The Universe Force & Mirror Universe Unified Theory: A Scalar–Tensor Braneworld Background Cosmology Framework (Version 2.1)

This work presents Version 2.1 of the Universe Force & Mirror Universe Unified Theory, reformulated as a scalar–tensor braneworld cosmology framework. The model replaces earlier speculative constructions with a mathematically consistent effective four-dimensional description based on a dynamical radion field representing the separation between two branes embedded in a higher-dimensional bulk. The radion evolves under a stabilizing potential and mediates interactions between the observable Universe and a hidden mirror sector. A localized field-space resonance associated with a Kaluza–Klein threshold crossing generates a transient Early Dark Energy (EDE) component during the pre-recombination epoch. This mechanism can reduce the cosmological sound horizon while naturally decaying at late times, providing a potential framework for investigating the Hubble tension. Numerical background-level simulations demonstrate stable radion relaxation, localized EDE generation, and recovery of cosmological-constant-like behavior at late times. Scientific Status: This work is a phenomenological background-cosmology framework and has not yet been tested against Cosmic Microwave Background anisotropies, Baryon Acoustic Oscillations, weak-lensing observations, or large-scale structure data. Linear perturbation theory, effective action derivations, and Einstein–Boltzmann analyses remain future work. Therefore, this manuscript should be considered an exploratory theoretical proposal rather than an observationally validated cosmological model. AI-assisted drafting, editing, and mathematical presentation tools were used during manuscript preparation. Version Notes (V2.1) Major revision of the theoretical framework, replacing earlier phenomenological Early Dark Energy (EDE) implementations with a radion-resonance braneworld mechanism and improved background cosmology treatment. This version introduces a stabilized radion potential, dynamic field-space resonance framework, corrected sound horizon calculations, and improved cosmological consistency relative to previous releases.