A Six-Parameter Teleparallel Cosmology Beyond $Λ$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 $Λ$CDM. A Markov Chain Monte Carlo (MCMC) analysis using Planck, DESI, and Type Ia supernova data yields a well-constrained transition redshift of $z_{\rm tr} \gtrsim 1.62$, accompanied by a transition in the effective equation of state to the phantom regime ($w < -1$) for $z < z_{\rm tr}$. 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 $Λ$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 $Λ$CDM in terms of the values of $χ^2$ of the bestfit parameters. We discuss the result among other issues related to CMB-BAO tension.