Similar to QCD, general relativity has a $\Theta$ sector due to large diffeomorphisms. We make explicit, for the first time, that the gravitational CP violating $\Theta$ parameter is non-perturbatively related to the cosmological constant. A gravitational pseudoscalar coupling to massive fermions gives rise to general relativity from a topological $B\wedge F$ theory through a chiral symmetry breaking mechanism. We show that a gravitational Peccei-Quinn like mechanism can dynamically relax the cosmological constant.
Categories: hep-th astro-ph gr-qc hep-ph