Universality in the quantum nature of spacetime in Wheeler-DeWitt black holes

The essential singularity in the interior spacetime of a black hole occurs as a vacuum solution of the Einstein field equation. This singularity emerges as a result of treating the spacetime classically, and it can be resolved upon treating the spacetime quantum mechanically with simple metric ansatzes. This leads to the obvious question: Is the singularity resolution a universal feature with respect to arbitrariness of the metric ansatz? In this paper, we address this question by representing the black hole interior with a generalized Kantowski-Sachs metric characterized by two inbuilt arbitrary parameters, in addition to including a Klein-Gordon field. We quantize this system and solve the resulting Wheeler-DeWitt equation exactly to obtain a general form of the wave function in the black hole interior. Upon analyzing this wave function in light of the DeWitt boundary condition, three types of solutions emerge exhibiting universal behavior in the vicinity of the singularity with respect to the arbitrary metric parameters. In two of those types of solutions, regular quantum black holes would always occur as a universal feature. Within the same context, the third type of solutions, having no regular quantum black holes, must also occur as another universal feature. Importantly, these wave functions are found to carry clear imprints of the arbitrary metric parameters, discernible prominently upon moving away from the singularity, and their near-singularity behaviors remain universal with respect to the arbitrariness in the metric ansatz. This is an emergent new feature of quantum gravity irrespective of how the metric ansatz is made. Furthermore, we show how the regularity conditions for the expectation values of the Kretschmann operator, as well as other relevant operators, carry imprints of the arbitrary metric parameters.