NUCLEOSYNTHESIS CONSTRAINTS ON SCALAR-TENSOR THEORIES OF GRAVITY

We study the cosmological evolution of massless single-field scalar-te nsor theories of gravitation from the time before the onset of e(+)e(- ) annihilation and nucleosynthesis up to the present. The cosmological evolution together with the observational bounds on the abundances of the lightest elements (those mostly produced in the early universe) p lace constraints on the coefficients of the Taylor series expansion of alpha(phi), which specifies the coupling of the scalar field to matte r and is the only free function in the theory. In the case when alpha( phi) has a minimum (i.e., when the theory evolves towards general rela tivity) these constraints translate into a stronger limit on the post- Newtonian parameters gamma and beta than any other observational test. Moreover, our bounds imply that, even at the epoch of annihilation an d nucleosynthesis, the evolution of the universe must be very close to that predicted by general relativity if we do not want to over-or und erproduce He-4. Thus the amount of scalar field contribution to gravit y is very small even at such an early epoch. [S0556-2821(97)04724-3].