Redshift, Horizons, and Vacuum as Limits of Null Accessibility

This work introduces an operational framework in which redshift, horizons, and vacuum structure emerge as limits of null accessibility rather than geometric primitives. Frequency, vacuum response, and horizon formation are unified under causal accessibility constraints imposed by finite signal propagation. The approach provides a physically transparent reinterpretation of relativistic redshift and horizon entropy, emphasizing operational limits over spacetime expansion or coordinate-dependent geometry. Keywords Causality; Strain; Finite Signal Speed; Redshift; Horizons; Vacuum Structure; Operational Physics