Redshift, Horizons, and Vacuum as Limits of Null Accessibility

Arc I — Accessibility Paper 2 Title Redshift, Horizons, and Vacuum as Limits of Null Accessibility Zenodo DOI 10.5281/zenodo.18180776 Description This paper reformulates gravitational redshift, vacuum structure, and horizon formation in purely operational terms, without presupposing spacetime geometry. Redshift is interpreted not as a geometric stretching of wavelengths, but as a consequence of degraded null accessibility between emission and reception events. By treating null signal exchange as the primitive coordination mechanism, the paper shows that redshift arises whenever finite signal speed and finite record resolution prevent perfect synchronization across regions. Horizons appear as limits of null accessibility: boundaries beyond which coordination fails irreversibly, independent of metric singularities or causal discontinuities. The vacuum is redefined operationally as the limiting medium that saturates null accessibility constraints. Rather than being empty or structureless, vacuum is characterized by the maximal achievable coordination consistent with finite signal speed and bounded inference capacity. This framework unifies redshift, time dilation, and horizon phenomena under a single operational principle and establishes horizons as accessibility failures rather than geometric entities. Keywords gravitational redshift; horizons; vacuum structure; null signals; operational causality; accessibility limits Notes Arc I — Accessibility (operational foundations) Redshift derived without spacetime geometry Horizons as null-accessibility boundaries Vacuum defined as maximal coordination medium Conceptual precursor to operational horizon theory