A Ratio-Preserving Approach to Cosmological Concordance

Cosmological observables are particularly sensitive to key ratios of energy densities and rates, both today and at earlier epochs of the Universe. Well-known examples include the photon-to-baryon and the matter-to-radiation ratios. Equally important, though less publicized, are the ratios of pressure-supported to pressureless matter and the Thomson scattering rate to the Hubble rate around recombination, both of which observations tightly constrain. Preserving these key ratios in theories beyond the Λ Cold-Dark-Matter (ΛCDM) model ensures broad concordance with a large swath of datasets when addressing cosmological tensions. We demonstrate that a mirror dark sector, reflecting a partial Z₂ symmetry with the Standard Model, in conjunction with percent level changes to the visible fine-structure constant and electron mass which represent a phenomenological change to the Thomson scattering rate, maintains essential cosmological ratios. Incorporating this ratio preserving approach into a cosmological framework significantly improves agreement to observational data (Δχ²=-35.72) and completely eliminates the Hubble tension with a cosmologically inferred H₀ = 73.80 ± 1.02 km/s/Mpc when including the SH₀ES calibration in our analysis. While our approach is certainly nonminimal, it emphasizes the importance of keeping key ratios constant when exploring models beyond ΛCDM.