Entropic route to Brown-York tensor: A unified framework for null and timelike hypersurfaces
Phys. Rev. D 114, 064083 – Published 28 September, 2026
DOI: https://doi.org/10.1103/9rb4-jp96
Abstract
Building on Padmanabhan’s entropy functional, originally introduced to derive Einstein’s equations and highlight the emergent nature of gravity, we demonstrate its robustness in a broader context. Using the same entropy density, we show that the Brown-York (BY) tensor is associated with a tangential projection of the entropy-functional polymomentum, thereby providing a common construction applicable to both timelike and null hypersurfaces. This perspective places the two (null and timelike) cases on the same variational footing and clarifies the structural differences of the null Brown-York tensor, including its generally nonsymmetric character, without resorting to separate or ad hoc prescriptions in the formulation. We further extend the analysis to scalar-tensor theories, showing that the entropy-based formulation reproduces the expected equations of motion along with the corresponding BY tensor. Our results provide a coherent variational interpretation of quasilocal gravitational quantities and reveal a common underlying structure linking bulk dynamics and boundary momentum.
