usually suffer from the Cauchy horizon instability. To overcome this drawback, we extended the analysis to include hyperbolic and toroidal horizon topologies within the framework of static, topologically maximally symmetric spacetimes. We show that both hyperbolic and toroidal black holes can be constructed without Cauchy horizons and without curvature singularities, thereby avoiding the mass inflation instability. These solutions exhibit asymptotic flatness in a generalized quasi-Minkowskian sense. The phenomenological aspects of these solutions are also studied by examining their thermodynamic properties, the photon sphere, and the effective potentials, ensuring consistency with observable properties such as black hole shadows. Lastly, we investigate a reconstruction technique within a scalar-tensor gravity framework, illustrating how the discussed metrics can arise from well-defined scalar field dynamics. Our investigation presents a viable pathway for constructing physically realistic, regular black holes in both general relativity and modified gravity, broadening the landscape of singularity-free spacetimes and offering models that may better reflect the nature of strong gravitational fields in astrophysical and cosmological settings.

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