Big bang nucleosynthesis and entropy evolution in f(R, T) gravity

The present article is devoted to constraining the model parameter chi for the f(R,T)=R+chi T gravity model by employing the constraints coming from big bang nucleosynthesis. We solve the field equations and constrain chi in the range -0.14 kappa 2 <=chi <= 0.84 kappa 2 (where kappa 2=8 pi Gc4) from the primordial abundances of light elements such as helium-4, deuterium and lithium-7. We found the abundances of helium-4 and deuterium agree with theoretical predictions; however, the lithium problem persists for the f(R, T) gravity model. We also investigate the evolution of entropy for the constrained parameter space of chi for the radiation and dust universe. We report that entropy is constant when chi=0 for the radiation-dominated universe, whereas for the dust universe, entropy increases with time. We finally use the constraints to show that chi has a negligible influence on the cold dark matter annihilation cross section.