Quasinormal Mode Shifts Induced by Regular-Core Structures in Black Holes

Description This record contains the manuscript “Quasinormal Mode Shifts Induced by Regular‑Core Structures in Black Holes” (May 2026). The study investigates how a finite‑size regular structure inside a black hole can produce small, localized deformations in the external effective potential governing gravitational‑wave perturbations. The work introduces two generic tendencies of internal structure—spreading (repulsive/delocalizing) and contracting (compressive/localizing)—and shows that their competition determines the shape of the potential deformation in a model‑independent way. Using first‑order perturbative quasinormal‑mode (QNM) theory, the manuscript evaluates how these deformations lead to small shifts in QNM frequencies. A numerical analysis based on Leaver’s continued‑fraction method demonstrates that the predicted frequency shifts fall within the sensitivity range of next‑generation gravitational‑wave detectors such as the Einstein Telescope and Cosmic Explorer. The results suggest that high‑precision ringdown spectroscopy may provide a new observational probe of possible internal structure in black holes. This version includes the full text, figures, and references. It is intended as a stable public record of the work and may be cited using the DOI assigned to this Zenodo entry. Keywords: black holes; quasinormal modes; gravitational waves; regular core; effective potential; ringdown spectroscopy; perturbation theory