Emergent Plasma–Radiation Equilibrium as a Viable Origin for the Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is traditionally interpreted as a relic radiation field originating from an early hot phase of the universe. While this interpretation is highly successful, the physical viability of alternative mechanisms—particularly those involving long-term plasma–radiation interaction—has remained an open question. This work presents a global, constraint-driven plasma–radiation simulation designed to test whether a CMB-like background can arise naturally as a stable equilibrium state of radiating intergalactic plasma, without imposing a primordial relic radiation field, a hot initial condition, or a pre-set background temperature. The system is treated as an open, inhomogeneous intergalactic plasma dynamically coupled to radiation through self-consistent radiative transfer, ionisation–recombination processes, non-equilibrium energy exchange, and entropy flow. Rather than enforcing a single cosmological trajectory, large ensembles of physically admissible histories are evolved over extended timescales that may exceed the currently inferred cosmic age. Across thousands of admissible histories, the system consistently converges toward a stable equilibrium radiation temperature close to the observed CMB value (~2.7 K), despite no temperature or spectral form being imposed. Attempts to stabilise at substantially lower temperatures fail under physically identifiable constraints, typically through spectral instability or breakdown of global causal coherence. These results do not challenge or replace the standard cosmological model. Instead, they demonstrate that a plasma-equilibrium origin for a CMB-like background is physically viable, dynamically stable, and naturally constrained in extended-timescale or old-universe frameworks. The observed CMB temperature emerges as a lower-bound attractor of the plasma–radiation system, rather than as an arbitrary or finely tuned parameter.