Quadratic energy–momentum squared gravity: Constraints from big bang nucleosynthesis

In this work, we extend the standard cosmological model within the quadratic energy–momentum squared gravity (qEMSG) framework, introducing a nonminimal interaction between the usual material field (T_μν) and its accompanying partner field (qEMSF, T_μν^qEMSF), defined by f(T²)=αT² with T²=T_μνT^μν. Adopting an analytical approach within the qEMSG framework, we present a comprehensive exploration of Big Bang Nucleosynthesis (BBN) dynamics. Our analysis selects the radiation-dominated universe solution compatible with the standard cosmological model limit as α→0 and reveals that qEMSF interaction model can modify the radiation energy density's evolution, potentially altering neutron–proton interconversion rates and consequently affecting ⁴He abundance in various ways. By explicitly defining modifications to the predicted primordial ⁴He mass fraction, Y_p, we establish the most stringent cosmological constraints on the parameter α based on recent measurements of Y_p: (−8.81≤α≤8.14)×10⁻²⁷eV⁻⁴ (68% CL) from Aver et al.’s primordial ⁴He abundance measurements, aligning with α=0. Additionally, (3.48≤α≤4.43)×10⁻²⁷eV⁻⁴ (68% CL) from Fields et al.’s estimates, utilizing the Planck-CMB estimated baryon density within the standard cosmological model framework, diverges from α=0, thereby lending support to the qEMSF interaction model. The study also highlights the bidirectional nature of energy–momentum/entropy transfer in qEMSF interaction model, depending on the sign of α. The implications of qEMSF in the presence of additional relativistic relics are also explored, showcasing the model's potential to accommodate deviations from standard cosmology and the Standard Model of particle physics.