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˙_N/G_N≪1/t. G_N could have been somewhat different, and possibly smaller, at the time of Big Bang nucleosynthesis.