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 nonperturbatively related to the cosmological constant. A gravitational pseudoscalar coupling to massive fermions gives rise to general relativity from a topological B A F theory through a chiral symmetry breaking mechanism. We show that a gravitational Peccei-Quinn like mechanism can dynamically relax the cosmological constant. (c) 2005 Published by Elsevier B.V.
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