Nucleosynthesis Constraints on Scalar-Tensor Theories of Gravity

TL;DR

This paper investigates how scalar-tensor theories of gravity are constrained by early universe nucleosynthesis, showing that deviations from general relativity are tightly limited to match observed light element abundances.

Contribution

It provides new bounds on scalar-tensor gravity theories based on primordial element abundances, especially when the coupling function has a minimum leading to general relativity.

Findings

Constraints on the Taylor series coefficients of the coupling function a(φ).

Stronger limits on Post-Newtonian parameters γ and β.

Early universe evolution must be close to general relativity.

Abstract

We study the cosmological evolution of massless single-field scalar-tensor 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) place constraints on the coefficients of the Taylor series expansion of $a(\phi)$, which specifies the coupling of the scalar field to matter and is the only free function in the theory. In the case when $a(\phi)$ has a minimum (i.e., when the theory evolves towards general relativity) 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 and nucleosynthesis, the evolution of the universe must be very close to that predicted by general relativity if we do not want to over- or underproduce $^{4}$He. Thus the amount of scalar field contribution to gravity is very small even at such an early epoch.