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arXiv · 2508.16530

Reconciling the ACT Preference in $f(T)$ Gravity: Inflation and Reheating Constraints

Feng-Yi Zhang·Rongrong Zhai·Li-Yang Chen
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Abstract

Compared with the results of Planck-only analyses, recent measurements from the Atacama Cosmology Telescope (ACT) indicate a preference for a slightly bluer scalar spectral index, placing canonical inflationary models in General Relativity (GR) under mild pressure. We demonstrate that $f(T)$ gravity systematically accommodates these dataset-dependent preferences by suppressing the tensor-to-scalar ratio in monomial and hilltop potentials, and by shifting the spectral index of E-models toward the ACT-favored region. Incorporating Big Bang Nucleosynthesis bounds, we break the degeneracy between the inflationary $e$-folding number and the post-inflationary thermal history. A direct side-by-side comparison reveals that reconciling models such as the Starobinsky potential with ACT data in GR strictly necessitates a non-standard, stiff (kinetic-dominated) reheating phase. In contrast, torsional corrections in $f(T)$ gravity significantly enlarge the viable parameter space, relaxing these stringent phenomenological requirements and establishing a coherent framework that jointly constrains CMB observables and reheating dynamics.

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Feng-Yi Zhang, Rongrong Zhai, Li-Yang Chen. 2025-08-22. Reconciling the ACT Preference in $f(T)$ Gravity: Inflation and Reheating Constraints. https://doi.org/10.1088/1674-1137%2Fae5806

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