A quantum gravitational relaxation of the cosmological constant

Similar to QCD, general relativity has a Θ sector due to large diffeomorphisms. We make explicit, for the first time, that the gravitational CP violating Θ parameter is nonperturbatively related to the cosmological constant. A gravitational pseudoscalar coupling to massive fermions gives rise to general relativity from a topological B∧F theory through a chiral symmetry breaking mechanism. We show that a gravitational Peccei-Quinn like mechanism can dynamically relax the cosmological constant.