We study the cosmology of ghost-free Hassan-Rosen bimetric gravity under the restrictive ansatz betaₙ = beta₄-₍, which we refer to as the discrete exchange symmetry g f. This ansatz coincides, in the case Mg = Mf, with the "symmetric models" introduced by Hassan, Schmidt-May and von Strauss (arXiv: 1407. 2772; special case alpha = 1 of their condition alpha^ (4-n) betaₙ = alphaⁿ beta₄-₍). It defines a predictive three-parameter subclass of bigravity. Whereas Hassan-Schmidt-May-von Strauss investigated the implications of this ansatz for the massive-gravity limit, we explore here its late-time cosmological phenomenology and identify two consequences that we believe to be new in this generality: (i) a universal linear-order CPL constraint wₐ = -3 (1+w₀) along trajectories converging to the de Sitter attractor at xi -> 1, and (ii) the universality of the linearised eigenvalue lambda = -3H of the FLRW background at xi = 1, independent of the bigravity parameters. These results bear not on the ansatz itself but on the constraints that dust-matter conservation and late-time dynamics impose upon it. We position the analysis explicitly as a late-time effective field theory valid in the post-recombination regime. The framework does not resolve the cosmological-constant problem: it shifts the fine-tuning from a direct adjustment of Lambda to an adjustment of the primordial deviation |xiCMB - 1| ~ 10^-11. This record contains: - HRₜheoremᵥ1. 17. pdf (English manuscript, 22 pages) - HRₜheoremᵥ1. 17FR. pdf (French translation, 15 pages) - HRₜheoremᵥ1. 17. tex / HRₜheoremᵥ1. 17FR. tex (LaTeX sources) - B5basinₛcan. py (ancillary: reproduces Appendix E symbolic + numerical scan, seed 42)
lakhdar sabri (Sat,) studied this question.
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