Nucleosynthesis bounds on scalar-tensor gravity: power-law couplings

A simple model of variation of primordial helium production is used to fix bounds for the exponent of power-law type of couplings (ω(Φ) α Φα) in scalar-tensor theories of gravitation. It is found that α ≥ 4 is compatible with the observed abundance. This lower limit is obtained for k = 0 models and for a constant Ωoh2 = 1 and varies for others values of this quantity.