D. F. P. Cruz, D. S. Pereira, F. S. N. Lobo, J. P. Mimoso
Abstract
We investigate Big Bang Nucleosynthesis (BBN) in the framework of f(T,Lm) gravity, where the gravitational Lagrangian depends on the torsion scalar T and the matter Lagrangian Lm. Working within a semi-analytical BBN strategy, we encode departures from GR through the expansion-rate ratio Z≡H/HGR evaluated at a characteristic freeze-out temperature and combine this with the freeze-out condition and the observationally inferred abundances of deuterium and helium-4 to constrain the free parameters of three representative EFT-motivated f(T,Lm) models. A distinctive aspect of f(T,Lm) cosmology is that the explicit Lm dependence can induce an effective energy exchange between the standard component and the modified-gravity sector; we therefore derive the corresponding interaction term Q and restrict our analysis to the adiabatic regime ∊≡|Qrad/(4Hρ)|≪1 throughout the BBN window, ensuring internal consistency of the temperature-based BBN mapping. Finally, to connect the radiation-era constraints with the late-time background, we present a two-fluid (dust+radiation) analysis showing how the Lm-dependent corrections decouple as ℓ∝(1+z)4→0, yielding torsion-only (f(T) or TEGR) cosmologies at late times on the GR-connected branch. Our results provide transparent first-pass BBN bounds on torsion─matter EFT corrections and identify viable parameter regions consistent with early-Universe data providing a controlled starting point for further early-Universe phenomenology in f(T,Lm) gravity.
The European Physical Journal C
Volume 86, Issue 717
2026 June
