ACT-Era Constraints on Single-Field Inflation in $f(T)$ Teleparallel Gravity

TL;DR

This paper investigates how modifications in $f(T)$ teleparallel gravity affect single-field inflation predictions, showing that torsional corrections can reconcile certain models with recent CMB observations.

Contribution

It provides a detailed analysis of how torsional corrections in $f(T)$ gravity influence inflationary observables, expanding the viability of models disfavored in general relativity.

Findings

Torsional corrections suppress the tensor-to-scalar ratio $r$.

Positive $ ext{delta}$ can restore viability to sub-quadratic models.

E-type plateau models remain compatible only within limited $ ext{delta}$ ranges.

Abstract

We reassess single-field slow-roll inflation in teleparallel gravity with $f(T)=C\,T^{2\delta+1}$, motivated by recent measurements from the Atacama Cosmology Telescope (ACT) that indicate a modest upward shift in the scalar spectral index $n_s$. Using analytic approximations together with high-precision numerical calculations, we compute primordial predictions for representative potentials: power-law monomials, hilltop models, and $E$-type plateaus. We find that torsional corrections controlled by $\delta$ generically suppress $r$ while keeping $n_s$ near its slow-roll value. As a result, modest positive $\delta$ can restore viability to sub-quadratic monomials and hilltop models that are disfavored in general relativity (GR), whereas $E$-type plateaus remain compatible only in a limited range of $\delta$: small $\delta$ may improve the fit but moderate $\delta$ drives the dynamics toward quadratic-like behaviour and increases $r$. These signatures are observationally testable: improved cosmic microwave background (CMB) B-mode measurements will further discriminate among potential classes and place quantitative bounds on torsional deviations from GR.