Early- and Late-Time Modifications to $\hat{I}›$CDM: Implications for the Hubble Tension

Abstract

We investigate an extension of $\hat{I}›$CDM in which a fraction of cold Dark Matter (DM) decays into invisible dark radiation (DR) around the radiation-matter equality epoch, together with a non-standard dark energy (DE) equation of state characterized by $w_0$. The decaying DM component modifies the early expansion history and reduces the sound horizon at baryon drag, while the DE alters the expansion rate at the late times. A comprehensive analysis combining \texttt{Planck 2018+ACT DR6+DESI DR2+CMB lensing} datasets has been carried out to explore the viability of this framework in addressing the $H_0$ tension. This model yields a Hubble constant of $H_0=69.83\pm 0.98\mathrm{km}{\mathrm{s}}^{-1}\mathrm{Mp}{\mathrm{c}}^{-1}$, reducing the discrepancy with SH0ES measurement to $\sim 2.2\ddot{I}ƒ$ and local distance network measurement (H0DN) to $\sim 2.9\ddot{I}ƒ$. Further, considering \texttt{SH0ES} and \texttt{Pantheon+}, the inferred value of the Hubble constant becomes $H_0=70.20\pm 0.66\mathrm{km}{\mathrm{s}}^{-1}\mathrm{Mp}{\mathrm{c}}^{-1}$. The Bayesian evidence suggests that this framework offers a fit to the relevant cosmological datasets at a statistically similar level as $\hat{I}›$CDM. It is observed that correlated early- and late-time modifications to the cosmological expansion history provide a more effective route to reducing the $H_0$ tension than either class of modification alone.

31 pages, 4 figures, 7 tables (Including Appendix)