  Based on an observer-centric methodology, we pinpoint the basic origin of the
spectral Planckianity of the asymptotic Hawking modes in the conventional
treatments of the evaporating horizons. By considering an observer who analyzes
a causal horizon in a generic spacetime, we first clarify how the asymptotic
Planckian spectrum is imposed on the exponentially redshifted Hawking modes
through a geometric dispersion mechanism developed by a semiclassical
environment which is composed by all the modes that build up the curvature of
the causal patch of the asymptotic observer. We also discuss the actual
microscopic phenomenon of the Hawking evaporation of generic causal horizons.
Our quantum description is based on a novel holographic scheme of gravitational
open quantum systems in which the degrees of freedom that build up the
curvature of the observer's causal patch interact with the radiated Hawking
modes, initially as environmental quanta, and after a crossover time, as
quantum defects. Planckian dispersion of the modes would only be developed in
the strict thermodynamic limit of this quantum environment, called optimal
disperser, which is nevertheless avoided holographically. Finally, we outline
and characterize how our microscopic formulation of the observer-centric
holography, beyond the AdS/CFT examples and for generic causal patches, does
realize the information-theoretic processing of unitarity.
