Relaxing nucleosynthesis constraints on Brans-Dicke theories

We reconsider constraints on Brans-Dicke theories arising from the requirement of successful big bang nucleosynthesis. Such constraints typically arise by imposing that the universe be radiation-dominated at early times, and therefore restricting the contribution that a Brans-Dicke scalar could make to the energy budget of the universe. However, in this paper we show how the dynamics of the Brans-Dicke scalar itself can mimic a radiation-dominated kinematics, thereby allowing successful nucleosynthesis with a sizable contribution to the total cosmic energy density. In other words Newton's constant may dynamically acquire values quite different from that today, even though the evolution mimics radiation domination. This possibility significantly relaxes the existing bounds on Brans-Dicke fields, and opens the door to new possibilities for early universe cosmology. The necessary fine tunings required by such an arrangement are identified and discussed.