Classical cancellation of the cosmological constant reconsidered

We revisit a scenario in which the cosmological constant is cancelled by the potential energy of a slowly evolving scalar field, or cosmon. The cosmon's evolution is tied to the cosmological constant by a feedback mechanism. This feedback is achieved by an unconventional coupling of the cosmon field to the Ricci curvature scalar. The solutions show that the effective cosmological constant evolves approximately as t(-2) and remains always of the same order as the density of ordinary matter and radiation. Newton's constant varies on cosmological time scales, with (G)over dot(N)/G(N) << 1/t. G(N) could have been somewhat different, and possibly smaller, at the time of Big Bang nucleosynthesis.