CPT-Symmetric Two-Sector Cosmology with Gravitational Relaxation: A Minimal Late-Time Effective Model

This preprint presents a minimal effective cosmological model based on two CPT-related matter sectors that share a common FLRW metric and interact only through gravity via a weak relaxation term. At the background level the model is degenerate with ΛCDM and reproduces the observed dark matter abundance (Ω₋ ≈ 5 Ω₊). At the linear perturbative level it predicts a damped isocurvature mode controlled by a single dimensionless parameter α, leading to percent-level deviations in the growth rate fσ₈(z) while remaining consistent with primary CMB constraints. The cosmological bounce mechanism is not modeled in this effective formulation and is left as an open problem requiring physics beyond canonical General Relativity. The model is falsifiable with upcoming large-scale structure surveys (Euclid, DESI, CMB-S4): α = 0 recovers ΛCDM exactly, while α > 0 implies observable deviations in structure growth. The framework aims to provide a testable, minimal alternative to ΛCDM that incorporates global CPT symmetry and gravitational coupling between sectors without additional fields or modified gravity at the fundamental level. Future work will address the bounce mechanism, microscopic origin of the relaxation term, full thermodynamic consistency, and detailed observational forecasts. Related documents and the full RDCC ecosystem are available via Zenodo:https://zenodo.org/records/18204087 Michael Lehmannmi.lehmann@gmx.de