Boundary as Unified Stage: From Time Translation Operator, Null--Modular Double Cover to GHY Boundary Term
Haobo Ma, Wenlin Zhang
Abstract
Construct unified framework with boundary as sole fundamental stage, gluing three mature but usually separate structures as three projections of same object: (i) Based on Birman--Krein--Friedel--Wigner--Smith framework, ``time translation operator'' characterized by scattering phase, spectral shift function, Wigner--Smith time delay matrix; (ii) Based on Tomita--Takesaki modular theory and recent results on causal diamond/null surface modular Hamiltonians, Markov property, QNEC, Null--Modular do