Cosmic Birefringence and the Governance of Contested Rotation Signals: A TRIDENT Case Analysis

Cosmic birefringence has been reported as a possible rotation of the polarisation plane of the cosmic microwave background, with interpretations ranging from parity-violating new physics to unresolved instrumental systematics. This work does not advance a physical claim. Instead, it applies the TRIDENT governance architecture to structure how such contested signals should be evaluated. The case analysis separates hypothesis classes into physical-mechanism candidates and systematic-origin candidates, preventing conflation between new-physics interpretation and calibration artefact. An ordered discriminator is constructed with explicit tier precedence: cross-instrument consistency, split stability, and frequency scaling. The evaluation logic enforces that lower-tier indicators cannot override ambiguity in higher-tier tests. A four-state classification output is formalised: PHYSICAL_CANDIDATE, SYSTEMATIC_CANDIDATE, INDETERMINATE, and MIXED. The MIXED state is treated as a legitimate outcome rather than a transitional weakness, and a non-elevation rule prevents ambiguous results from being operationally reinterpreted as physical candidates without new Tier-1 or Tier-2 evidence. A re-entry condition defines the only valid upgrade path. Adversarial simulation confirms that under realistic partial-agreement scenarios the system returns honest ambiguity rather than forced resolution. This demonstrates portability of the governance spine previously applied to dynamical inference (Palomar 5) into a domain characterised by contested signal detection and instrumental systematics. The contribution is architectural rather than theoretical: it provides a disciplined evaluation framework for high-sensitivity cosmological rotation claims, preserving falsifiability while preventing threshold creep and interpretive drift. Keywords Cosmic birefringence CMB polarisation rotation Systematics propagation Hypothesis-class separation Ordered discriminator logic Multi-state classification Governance architecture TRIDENT framework Scientific falsifiability Ambiguity handling