A Six-Parameter Teleparallel Cosmology Beyond $\Lambda$CDM: Sign-Changing Torsional Dark Energy and Implications for $H_0$

Abstract

We investigate a particular case of the extended exponential infrared f(T) teleparallel gravity, in which the geometric sector naturally produces an effective dark-energy density that evolves from negative values in the past to positive values at late times. This behaviour could be motivated by observational results providing a compelling motivation for a geometric, sign-changing dark-energy scenario within modified gravity. We demonstrate that the parameter space of the present model contains only six parameters similar to $\Lambda$CDM. A Markov Chain Monte Carlo (MCMC) analysis using Planck, DESI, and Type Ia supernova data yields a well-constrained transition redshift of $z_{\mathrm{tr}}\gtrsim 1.62$, accompanied by a transition in the effective equation of state to the phantom regime ($w<-1$) for $z<z_{\mathrm{tr}}$. Since the effective dark energy originates from modified geometric degrees of freedom, no instabilities or violations of energy conditions arise. The model naturally accounts for the $H_0$ tension, where Planck+DESI data combination gives $H_0=72.13\pm 0.28{\text{ km s}}^{-1}{\text{ Mpc}}^{-1}$ in a better agreement with local measurements than $\Lambda$CDM which gives $H_0=68.46\pm 0.30{\text{ km s}}^{-1}{\text{ Mpc}}^{-1}$. However, the model is disfavored in comparison to $\Lambda$CDM in terms of the values of ${\chi }^2$ of the bestfit parameters. We discuss the result among other issues related to CMB-BAO tension.

24 pages,15 figures