The Causal Archipelago: Communication and Coexistence in a Light-Bounded Universe

This is the authors' abstract. We don't add key points for this paper.

For three decades, cosmic silence has been interpreted through Hanson's Great Filter and Bostrom's existential hazard hypothesis as an empirical warning of biological rarity or civilizational self-destruction. This deduction rests on an unexamined premise: treating civilizational longevity as synonymous with astronomical visibility. Recent work in theoretical astrobiology (incorporating percolation, steady-state settlement, and ecological thermodynamics) has established that interstellar expansion is non-homogeneous, leaving large uncolonized voids and favouring homeostatic equilibria over exponential growth. Building upon this synthesis, we formalize the physical-optical, relativistic, and epistemological constraints that enforce observational isolation, a framework termed the Causal Archipelago. Directed optical signalling requires prior coordinates because diffraction collimation partitions the sky into ∼10¹⁴ pointing cells, yielding a blind interception probability of ∼10⁻¹⁵. Within the galactic plane, star-planet contrast (C_opt ∼ 10⁻¹⁰) and dust extinction restrict atmospheric characterization to a spectroscopic horizon of d_obs ≤ 1,000 ly, obscuring >99.97% of the Milky Way. Combined with light latency (3d/c) and duty cycles (η ∼ 10⁻⁶), deliberate exchange fails across galactic distances for realistic lifespans (L ∼ 10³–10⁴ yr). Extending this analysis extragalactically via M/G/∞ queueing theory, we show that the Bayes factor relating silence to civilizational collapse is unity (Λ ≈ 1.0). Placing these physical bounds alongside Weinberg's clearing of teleological expectations reframes cosmic silence: far from signalling an empty universe or catastrophe, technological civilizations persist as an archipelago of autonomous, self-sufficient local worlds.