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 $\mathrm{\ensuremath{\Lambda}}$ Cold-Dark-Matter ($\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$) model ensures broad concordance with a large swath of datasets when addressing cosmological tensions. We demonstrate that a mirror dark sector, reflecting a partial ${\mathbb{Z}}_{2}$ symmetry with the Standard Model, in conjunction with percentage 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 ($\mathrm{\ensuremath{\Delta}}{\ensuremath{\chi}}^{2}=\ensuremath{-}35.72$) and completely eliminates the Hubble tension with a cosmologically inferred ${H}_{0}=73.80\ifmmode\pm\else\textpm\fi{}1.02\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ when including the $\mathrm{S}{H}_{0}\mathrm{ES}$ calibration in our analysis. While our approach is certainly nonminimal, it emphasizes the importance of keeping key ratios constant when exploring models beyond $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$.