CPT-Symmetric Two-Sector Dynamics within Ghost-Free Bimetric Cosmology

Abstract: We investigate a CPT-symmetric two-sector configuration within ghost-free Hassan–Rosen bimetric gravity, in which each matter sector couples minimally and exclusively to its own metric. The construction does not modify the underlying dRGT action and preserves positive-definite stress-energy tensors in both sectors. Effective cross-sector gravitational repulsion arises as an emergent macroscopic feature of the bimetric interaction potential, rather than from the introduction of negative inertial mass or violations of local energy conditions. Imposing an exact CPT-symmetric initial state enforces a proportional FLRW background with matched Hubble rates and energy densities in the early Universe. On this branch, the linear spectrum decomposes into massless and massive spin-2 modes with algebraically fixed effective mass, allowing the Higuchi condition to be satisfied within viable parameter space and suppressing early-time scalar instabilities during the radiation-dominated epoch. Deviations from exact proportionality arise only at late times during nonlinear structure formation. At low redshift, the framework predicts a quantitative phenomenological signature. Within parameter ranges consistent with background expansion constraints, the modified Poisson structure generically yields an O(10%) enhancement in the radial slope of the weak-lensing convergence profile κ(r) at the compensation radius of cosmic voids relative to ΛCDM expectations. This provides a falsifiable observable for stacked void lensing surveys.The scenario is presented as a speculative phenomenological extension within established ghost-free bimetric gravity. It does not claim observational confirmation, but offers a self-consistent construction together with empirically testable consequences.