The GW190521 Event: Echo Windows, Tidal Deformability, and Ringdown Signatures within the BRISM Core–Interface Framework

This work positions the BRISM (Brane Interface Substrate Model) framework as a structural and geometrically motivated approach for interpreting horizon-scale phenomenology in gravitational-wave observations. The model treats the measurement arena as a two-sided interface embedded within a Naimark–Stinespring dilation space, implying a semipermeable near-horizon layer. Under this assumption, several observationally relevant signatures arise, including cavity-like echo windows in the post-merger signal, finite tidal deformability characteristic of nonzero Love numbers, and ringdown modulations resulting from mode competition between photon-sphere and echo families. A structural coherence scale emerges naturally within the framework, representing a minimal interface resolution that plays a central role across BRISM applications. This scale has previously been shown to reproduce the infrared fine-structure constant with an internal accuracy at the 10⁻⁹ level (approximately 99.9999996%), illustrating the internal consistency of the model. Based on the same structural element, indicative windows for echo delays, tidal deformability, and cavity-enhanced ringdown damping are outlined. These expressions are presented as hypothesis windows compatible with the BRISM interface picture, and the gravitational-wave event GW190521 is used as an illustrative example for their potential application.